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Chapter Presentation
Bellringer
Transparencies
Sample Problems
Visual Concepts
Standardized Test Prep
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Chapter 3
Atoms and Moles
Table of Contents
Section 1 Substances Are Made of Atoms
Section 2 Structure of Atoms
Section 3 Electron Configuration
Section 4 Counting Atoms
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Chapter 3
Section 1 Substances Are Made
of Atoms
Bellringer
• Make a list of inferences about any properties of
objects in the box.
• How could you learn more about the objects in the
box without opening the box?
• Scientist face these same questions as they try to
learn more about atoms.
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Chapter 3
Section 1 Substances Are Made
of Atoms
Objectives
• State the three laws that support the existence of
atoms.
• List the five principles of John Dalton’s atomic
theory.
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Chapter 3
Section 1 Substances Are Made
of Atoms
Atomic Theory
• The idea of an atomic theory is more than 2000 years
old.
• Until recently, scientists had never seen evidence of
atoms.
• The law of definite proportions, the law of
conservation of mass and the law of multiple
proportions support the current atomic theory.
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Chapter 3
Section 1 Substances Are Made
of Atoms
Atomic Theory, continued
• The figure on the right is a more accurate
representation of an atom than the figure on the left.
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Chapter 3
Section 1 Substances Are Made
of Atoms
Atomic Theory, continued
The Law of Definite Proportions
• The law of definite proportions states that a
chemical compound always contains the same
elements in exactly the same proportions by weight or
mass.
• The law of definite proportions also states that every
molecule of a substance is made of the same number
and types of atoms.
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Chapter 3
Visual Concepts
Law of Definite Proportions
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Visual Concept
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Chapter 3
Section 1 Substances Are Made
of Atoms
Atomic Theory, continued
The Law of Conservation of Mass
• The law of conservation of mass states that mass
cannot be created or destroyed in ordinary chemical
and physical changes.
• The mass of the reactants is equal to the mass of the
products.
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Chapter 3
Section 1 Substances Are Made
of Atoms
Law of Conservation of Mass
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Chapter 3
Section 1 Substances Are Made
of Atoms
Law of Conservation of Mass
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Chapter 3
Visual Concepts
Law of Conservation of Mass
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Visual Concept
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Chapter 3
Section 1 Substances Are Made
of Atoms
Atomic Theory, continued
The Law of Multiple Proportions
• The law of multiple proportions states that when
two elements combine to form two or more
compounds, the mass of one element that combines
with a given mass of the other is in the ratio of small
whole numbers.
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Chapter 3
Section 1 Substances Are Made
of Atoms
Law of Multiple Proportions
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Chapter 3
Visual Concepts
Law of Multiple Proportions
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Chapter 3
Section 1 Substances Are Made
of Atoms
Dalton’s Atomic Theory
• In 1808, John Dalton developed an atomic theory.
• Dalton believed that a few kinds of atoms made up all
matter.
• According to Dalton, elements are composed of only
one kind of atom and compounds are made from two
or more kinds of atoms.
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Chapter 3
Section 1 Substances Are Made
of Atoms
Dalton’s Atomic Theory , continued
Dalton’s Theory Contains Five Principles
1. All matter is composed of extremely small particles
called atoms, which cannot be subdivided, created,
or destroyed.
2. Atoms of a given element are identical in their
physical and chemical properties.
3. Atoms of different elements differ in their physical
and chemical properties.
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Chapter 3
Section 1 Substances Are Made
of Atoms
Dalton’s Atomic Theory , continued
Dalton’s Theory Contains Five Principles, continued
4. Atoms of different elements combine in simple,
whole-number ratios to form compounds.
5. In chemical reactions, atoms are combined,
separated, or rearranged but never created,
destroyed, or changed.
•
Data gathered since Dalton’s time shows that the
first two principles are not true in all cases.
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Chapter 3
Section 2 Structure of Atoms
Bellringer
• Look at the following terms:
electron, nucleus, proton, neutron, atomic number,
mass number, isotope
• Make a list of the terms that are unfamiliar to you?
• After completing this section, look over your list to
check that you are familiar with and understand all of
the terms.
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Chapter 3
Section 2 Structure of Atoms
Objectives
• Describe the evidence for the existence of electrons,
protons, neutrons, and describe the properties of
these subatomic particles.
• Discuss atoms of different elements in terms of their
numbers of electrons, protons, neutrons, and define
the terms atomic number and atomic mass.
• Define isotope, and determine the number of
particles in the nucleus of an isotope.
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles
• Experiments by several scientists in the mid-1800s
led to the first change to Dalton’s atomic theory.
Scientists discovered that atoms can be broken into
pieces after all.
• The smaller parts that make up atoms are called
subatomic particles.
• The three subatomic particles that are most important
for chemistry are the electron, the proton, and the
neutron.
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
Electrons Were Discovered Using Cathode Rays
• To study current, J. J. Thomson pumped most of the
air out of a glass tube. He applied a voltage to two
metal plates, called electrodes, which were placed at
either end of the tube.
• One electrode, called the anode, was attached to the
positive terminal of the voltage source, so it had a
positive charge.
• The other electrode, called a cathode, had a negative
charge because it was attached to the negative
terminal of the voltage source.
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
Electrons Were Discovered Using Cathode Rays,
continued
• Thomson observed a glowing beam that came out of
the cathode and struck the anode and the nearby
glass walls of the tube.
• He called these rays cathode rays.
• The glass tube Thomson used is known as a
cathode-ray tube (CRT).
• CRTs are used in television sets, computer monitors,
and radar displays.
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
An Electron Has a Negative Charge
• Because the cathode ray came from the negatively
charged cathode, Thomson reasoned that the ray was
negatively charged.
• Thomson confirmed this prediction by seeing how electric and
magnetic fields affected the cathode ray.
• Thomson also observed that when a small paddle
wheel was placed in the path of the rays, the wheel
would turn.
• This suggested that the cathode rays consisted of tiny
particles that were hitting the paddles of the wheel.
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
An Electron Has a Negative Charge, continued
• Thomson’s experiments showed that a cathode ray
consists of particles that have mass and a negative
charge.
• These particles are called electrons.
• An electron is a subatomic particle that has a negative
electric charge.
• Electrons are negatively charged, but atoms have no
charge.
• Atoms contain some positive charges that
balance the negative charges of the electrons.
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
An Electron Has a Negative Charge, continued
• Properties of Electrons
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Chapter 3
Visual Concepts
Thompson’s Cathode Ray Tube Experiment
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Visual Concept
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
Rutherford Discovered the Nucleus
• Thomson proposed that the electrons of an atom
were embedded in a positively charged ball of matter.
His model of an atom was named the plum-pudding
model.
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
Rutherford Discovers the Nucleus, continued
• Ernest Rutherford performed the gold foil experiment,
which disproved the plum-pudding model of the atom.
• A beam of small, positively charged particles, called alpha
particles, was directed at a thin gold foil.
• Rutherford’s team measured the angles at which the
particles were deflected from their former straight-line paths
as they came out of the foil.
• Rutherford found that most of the alpha particles shot
at the foil passed straight through the foil. But very
few were deflected, in some cases backward.
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Chapter 3
Gold Foil Experiment
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
Rutherford Discovers the Nucleus, continued
• Rutherford reasoned that only a very concentrated
positive charge in a tiny space within the gold atom
could possibly repel the fast-moving, alpha particles
enough to reverse the alpha particles’ direction.
• Rutherford also hypothesized that the mass of this
positive-charge containing region, called the nucleus,
must be larger than the mass of the alpha particle.
• Rutherford argued that the reason most of the alpha
particles were undeflected, was that most parts of the
atoms in the gold foil were empty space.
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Chapter 3
Gold Foil Experiment on the Atomic Level
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
Rutherford Discovers the Nucleus, continued
• The nucleus is the dense, central portion of the atom.
• The nucleus is made up of protons and neutrons.
• The nucleus has all of the positive charge, nearly all
of the mass, but only a very small fraction of the
volume of the atom.
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Chapter 3
Visual Concepts
Rutherford’s Gold Foil Experiment
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Visual Concept
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
Proton and Neutrons Compose the Nucleus
• Protons are the subatomic particles that have a
positive charge and that is found in the nucleus of an
atom.
• The number of protons of the nucleus is the atomic number,
which determines the identity of an element.
• Because protons and electrons have equal but opposite
charges, a neutral atom must contain equal numbers of
protons and electrons.
• Neutrons are the subatomic particles that have no
charge and that is found in the nucleus of an atom.
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
Proton and Neutrons Compose the Nucleus, continued
• Properties of a Proton and a Neutron
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Chapter 3
Visual Concepts
Parts of an Atom
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Visual Concept
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
Protons and Neutrons Can Form a Stable Nucleus
• Coulomb’s law states that the closer two charges are,
the greater the force between them.
• The repulsive force between two protons is large
when two protons are close together.
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
Protons and Neutrons Can Form a Stable Nucleus
• Protons form stable nuclei despite the repulsive force
between them.
• A strong attractive force between these protons
overcomes the repulsive force at small distances.
• Because neutrons also add attractive forces, some
neutrons can help stabilize a nucleus.
• All atoms that have more than one proton also
have neutrons.
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Chapter 3
Section 2 Structure of Atoms
Atomic Number and Mass Number
Atomic Number Is the Number of Protons of the
Nucleus
• The number of protons that an atom has is known as
the atom’s atomic number.
• The atomic number is the same for all atoms of an
element.
• Because each element has a unique number of
protons in its atoms, no two elements have the
same atomic number.
• Example: the atomic number of hydrogen is 1 because
the nucleus of each hydrogen atom has one proton.
The atomic number of oxygen is 8.
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Chapter 3
Section 2 Structure of Atoms
Atomic Number and Mass Number, continued
Atomic Number Is the Number of Protons of the
Nucleus, continued
• Atomic numbers are always whole numbers.
• The atomic number also reveals the number of
electrons in an atom of an element.
• For atoms to be neutral, the number of negatively charged
electrons must equal the number of positively charged
protons.
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Chapter 3
Section 2 Structure of Atoms
Atomic Number and Mass Number, continued
Atomic Number Is the Number of Protons of the
Nucleus, continued
• The atomic number for oxygen tells you that the
oxygen atom has 8 protons and 8 electrons.
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Chapter 3
Visual Concepts
Atomic Number
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Chapter 3
Section 2 Structure of Atoms
Atomic Number and Mass Number, continued
Mass Number Is the Number of Particles of the
Nucleus, continued
• The mass number is the sum of the number of
protons and neutrons in the nucleus of an atom.
• You can calculate the number of neutrons in an atom
by subtracting the atomic number (the number of
protons) from the mass number (the number of
protons and neutrons).
mass number – atomic number = number of neutrons
• Unlike the atomic number, the mass number can vary
among atoms of a single element.
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Chapter 3
Section 2 Structure of Atoms
Atomic Number and Mass Number, continued
Mass Number Is the Number of Particles of the
Nucleus, continued
• Example: a particular atom of neon has a mass
number of 20.
• Because the atomic number for an atom of neon is
10, neon has 10 protons.
number of protons and neutrons (mass number) = 20
 number of protons (atomic number) = 10
number of neutrons = 10
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Chapter 3
Section 2 Structure of Atoms
Atomic Number and Mass Number, continued
Mass Number Is the Number of Particles of the
Nucleus, continued
• The neon atom has 10 protons, 10 electrons, and
10 neutrons. The mass number is 20.
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Chapter 3
Visual Concepts
Mass Number
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Chapter 3
Section 2 Structure of Atoms
Determining the Number of Particle In An
Atom
Sample Problem A
How many protons, electrons, and neutrons are
present in an atom of copper whose atomic number is
29 and whose mass number is 64?
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Chapter 3
Section 2 Structure of Atoms
Sample Problem A Solution
• The atomic number indicates the number of protons in
the nucleus of a copper atom.
atomic number (29) = number of protons = 29
• A copper atom must be electrically neutral, so the
number of electrons equals the number of protons.
number of protons = number of electrons = 29
• The mass number indicates the total number of protons
and neutrons
mass number (64) - atomic number (29) =
number of neutrons = 35
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Chapter 3
Section 2 Structure of Atoms
Atomic Number and Mass Number, continued
Atomic Structures Can Be Represented by Symbols
• Each element has a name, and the same name is
given to all atoms of an element.
• Example: sulfur is composed of sulfur atoms.
• Each element has a symbol, and the same symbol is
used to represent one of the element’s atoms.
• Atomic number and mass number are sometimes
written with an element’s symbol.
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Section 2 Structure of Atoms
Chapter 3
Atomic Number and Mass Number, continued
Atomic Structures Can Be Represented by Symbols
• The atomic number always appears on the lower left
side of the symbol.
H
1
He 3Li
Be
2
4
B
5
• Mass numbers are written on the upper left side of the
symbol.
1
H
2
H
3
He
4
He
6
Li
7
Li
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9
Be
10
B
11
B
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Section 2 Structure of Atoms
Chapter 3
Atomic Number and Mass Number, continued
Atomic Structures Can Be Represented by Symbols
• Both numbers may be written with the symbol.
1
1
H
4
2
He
7
3
Li
9
4
Be
11
5
B
• An element may be represented by more than one
notation.
1
1
H
4
2
H
3
1
H
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Chapter 3
Section 2 Structure of Atoms
Atomic Number and Mass Number, continued
Isotopes of an Element Have the Same Atomic
Number
• All atoms of an element have the same atomic
number and the same number of protons. Atoms do
not necessarily have the same number of neutrons.
• Atoms of the same element that have different
numbers of neutrons are called isotopes.
• One standard method of identifying isotopes is to
write the mass number with a hyphen after the name
of an element.
helium-3 or helium-4
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Chapter 3
Section 2 Structure of Atoms
Atomic Number and Mass Number, continued
Isotopes of an Element Have the Same Atomic
Number, continued
• The second method of identifying isotopes shows the
composition of a nucleus as the isotope’s nuclear
symbol.
3
2
He or 24He
• All isotopes of an element have the same atomic
number. However, their atomic masses are not the
same because the number of neutrons of the atomic
nucleus of each isotope varies.
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Chapter 3
Section 2 Structure of Atoms
Atomic Number and Mass Number, continued
Isotopes of an Element Have the Same Atomic
Number, continued
• The two stable helium isotopes are helium-3 and
helium-4.
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Chapter 3
Section 2 Structure of Atoms
Atomic Number and Mass Number, continued
Isotopes of an Element Have the Same Atomic
Number, continued
• The Stable Isotopes of Lead
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Chapter 3
Visual Concepts
Isotopes and Nuclides
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Chapter 3
Section 2 Structure of Atoms
Determining the Number of Particle In An
Isotope
Sample Problem B
Calculate the numbers of protons, electrons, and
neutrons in oxygen-17 and in oxygen-18.
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Chapter 3
Section 2 Structure of Atoms
Sample Problem B Solution
• atomic number = number of protons =
number of electrons = 8
• mass number - atomic number = number of neutrons
For oxygen-17, 17 - 8 = 9 neutrons
For oxygen-18, 18 - 8 = 10 neutrons
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Chapter 3
Section 3 Electron Configuration
Bellringer
• Make a drawing of what you think an atom looks like.
• As you study this section, compare your drawing of
an atom to the different models of atoms.
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Chapter 3
Section 3 Electron Configuration
Objectives
• Compare the Rutherford, Bohr, and quantum models
of an atom.
• Explain how the wavelengths of light emitted by an
atom provide information about electron energy
levels.
• List the four quantum numbers, and describe their
significance.
• Write the electron configuration of an atom by using
the Pauli exclusion principle and the the aufbau
principle.
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Chapter 3
Section 3 Electron Configuration
Atomic Models
Rutherford’s Model Proposed Electron Orbits
• The experiments of Rutherford’s team led to the
replacement of the plum pudding model of the atom
with a nuclear model of the atom.
• Rutherford suggested that electrons, like planets orbiting the
sun, revolve around the nucleus in circular or elliptical orbits.
• Rutherford’s model could not explain why electrons did not
crash into the nucleus.
• The Rutherford model of the atom was replaced only
two years later by a model developed by Niels Bohr.
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Chapter 3
Section 3 Electron Configuration
Atomic Models, continued
Bohr’s Model Confines Electrons to Energy Levels
• According to Bohr’s model, electrons can be only certain
distances from the nucleus. Each distance corresponds to
a certain quantity of energy that an electron can have.
• An electron that is as close to the nucleus as it can be is in its
lowest energy level.
• The farther an electron is from the nucleus, the higher the
energy level that the electron occupies.
• The difference in energy between two energy levels
is known as a quantum of energy.
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Chapter 3
Section 3 Electron Configuration
Atomic Models, continued
Bohr’s Model Confines Electrons to Energy Levels
• Rutherford’s model of an
atom
• electrons orbit the
nucleus just as planets
orbit the sun
• Bohr’s model of an atom
• electrons travel
around the nucleus in
specific energy levels
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Chapter 3
Section 3 Electron Configuration
Atomic Models, continued
Electrons Act Like Both Particles and Waves
• Thomson’s experiments demonstrated that electrons
act like particles that have mass.
• In 1924, Louis de Broglie pointed out that the
behavior of electrons according to Bohr’s model was
similar to the behavior of waves.
• De Broglie suggested that electrons could be
considered waves confined to the space around a
nucleus.
• As waves, electrons could have only certain frequencies
which correspond to the specific energy levels.
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Chapter 3
Visual Concepts
De Broglie and the Wave-Particle Nature of
Electrons
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Visual Concept
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Chapter 3
Section 3 Electron Configuration
Atomic Models, continued
Electrons Act Like Both Particles and Waves,
continued
• The present-day model of the atom takes into account
both the particle and wave properties of electrons.
• In this model, electrons are located in orbitals, regions
around a nucleus that correspond to specific energy
levels.
• Orbitals are regions where electrons are likely to be found.
• Orbitals are sometimes called electron clouds because they
do not have sharp boundaries. Because electrons can be in
other places, the orbital has a fuzzy boundary like a cloud.
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Chapter 3
Section 3 Electron Configuration
Atomic Models, continued
Electrons Act Like Both Particles and Waves,
continued
• According to the current model of an atom, electrons
are found in orbitals.
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Chapter 3
Visual Concepts
Orbital
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Chapter 3
Visual Concepts
Comparing Models of Atoms
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Visual Concept
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Chapter 3
Section 3 Electron Configuration
Electrons and Light
• By 1900, scientists knew that light could be thought of
as moving waves that have given frequencies,
speeds, and wavelengths.
• In empty space, light waves travel at 2.998  108 m/s.
• The wavelength is the distance between two
consecutive peaks or troughs of a wave.
• The distance of a wavelength is usually measured in
meters.
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Chapter 3
Visual Concepts
Characteristics of a Wave
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Visual Concept
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Chapter 3
Section 3 Electron Configuration
Electromagnetic Spectrum
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Chapter 3
Visual Concepts
Electromagnetic Spectrum
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Visual Concept
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Chapter 3
Section 3 Electron Configuration
Electrons and Light, continued
• The electromagnetic spectrum is all of the
frequencies or wavelengths of electromagnetic
radiation.
• The wavelength of light can vary from 105 m to less
than 10–13 m.
• In 1905, Albert Einstein proposed that light also has
some properties of particles.
• His theory would explain a phenomenon known as the
photoelectric effect.
• This effect happens when light strikes a metal and electrons
are released.
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Chapter 3
Section 3 Electron Configuration
Electrons and Light, continued
• Einstein proposed that light has the properties of both
waves and particles.
• Light can be described as a stream of particles,
the energy of which is determined by the light’s
frequency.
Light is an electromagnetic wave.
• Red light has a low frequency and a long wavelength.
• Violet light has a high frequency and a short wavelength.
• The frequency and wavelength of a wave are
inversely related.
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Chapter 3
Section 3 Electron Configuration
Electrons and Light, continued
Light is an electromagnetic Wave, continued
• The frequency and wavelength of a wave are
inversely related.
• As frequency increases, wavelength decreases.
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Chapter 3
Section 3 Electron Configuration
Wavelength and Frequency
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Chapter 3
Section 3 Electron Configuration
Electrons and Light, continued
Light Emission
• When a high-voltage current is passed through a tube
of hydrogen gas at low pressure, lavender-colored
light is seen. When this light passes through a prism,
you can see that the light is made of only a few
colors. This spectrum of a few colors is called a lineemission spectrum.
• Experiments with other gaseous elements show that
each element has a line-emission spectrum that is
made of a different pattern of colors.
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Chapter 3
Section 3 Electron Configuration
Electrons and Light, continued
Light Emission, continued
• In 1913, Bohr showed that hydrogen’s lineemission spectrum could be explained by assuming
that the hydrogen atom’s electron can be in any
one of a number of distinct energy levels.
• An electron can move from a low energy level to a high
energy level by absorbing energy.
• Electrons at a higher energy level are unstable and can
move to a lower energy level by releasing energy. This
energy is released as light that has a specific wavelength.
• Each different move from a particular energy level to a
lower energy level will release light of a different
wavelength.
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Chapter 3
Section 3 Electron Configuration
Electrons and Light, continued
Light Provides Information About Electrons
• An electron in a state of its lowest possible energy, is
in a ground state.
• The ground state is the lowest energy state of a quantized
system
• If an electron gains energy, it moves to an excited
state.
• An excited state is a state in which an atom has more
energy than it does at its ground state
• An electron in an excited state will release a specific
quantity of energy as it quickly “falls” back to its
ground state.
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Chapter 3
Section 3 Electron Configuration
Electrons and Light, continued
Light Provides Information About Electrons, continued
• An electron in a hydrogen
atom can move between
only certain energy states,
shown as n = 1 to n = 7.
• In dropping from a higher
energy state to a lower
energy state, an electron
emits a characteristic
wavelength of light.
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Chapter 3
Section 3 Electron Configuration
Hydrogen’s Line-Emission Spectrum
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Chapter 3
Visual Concepts
Absorption and Emission Spectra
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Visual Concept
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Chapter 3
Section 3 Electron Configuration
Quantum Numbers
• The present-day model of the atom is also known as
the quantum model.
• According to this model, electrons within an energy level
are located in orbitals, regions of high probability for
finding a particular electron.
• The model does not explain how the electrons move
about the nucleus to create these regions.
• To define the region in which electrons can be found,
scientists have assigned four quantum numbers that
specify the properties of the electrons.
• A quantum number is a number that specifies the
properties of electrons.
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Chapter 3
Section 3 Electron Configuration
Quantum Numbers, continued
• The principal quantum number, symbolized by n,
indicates the main energy level occupied by the
electron.
• Values of n are positive integers, such as 1, 2, 3,
and 4.
• As n increases, the electron’s distance from the
nucleus and the electron’s energy increases.
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Chapter 3
Visual Concepts
Principal Quantum Number
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Chapter 3
Section 3 Electron Configuration
Quantum Numbers, continued
• The main energy levels can be divided into sublevels.
These sublevels are represented by the angular
momentum quantum number, l.
• This quantum number indicates the shape or type
of orbital that corresponds to a particular sublevel.
• A letter code is used for this quantum number.
• l = 0 corresponds to an s orbital
• l = 1 to a p orbital
• l = 2 to a d orbital
• l = 3 to an f orbital
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Chapter 3
Section 3 Electron Configuration
Quantum Numbers, continued
• The magnetic quantum number, symbolized by m, is a
subset of the l quantum number.
• It also indicates the numbers and orientations of
orbitals around the nucleus.
• The value of m takes whole-number values,
depending on the value of l.
• The number of orbitals includes
•
•
•
•
one s orbital
three p orbitals
five d orbitals
seven f orbitals
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Chapter 3
Section 3 Electron Configuration
Quantum Numbers, continued
• The spin quantum number, indicates the orientation of
an electron’s magnetic field relative to an outside
magnetic field.
• The spin quantum number is represented by:
1
1
+ or 
or (  or  )
2
2
• A single orbital can hold a maximum of two
electrons, which must have opposite spins.
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Chapter 3
Orbital Notation
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Visual Concept
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Chapter 3
Visual Concepts
Quantum Numbers and Orbitals
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Visual Concept
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Chapter 3
Section 3 Electron Configuration
Quantum Numbers, continued
• Quantum Numbers of the First 30 Atomic Orbitals
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Chapter 3
Section 3 Electron Configuration
Electron Configurations
• In 1925 the German chemist Wolfgang Pauli
established a rule is known as the Pauli exclusion
principle.
• The Pauli exclusion principle states that two
particles of a certain class cannot be in the exact
same energy state.
• This means that that no two electrons in the same
atom can have the same four quantum numbers.
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Pauli Exclusion Principle
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Chapter 3
Section 3 Electron Configuration
Electron Configurations, continued
• Two electrons can have the same value of n by
being in the same main energy level.
• These two electrons can also have the same value
of l by being in orbitals that have the same shape.
• These two electrons may also have the same value
of m by being in the same orbital.
• But these two electrons cannot have the same spin
quantum number.
• If one electron has the value of 1/2, then the
other electron must have the value of –1/2.
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Chapter 3
Section 3 Electron Configuration
Electron Configurations, continued
• The arrangement of electrons in an atom is usually
shown by writing an electron configuration.
• Like all systems in nature, electrons in atoms tend to
assume arrangements that have the lowest possible
energies.
• An electron configuration of an atom shows the
lowest-energy arrangement of the electrons for the
element.
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Chapter 3
Section 3 Electron Configuration
Shapes of s, p, and d Orbitals
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Visual Concepts
s Orbitals
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p Orbitals
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Chapter 3
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d Orbitals
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Visual Concept
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Chapter 3
Section 3 Electron Configuration
Electron Configurations, continued
An Electron Occupies the Lowest Energy Level
Available
• The aufbau principle states that electrons fill orbitals
that have the lowest energy first.
• Aufbau is the German word for “building up.”
• The smaller the principal quantum number, the lower
the energy. Within an energy level, the smaller the l
quantum number, the lower the energy.
• So, the order in which the orbitals are filled
matches the order of energies.
1s < 2s < 2p < 3s < 3p
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Chapter 3
Section 3 Electron Configuration
Electron Configurations, continued
An Electron Occupies the Lowest Energy Level
Available, continued
• The energy of the 3d orbitals is slightly higher than
the energy of the 4s orbitals.
• As a result, the order in which the orbitals are
filled is as follows:
1s < 2s < 2p < 3s < 3p < 4s < 3d
• Additional irregularities occur at higher energy levels.
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Chapter 3
Section 3 Electron Configuration
Electron Configurations, continued
An Electron Occupies the Lowest Energy Level
Available, continued
• This diagrams shows how
the energy of the orbitals
can overlap.
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Aufbau Principle
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Chapter 3
Section 3 Electron Configuration
Electron Configurations, continued
An Electron Configuration Is a Shorthand Notation
• Based on the quantum model of the atom, the
arrangement of the electrons around the nucleus can
be shown by the nucleus’s electron configuration.
• Example: sulfur has sixteen electrons.
Its electron configuration is written as
1s22s22p63s23p4.
• Two electrons are in the 1s orbital, two electrons
are in the 2s orbital, six electrons are in the 2p
orbitals, two electrons are in the 3s orbital, and
four electrons are in the 3p orbitals.
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Chapter 3
Section 3 Electron Configuration
Electron Configurations, continued
An Electron Configuration Is a Shorthand Notation,
continued
• Each element’s configuration builds on the previous
elements’ configurations.
• To save space, one can write this configuration by
using a configuration of a noble gas.
• neon, argon, krypton, and xenon
• The neon atom’s configuration is 1s22s22p6, so the
electron configuration of sulfur is
[Ne] 3s23p4
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Electron Configuration
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Chapter 3
Visual Concepts
Noble Gas Notation
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Section 3 Electron Configuration
Chapter 3
Electron Configurations, continued
An Electron Configuration Is a Shorthand Notation,
continued
• Electron orbitals are filled according to Hund’s Rule.
• Hund’s rule states that orbitals of the same n and l
quantum numbers are each occupied by one electron
before any pairing occurs.
• Orbital diagram for sulfur
  

3p
   3s
2p

 2s
1s
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Chapter 3
Section 3 Electron Configuration
Sample Problem C
Writing Electron Configurations
Write the electron configuration for an atom whose
atomic number is 20.
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Chapter 3
Section 3 Electron Configuration
Sample Problem C Solution
• atomic number = number of protons =
number of electrons = 20
• According to the aufbau principle, the order of orbital
filling is 1s,2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on.
• The electron configuration for an atom of this element
is written as follows:
1s22s22p63s23p64s2
• This electron configuration can be abbreviated as
follows:
[Ar]4s2
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Chapter 3
Section 4 Counting Atoms
Bellringer
• A penny has 2.97  1022 copper atoms.
• On a sheet of paper, write out this number in regular
notation with all the zeros.
• What does this tell you about the size of an atom?
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Chapter 3
Section 4 Counting Atoms
Objectives
• Compare the mass quantities and units for atomic
mass with those for molar mass.
• Define mole, and explain why this unit is used to
count atoms.
• Calculate either mass with molar mass or number
with Avogadro’s number given an amount in moles.
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Chapter 3
Section 4 Counting Atoms
Atomic Mass
• You would not expect something as small as an atom
to have much mass.
• Example: copper atoms have an average mass of only
1.0552  10-25 kg.
• Each penny has an average mass of 3.13  10-3 kg and
contains copper.
• How many copper atoms are there in one penny?
• Assuming that a penny is pure copper, you can find the
number of copper atoms using the following conversion
factor: 1 atom Cu/1.0552  10-25 kg
1 atom Cu
22
2.97

10
Cu atoms
3.13  10 kg 
=
-25
1.0552  10 kg
-3
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Chapter 3
Section 4 Counting Atoms
Atomic Mass, continued
Masses of Atoms Are Expressed in Atomic Mass
Units
• A special mass unit is used to express atomic mass.
• This unit has two names—the atomic mass unit
(amu) and the Dalton (Da).
• Mass number and atomic mass units would be the
same because a proton and a neutron each have a
mass of about 1.0 amu.
• Example: A copper-63 atom has an atomic mass of 62.940.
A copper-65 atom has an atomic mass of 64.928.
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Chapter 3
Section 4 Counting Atoms
Atomic Mass, continued
Masses of Atoms Are Expressed in Atomic Mass
Units, continued
• In the periodic table, the mass shown is an average of
the atomic masses of the naturally occurring isotopes.
• Example: copper is listed as 63.546 instead of
62.940 or 64.928.
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Chapter 3
Section 4 Counting Atoms
Introduction to the Mole
• Most samples of elements have great numbers of
atoms.
• A mole is defined as the number of atoms in
exactly 12 grams of carbon-12.The mole is the SI
unit for the amount of a substance.
• The molar mass of an element is the mass in
grams of one mole of the element. Molar mass has
the unit grams per mol (g/mol).
• The mass in grams of 1 mol of an element is
numerically equal to the element’s atomic mass
from the periodic table in atomic mass units.
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Molar Mass
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Chapter 3
Section 4 Counting Atoms
Introduction to the Mole, continued
• Scientists have also determined the number of
particles present in 1 mol of a substance, called
Avogadro’s number.
• One mole of pure substance contains
6.022 1367  1023 particles.
• Avogadro’s number may be used to count any
kind of particle, including atoms and molecules.
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Chapter 3
Section 4 Counting Atoms
Determining the Mass from the Amount In Moles
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Chapter 3
Section 4 Counting Atoms
Converting from Amount in Moles to Mass
Sample Problem D
Determine the mass in grams of 3.50 mol of copper.
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Chapter 3
Section 4 Counting Atoms
Sample Problem D Solution
• First, make a set-up that shows what is given and
what is desired.
3.50 mol Cu  ? = ? g Cu
• Use a conversion factor that has g Cu in the numerator
and mol Cu in the denominator.
? g Cu
3.50 mol Cu 
= ? g Cu
1 mol
• The correct conversion factor is the molar mass of
Cu, 63.55 g/mol.
63.55 g Cu
3.50 mol Cu 
= 222 g Cu
1 mol
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Chapter 3
Section 4 Counting Atoms
Converting from Amount in Moles to
Number of Atoms
Sample Problem E
Determine the number of atoms in 0.30 mol of fluorine
atoms.
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Chapter 3
Section 4 Counting Atoms
Sample Problem E Answer
• To determine the number of atoms, select the
conversion factor that will take you from the amount
in moles to the number of atoms.
amount (mol)  6.022  1023 atoms/mol =
number of atoms
6.022  1023 F atoms
3.50 mol F 
=
1 mol F
1.8  1023 F atoms
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Chapter 3
Section 4 Counting Atoms
Introduction to the Mole, continued
Chemists and Physicists Agree on a Standard
• Originally, atomic masses expressed the ratio of the
mass of an atom to the mass of a hydrogen atom.
• Using hydrogen as the standard turned out to be
inconvenient because hydrogen does not react
with many elements.
• Because oxygen combines with almost all other
elements, oxygen became the standard of
comparison.
• This choice also led to difficulties because
oxygen has as three isotopes.
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Chapter 3
Section 4 Counting Atoms
Introduction to the Mole, continued
Chemists and Physicists Agree on a Standard,
continued
• In 1962, a conference of chemists and physicists
agreed on a scale based on an isotope of carbon.
• Used by all scientists today, this scale defines the
atomic mass unit as exactly one-twelfth of the mass
of one carbon-12 atom.
• As a result, one atomic mass unit is equal to
1.600 5402  10-27 kg.
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Chapter 3
Standardized Test Preparation
Understanding Concepts
1. Which of the following represents an electron
configuration of a calcium atom, whose atomic
number is 20?
A. 1s22s22p63s23p64s2
B. 1s22s22p63s23p64s3
C. 1s22s22p63s13p64s23d1
D. 1s22s22p63s23d8
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Chapter 3
Standardized Test Preparation
Understanding Concepts
1. Which of the following represents an electron
configuration of a calcium atom, whose atomic
number is 20?
A. 1s22s22p63s23p64s2
B. 1s22s22p63s23p64s3
C. 1s22s22p63s13p64s23d1
D. 1s22s22p63s23d8
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Chapter 3
Standardized Test Preparation
Understanding Concepts
2. Which of these is always equal to the number of
protons in an atom?
F. the mass number
G. the number of isotopes
H. the number of neutrons
I. the number of electrons
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Understanding Concepts
2. Which of these is always equal to the number of
protons in an atom?
F. the mass number
G. the number of isotopes
H. the number of neutrons
I. the number of electrons
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Chapter 3
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Understanding Concepts
3. Which of these events occurs when an electron in an
excited state returns to its ground state?
A. Light energy is emitted.
B. Energy is absorbed by the atom.
C. The atom undergoes spontaneous decay.
D. The electron configuration of the atom changes.
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Chapter 3
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Understanding Concepts
3. Which of these events occurs when an electron in an
excited state returns to its ground state?
A. Light energy is emitted.
B. Energy is absorbed by the atom.
C. The atom undergoes spontaneous decay.
D. The electron configuration of the atom changes.
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Understanding Concepts
4. What is the electron configuration of bromine, whose
atomic number is 35?
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Understanding Concepts
4. What is the electron configuration of bromine, whose
atomic number is 35?
Answer: 1s22s22p63s23p63d104s24p5
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Understanding Concepts
5. Electrons do not always act like particles. What
electron behavior did de Broglie observe, and what
evidence did he use to support his ideas?
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Understanding Concepts
5. Electrons do not always act like particles. What
electron behavior did de Broglie observe, and what
evidence did he use to support his ideas?
Answer: De Broglie suggested that electrons could
behave as waves. For evidence he used the
observation that electrons have specific frequencies,
and they diffract.
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Understanding Concepts
6. Only materials with unpaired electrons can exhibit
magnetic properties. Can the element xenon be
highly magnetic? Explain your answer.
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Understanding Concepts
6. Only materials with unpaired electrons can exhibit
magnetic properties. Can the element xenon be
highly magnetic? Explain your answer.
Answer: No. Xenon is a noble gas, so all of its electron
orbitals are filled. Therefore, there can be no
unpaired electrons.
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Chapter 3
Standardized Test Preparation
Reading Skills
Read the passage below. Then answer the questions.
Although there is no detector that allows us to see
the inside of an atom, scientists infer its structure from
the properties of its components. Rutherford's model
shows electrons orbiting the nucleus like planets
around the sun. In Bohr's model the electrons travel
around the nucleus in specific energy levels.
According to the current model, electron orbitals do
not have sharp boundaries and the electrons are
portrayed as a cloud.
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Reading Skills
7. The model of the atom has changed over time
because
F. earlier models were proven to be wrong.
G. electrons do not revolve around the nucleus.
H. as new properties of atoms were discovered,
models had to be revised to account for those
properties.
I. new particles were discovered, so the model had
to be changed to explain how they could exist.
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Reading Skills
7. The model of the atom has changed over time
because
F. earlier models were proven to be wrong.
G. electrons do not revolve around the nucleus.
H. as new properties of atoms were discovered,
models had to be revised to account for those
properties.
I. new particles were discovered, so the model had
to be changed to explain how they could exist.
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Reading Skills
8. Why do scientists need models as opposed to directly
observing electrons?
A. Models can be changed.
B. There is no technology that allows direct
observation of electrons.
C. The charges on the electrons and protons
interfere with direct observation of the atom.
D. Scientists cannot measure the speed of electrons
with sufficient accuracy to determine which model
is correct.
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Reading Skills
8. Why do scientists need models as opposed to directly
observing electrons?
A. Models can be changed.
B. There is no technology that allows direct
observation of electrons.
C. The charges on the electrons and protons
interfere with direct observation of the atom.
D. Scientists cannot measure the speed of electrons
with sufficient accuracy to determine which model
is correct.
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Chapter 3
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Reading Skills
9. What would cause scientists to change the current
model of the atom?
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Reading Skills
9. What would cause scientists to change the current
model of the atom?
Answer: The current model of the atom would change if
new data were found about atoms that could not be
explained by the existing model. The new model would
explain both the new data and all earlier observations.
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Standardized Test Preparation
Interpreting Graphics
Use the diagram below to answer questions 10 through 13.
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Chapter 3
Standardized Test Preparation
Interpreting Graphics
10. Potassium has 19 protons. According to this diagram
of energy levels, what is the energy level of the most
energetic electrons in a potassium atom at its ground
state?
F. 1s
G. 3d
H. 3p
I. 4s
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Interpreting Graphics
10. Potassium has 19 protons. According to this diagram
of energy levels, what is the energy level of the most
energetic electrons in a potassium atom at its ground
state?
F. 1s
G. 3d
H. 3p
I. 4s
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11. Which of these electron transitions emits the largest
amount of energy?
A. 2s to 3d
B. 2s to 4s
C. 3d to 2s
D. 4s to 2s
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11. Which of these electron transitions emits the largest
amount of energy?
A. 2s to 3d
B. 2s to 4s
C. 3d to 2s
D. 4s to 2s
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Standardized Test Preparation
Reading Skills
12. Why is the 4s level below the 3d level on this chart?
F. There are ten 3d electrons but only two 4s
electrons.
G. The 4s electrons have lower energy than the 3d
electrons.
H. It is just a convention to save space when
drawing the chart.
I. There is a smaller transition between 4s and 3p
than between 4s and 3d.
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Copyright © by Holt, Rinehart and Winston. All rights reserved.
Chapter 3
Standardized Test Preparation
Reading Skills
12. Why is the 4s level below the 3d level on this chart?
F. There are ten 3d electrons but only two 4s
electrons.
G. The 4s electrons have lower energy than the 3d
electrons.
H. It is just a convention to save space when
drawing the chart.
I. There is a smaller transition between 4s and 3p
than between 4s and 3d.
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Resources
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Chapter 3
Standardized Test Preparation
Reading Skills
13. The element, titanium, has two electrons in the 3d
orbital. What is the atomic number of titanium?
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Copyright © by Holt, Rinehart and Winston. All rights reserved.
Chapter 3
Standardized Test Preparation
Reading Skills
13. The element, titanium, has two electrons in the 3d
orbital. What is the atomic number of titanium?
Answer: 22
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Copyright © by Holt, Rinehart and Winston. All rights reserved.