Download Chapter 4 Atomic Structure

4/26/2017
4.1 Studying Atoms
Pearson Prentice Hall
Physical Science: Concepts
in Action
Chapter 4
Atomic Structure
Democritus (4th century B.C) said the
universe was made of invisible units
called atoms
In 1808, an Englishman named John
Dalton proposed a theory:
1. every element is made of tiny
unique particles called atoms that
cannot be subdivided
2. atoms of the same element have
the same mass & atoms of different
elements have different masses
3. compounds contain atoms of more
than one element
4. in any particular compound, atoms
of different elements always combine
in the same way
Objectives:
1. Explain Dalton’s atomic
theory
2. Explain the contributions of
Thomson and Rutherford
Structure of the Atom
Atomic Components
• Atom-smallest piece of matter that still
retains the property of the element.
• Examples?
• Hydrogen
• Oxygen
• Helium
• Iron
Click image to view movie
• Gold
Structure of the Atom
Scientific Shorthand
Thomson & Rutherford
Q: What did Dalton notice that
all compounds have in
common?
A: Compounds contain atoms
of more than one element
JJ Thomson (1856-1940) used
a sealed tube of gas to show
that particles are charged
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In Thomson’s experiment, the
particles repelled the negative
plate and were attracted to
the positive plate
Q: What was the charge on the
particles in the beam of the
gas?
Thomson’s experiments
provided the first evidence
that atoms are made of even
smaller particles
Q: How do objects of the same
charge behave when they
come close to one another?
Thomson developed the “plum
pudding” model of the atom
His model suggested that
negative charges were evenly
scattered throughout an atom
filled with a positively charged
mass of matter
Ernest Rutherford (1871-1937)
discovered fast moving particles in
uranium that he called alpha
particles
Uranium is a radioactive substance
He used a zinc sulfide screen, gold
foil and alpha particles to produce a
very famous experiment in which he
showed that the positive charges
are not evenly dispersed throughout
the atom
Rutherford concluded that atoms
must have a concentrated central
core
Definition: a nucleus is a dense,
positively charged mass located
in the center of the atom
According to Rutherford’s model,
all of an atom’s positive charge is
concentrated in its nucleus
Today, much of an atom consists
of empty space outside the
positively charged nucleus
4.2 The Structure of an Atom
Objectives:
1. Describe the three subatomic
particles
2. Compare protons, electrons and
neutrons in terms of physical
properties
3. Explain how atoms of one
element are different from atoms of
other elements
4. Describe the difference between
2 isotopes of the same element
Three Subatomic Particles
Protons, neutrons & electrons are
subatomic particles
Definition: a proton is a positively
charged subatomic particle that is
found in the nucleus of an atom
Definition: an electron is a negatively
charged subatomic particle found in
the space OUTSIDE THE NUCLEUS
(ORBITALS)
Definition: a neutron is a neutral
subatomic particle found in the
nucleus of an atom
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Structure of the Atom
Atomic Components
• Nucleus- positively charged center of the
atom that contains protons and neutrons.
Surrounded by electron cloud. Ernest
Rutherford provided evidence for this.
• Protonsparticles
with an
electrical
charge of
1+.
Structure of the Atom
Quarks—Even Smaller Particles
• Protons and neutrons are made up of
smaller particles called quarks.
• So far, scientists have confirmed the
existence of six uniquely different
quarks.
Comparing Protons, Neutrons &
Electrons
Protons, neutrons and electrons
can be distinguished by mass,
charge & location in an atom
The mass of a neutron is almost
exactly equal to the mass of a
proton
Electrons are nearly 2,000 times
less massive than either a
proton or a neutron
Structure of the Atom
Atomic Components
• Electrons- particles with an electrical
charge of 1–. J.J. Thomson provided
evidence that an atom contains
negatively charged particles.
• Neutrons -are neutral particles that do
not have an electrical charge.
Structure of the Atom
Quarks—Even Smaller Particles
• Scientists theorize that an arrangement
of three quarks held together with the
strong nuclear force produces a proton.
• Another arrangement of three quarks
produces a neutron
Isotopes
Definition: isotopes are atoms of the
same element that have different
numbers of neutrons and different
mass numbers
Isotopes: element has same atomic
number, same # protons, different #
neutrons, different mass #
Q: Why do isotopes have different
mass numbers for the same element?
A: Because they have different
numbers of neutrons
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Masses of Atoms
Isotopes
• Not all the atoms of an element have the
same number of neutrons.
• isotopes Atoms of the same element
that have different numbers of neutrons
Masses of Atoms
Identifying Isotopes
• average atomic mass- is the weightedaverage mass of the mixture of its isotopes.
• For example, four out of five atoms of
boron are boron-11, and one out of five is
boron-10.
• To find the weighted-average or the average
atomic mass of boron, you would solve the
following equation:
Atoms of Different Elements
Atoms of any given element
ALWAYS have the same number of
PROTONS
Definition: atomic number of an
element equals the number of
protons
Atoms of different elements
ALWAYS have different numbers of
PROTONS
The number of PROTONS
determines which element you have
Masses of Atoms
Identifying Isotopes
• Models of two isotopes of boron are
shown. Because the numbers of
neutrons in the isotopes are different, the
mass numbers are also different.
• You use the name of
the element followed
by the mass number
of the isotope to
identify each
isotope: boron-10
and boron-11.
The mass of an electron is
insignificant
Thomson’s experiment with the gas
tube detected electrons
Rutherford discovered the positive
central core
The mass of each proton or neutron
equals one atomic mass unit (amu)
Q: What is the charge of a proton?
(neutron? electron?)
Q: Where is the proton located in an
atom? (neutron? electron?)
Masses of Atoms
Atomic Mass
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Definition: mass number of an
atom is the sum of the protons
plus the neutrons
Neutrons + Protons = Mass #
Q: Why aren’t electrons considered
in the mass number of an atom?
# neutrons = mass # - atomic #
Q: What subatomic particle could
be substituted for atomic number?
Q: What 2 values represent mass#?
Masses of Atoms
Atomic Mass
• The mass of each
is approximately
1,836 times
greater than the
mass of the
electron.
Masses of Atoms
Protons Identify the Element
• The number of protons tells you
what type of atom you have
Masses of Atoms
Atomic Mass
• The mass of a
proton is about the
same as that of a
neutron
Masses of Atoms
Atomic Mass
• atomic mass unit (amu)- unit of
measurement used for atomic particles
• proton and neutron = 1 amu.
Masses of Atoms
Mass Number
• mass number- sum of the number of
protons and the number of neutrons in
the atom.
For example, every carbon atom has six
protons. Also, all atoms with six protons
are carbon atoms.
• atomic number- The number of protons
in an atom
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Masses of Atoms
Mass Number
• If you know the mass number and the
atomic number of an atom, you can
calculate the number of neutrons.
number of neutrons = mass number – atomic number
Losing and Gaining Energy
Since the time of Democritus &
Dalton the model of the atom has
been revised
In 1913, Niels Bohr theorized that
atoms move in set paths around the
nucleus like planets orbiting the sun
The electron path is an electron’s
energy level
Energy level: any of the possible
energies an electron may have in an
atom
How Electrons Behave in Atoms
Definition: an electron cloud is
a visual model of the most
likely locations for electrons in
an atom
Scientists use the electron
cloud model to describe the
possible locations of electrons
around the nucleus
Instead of set paths like orbits,
electrons are found in orbitals
which have unique shapes
4.3 Modern Atomic Theory
Objectives:
1. Explain what can happen
when atoms gain or lose
energy
2. Explain the model that
scientists use to describe how
electrons behave in atoms
3. Describe the most stable
configuration of electrons in
an atom
Electrons in atoms must gain
energy to move to a higher energy
level or lose energy to move to a
lower energy level
The amount of energy gained
(absorbed) to go higher or energy
lost (released) can be measured
Each element has its own unique
set of energy levels
The element itself determines the
amount of energy gained or lost
Bohr’s model does not explain all
chemical observations
Structure of the Atom
The Electron Cloud Model
• By 1926, scientists had developed
the electron cloud model of the atom
that is in use today.
• electron cloudarea around the
nucleus of an
atom where
electrons are
found.
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Structure of the Atom
The Electron Cloud Model
• The electron cloud is 100,000 times
larger than the diameter of the nucleus.
• In contrast, each electron in the cloud is
much smaller than a single proton.
• Because an electron's mass is small and
the electron is moving so quickly around
the nucleus, it is impossible to describe
its exact location in an atom.
An electron cloud is a good
approximation of how electrons
behave in their orbitals
Definition: an orbital is a region
of space around the nucleus of
an atom where there is a high
probability of finding an electron
The level with the least energy
has only one orbital
Higher energy levels have more
than one orbital
Section Check
Electron Configurations
Definition: an electron configuration is
the arrangement of electrons in the
orbitals of an atom
The most stable configuration is the
one in which the electrons are in
orbitals with the lowest possible
energies
Definition: the ground state is when
all electrons in an atom have the
lowest possible energies
Definition: excited state is when an
electron is in an orbital with higher
energy & less stability
Question 1
Which is the smallest piece of matter
that still retains the property of the
element?
A. atom
B. quark
C. neutron
D. proton
Section Check
Section Check
Answer
Question 2
The answer is A. An atom is the smallest
piece of matter that still retains the
property of the element.
What particles are found in the nucleus
of an atom?
A. protons and electrons
B. protons and neutrons
C. neutrons and electrons
D. quarks and electrons
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Section Check
Section Check
Answer
Question 3
The answer is B.
Electrons are
located in an
electron cloud
surrounding the
nucleus of the
atom.
What is the name of the small particles
that make up protons and neutrons?
Answer
Protons and neutrons are made of
smaller particles called quarks.
Section Check
Section Check
Question 1
Question 2
How is the atomic number of an element
determined?
The element helium has a mass number
of 4 and atomic number of 2. How many
neutrons are in the nucleus of a helium
atom?
Answer
The atomic number of an element is
equal to the number of protons in an
atom of that element.
Section Check
Section Check
Answer
Question 3
Recall that the atomic number is equal
to the number of protons in the nucleus.
Since the mass number is 4 and the
atomic number is 2, there must be 2
neutrons in the nucleus of a helium
atom.
How much of the mass of an atom is
contained in an electron?
Answer
The electron’s mass is so small that it is
considered negligible when finding the
mass of an atom.
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