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Chapter 10
Introduction to Atoms
Preview
Section 1 Development of the Atomic Theory
Section 2 The Atom
Concept Mapping
Chapter 10
Section 1 Development of the
Atomic Theory
Objectives
• Describe some of the experiments that led to the
current atomic theory.
• Compare the different models of the atom.
• Explain how the atomic theory has changed as
scientists have discovered new information about the
atom.
Chapter 10
Section 1 Development of the
Atomic Theory
The Beginning of the Atomic Theory
• What Is an Element? Around 440 BC, a Greek
philosopher named Democritus thought that you
would eventually end up with a particle that could not
be cut. He called this particle an atom.
• Democritus was a philosopher, not a scientist.
• Aristotle, another Greek philosopher, disagreed with
Democritus’s ideas. He believed that you would never
end up with a particle that could not be cut.
Chapter 10
Section 1 Development of the
Atomic Theory
The Beginning of the Atomic Theory,
continued
• Democritus was right, though:
• Matter is made of particles, which we call atoms.
• An atom is the smallest unit of an element that
maintains the chemical properties of that element.
Chapter 10
Section 1 Development of the
Atomic Theory
• Dalton’s Theory
•John Dalton was a scientist. Democritus was not.
• John Dalton published his atomic theory in 1803. His
theory stated that all substances are made of atoms.
• Atoms are small particles that cannot be created,
divided, or destroyed (divided into smaller pieces).
•He got that last bit wrong. Atoms CAN be divided:
•Into protons, neutrons, electrons
Chapter 10
Section 1 Development of the
Atomic Theory
• Dalton’s Theory , continued
• Atoms of the same element are exactly alike.
• Atoms of different elements are different.
•Different numbers of protons, neutrons, and electrons
• Atoms join with other atoms to make new
substances.
Chapter 10
Section 1 Development of the
Atomic Theory
Dalton’s Atomic Theory Based on
Experiments
• Dalton Not Quite Correct
• So…the atomic theory has changed, as more has
been discovered, to describe the atom more correctly.
Chapter 10
Section 1 Development of the
Atomic Theory
Thomson’s Discovery of Electrons
• Negatively Charged Particles JJ Thomson
experimented with a cathode-ray tube like the one
shown on the next slide. He discovered negatively
charged particles that are now known as electrons.
• Plum Pudding Model After learning that atoms
contain electrons, Thomson proposed a new model of
the atom. Thomson thought that electrons were
mixed throughout an atom, like plums in a
pudding.
Chapter 10
Section 1 Development of the
Atomic Theory
Chapter 10
Section 1 Development of the
Atomic Theory
Rutherford’s Atomic “Shooting Gallery”
• Negatively Charged Particles In 1909, Ernest
Rutherford aimed a beam of small, positively charged
particles at a thin sheet of gold foil. The next slide
shows his experiment.
• Surprising Results Rutherford expected the
particles to pass right through the gold in a straight
line. To Rutherford’s great surprise, some of the
particles were deflected.
• The particles had hit something, unknown and big!
Chapter 10
Section 1 Development of the
Atomic Theory
Chapter 10
Section 1 Development of the
Atomic Theory
Where Are the Electrons?
• Electrons are Far from the
Nucleus!
•Rutherford proposed that in the
center of the atom is a tiny,
positively charged part called the
nucleus.
• Bohr’s Electron Levels In
1913, Niels Bohr proposed that
electrons orbit the nucleus in
certain paths, or energy levels.
Chapter 10
Section 1 Development of the
Atomic Theory
Where Are the Electrons?
• Bohr’s Electron Levels In
1913, Niels Bohr proposed that
electrons orbit the nucleus in
certain paths, or energy levels.
•We now know that electrons are
found in clouds, known as orbitals.
They don’t follow specific orbits
like planets around the sun.
• Werner Heisenberg. Mid-1900s.
Orbitals.
Chapter 10
Section 1 Development of the
Atomic Theory
Where Are the Electrons?, continued
• The Modern Atomic
Theory According to the
current theory, there are
regions inside the atom
where electrons are likely
to found. These regions
are called electron
clouds, or orbitals.
Chapter 10
Section 1 Development of the
Atomic Theory
Comparing Models of the Atom
Click below to watch the Visual Concept.
Visual Concept
Chapter 10
Section 2 The Atom
Bellringer
Answer the following question: An atom is the smallest
particle into which an element can be divided and still
be that element. Now that scientists have learned that
an atom is made up of even smaller particles, is this
definition still accurate?
Explain your answer in your science journal.
Chapter 10
Section 2 The Atom
Objectives
• Describe the size of an atom.
• Name the parts of an atom.
• Describe the relationship between numbers of
protons and neutrons and atomic number.
• State how isotopes differ.
• Calculate atomic masses.
• Describe the forces within an atom.
Chapter 10
Section 2 The Atom
How Small Is an Atom?
• Three One-Hundred-Millionths of a Centimeter!
Scientists know that aluminum is made of averagesized atoms. An aluminum atom has a diameter of
about 0.00000003 cm.
Chapter 10
Section 2 The Atom
What Is an Atom Made Of?
• Inside The Nucleus: Protons and Neutrons
• Protons are positively charged particles
• Neutrons have no electrical charge.
• Outside the Nucleus: Electrons
Electrons are the negatively charged particles
Electrons are found around/outside the nucleus within
orbitals.
Chapter 10
Section 2 The Atom
Chapter 10
Section 2 The Atom
How Do Atoms of Different Elements Differ?
• Starting Simply
• A hydrogen atom
(atomic number = 1) has
one proton and one
electron.
• The helium atom (atomic
number = 2) has two
protons, two neutrons, and
two electrons.
Chapter 10
Section 2 The Atom
How Do Atoms of Different Elements Differ?
• Building Bigger Atoms For bigger atoms, simply
add protons, neutrons, and electrons.
• Protons and Atomic Number
• The number of protons tells you what the element is.
• The number of protons in the nucleus = the atomic
number of that atom.
• All atoms of an element have the same atomic
number.
Chapter 10
Section 2 The Atom
Isotopes
• Isotopes are atoms of an element that have the
same number of protons but different numbers of
neutrons.
Chapter 10
Section 2 The Atom
Isotopes, continued
• Properties of Isotopes An unstable atom is an atom
with a nucleus that will change over time. This type of
isotope is radioactive.
• Telling Isotopes Apart You can identify each
isotope of an element by its mass number. The mass
number is the sum of the protons and neutrons in an
atom.
Chapter 10
Section 2 The Atom
Isotopes, continued
Chapter 10
Section 2 The Atom
Isotopes, continued
• Naming Isotopes To identify a specific isotope of an
element, write the name of the element followed by a
hyphen and the mass number of the isotope.
• Calculating the Mass of an Element The atomic
mass of an element is the weighted average of the
masses of all the naturally occurring isotopes of that
element.
Chapter 10
Section 2 The Atom
Chapter 10
Section 2 The Atom
Forces in Atoms
• Four Basic Forces Four basic forces are at work
everywhere, even within the atom. These forces are
gravitational force, electromagnetic force, strong force,
and weak force.
• These forces work together to give an atom its
structure and properties.
Chapter 10
Section 2 The Atom
Chapter 10
Introduction to Atoms
Concept Mapping
Use the terms below to complete the concept map
on the next slide.
a nucleus
mass number
isotopes
protons
atoms
electrons
atomic number
Chapter 10
Introduction to Atoms
Chapter 10
Introduction to Atoms