ATOMS
Chapter Fourteen: Atoms
14.1 The Structure of the Atom
14.2 Electrons
Chapter 14.1 Learning Goals
Identify and describe particles which
comprise atoms.
Describe the effects of radioactivity.
Compare and contrast forces inside
atoms
Investigation 14A
Atomic Structure
Key Question:
What is inside an atom?
14.1 Structure of the Atom
In order to understand atoms, we
need to understand the idea of
electric charge.
We know of two
different kinds of
electric charge and we
call them positive and
negative.
14.1 Electric charge in matter
We say an object is electrically neutral
when its total electric charge is zero.
14.1 An early model
In 1897 English physicist J.
J. Thomson discovered
that electricity passing
through a gas caused the
gas to give off particles
that were too small to be
atoms.
These negative particles
were eventually called
“electrons.”
14.1 The nuclear model
In 1911, Ernest
Rutherford, Hans
Geiger, and Ernest
Marsden did a clever
experiment to test
Thomson’s model.
We now know that
every atom has a tiny
nucleus, which
contains more than
99% of the atom’s
mass.
14.1 Inside an atom
The mass of the nucleus
determines the mass of
an atom because protons
and neutrons are much
larger and more massive
than electrons.
In fact, a proton is 1,836
times heavier than an
electron.
14.1 Force inside atoms
Electrons are bound
to the nucleus by the
attractive force
between electrons (-)
and protons (+).
14.1 Force inside atoms
What holds the
nucleus together?
There is another
force that is even
stronger than the
electric force.
We call it the strong
nuclear force.
14.1 How atoms of various
elements are different
The atoms of different
elements contain
different numbers of
protons in the nucleus.
Because the number of
protons is so important,
it is called the atomic
number.
14.1 How atoms of various
elements are different
Isotopes are atoms of the
same element that have
different numbers of
neutrons.
How are these carbon
isotopes different?
The mass number of an
isotope tells you the
number of protons plus
the number of neutrons.
14.1 Radioactivity
Almost all elements have
one or more isotopes
that are stable.
“Stable” means the
nucleus stays together.
Carbon-14 is radioactive
because it has an
unstable nucleus.
Solving Problems
How many neutrons are present in an
aluminum atom that has an atomic
number of 13 and a mass number of 27?
Solving Problems
1. Looking for:
 …number of neutrons in aluminum-27
2. Given
 … atomic no. = 13; mass no. = 27
3. Relationships:
 Periodic table says atomic no. = proton no.
 protons + neutrons = mass no.
4. Solution
 neutrons = mass no. – protons
 neutrons = 27 – 13 = 14
Chapter Fourteen: Atoms
14.1 The Structure of the Atom
14.2 Electrons
Chapter 14.2 Learning Goals
Compare spectra of elements.
Explain the Bohr atom model.
Apply principles of quantum
theory to explain the behavior of
electrons in atoms.
Investigation 14B
Atomic Challenge
Key Question:
How were the elements created?
14.2 Electrons in the atom
Each different element has
its own characteristic
pattern of colors called a
spectrum.
The colors of clothes,
paint, and everything else
around you come from
this property of elements
to emit or absorb light of
only certain colors.
14.2 Electrons in atoms
Each individual color in a spectrum is
called a spectral line because each color
appears as a line in a spectroscope.
A spectroscope is a device that spreads
light into its different colors.
14.2 Bohr model of the atom
Danish physicist Neils Bohr
proposed the concept of
energy levels to explain the
spectrum of hydrogen.
When an electron moves from
a higher energy level to a
lower one, the atom gives up
the energy difference between
the two levels.
The energy comes out as
different colors of light.
14.2 The quantum theory
Quantum theory says that when
things get very small, like the size of
an atom, matter and energy do not
obey Newton’s laws or other laws of
classical physics.
14.2 The quantum theory
According to quantum
theory, particles the
size of electrons are
fundamentally different
An electron appears in a
wave-like “cloud and
has no definite position.
14.2 The quantum theory
The work of German physicist Werner
Heisenberg (1901–1976) led to Heisenberg’s
uncertainty principle.
The uncertainty principle explains why a
particle’s position, momentum or energy
can never be precisely determined.
The uncertainty principle exists because
measuring any variable disturbs the others
in an unpredictable way.
14.2 The uncertainty principle
14.2 Electrons and energy levels
In the current model of the atom, we think
of the electrons as moving around the
nucleus in an area called an electron cloud.
The energy levels occur because electrons
in the cloud are at different average
distances from the nucleus.
14.2 Rules for energy levels
Inside an atom, electrons always obey these
rules:
1. The energy of an electron must match one
of the energy levels in the atom.
2. Each energy level can hold only a certain
number of electrons, and no more.
3. As electrons are added to an atom, they
settle into the lowest unfilled energy level.
14.2 Models of energy levels
While Bohr’s model of electron energy
levels explained atomic spectra and
the periodic behavior of the elements,
it was incomplete.
Energy levels are predicted by
quantum mechanics, the branch of
physics that deals with the
microscopic world of atoms.
14.2 Energy levels
In the Bohr model of the
atom, the first energy
level can accept up to
two electrons.
The second and third
energy levels hold up to
eight electrons each.
The fourth and fifth
energy levels hold 18
electrons.
14.2 Electrons and energy levels
The first energy level can accept up to two electrons.
The second energy levels hold up to eight electrons.
Investigation 14C
Energy and Quantum Theory
Key Question:
How do atoms absorb
and emit light
energy?
Bioluminescence- Glow Live!
Like a glow stick, living
things produce their own
light using a chemical
reaction. Bioluminescence is
“cold light” because it doesn’t
produce a lot of heat. While it
takes a lot of energy for a
living thing to produce light,
almost 100 percent of the
energy becomes visible light.