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4.80
Chapter 4: Atomic Structure
What are atoms and how are they put together?
All matter is formed from atoms. Atoms, by themselves or combined
with other atoms in molecules, make up everything that we see, hear,
feel, smell, and touch. An individual atom is so small that one cell in
your body contains 100 trillion atoms, and a speck of dust contains
many more atoms than that.
As small as they are, atoms and molecules are the building blocks of
every type of matter. A few hundred incredibly tiny atoms
of gold have the same density as a bar of gold. A few
hundred very tiny molecules of water have the same
density as a cup of water. (We would not, of course, be
able to see or notice in any way those atomic clumps of
gold nor those molecular masses of water!)



Why are atoms the smallest piece that is still
recognizably matter?
What do you find when you break apart an atom?
How big is it?
What do
atoms look
like?
How big are atoms?
Atoms are very small!
An atom and its parts are much smaller than a meter. The diameter of
an atom is 10-10 (0.0000000001) meter, whereas an electron is smaller
than 10-18 (0.000000000000000001) meter. Comparatively, this means
that an electron is 10 million times smaller than an atom!
Most of the atom is empty space!
You may be surprised to learn that most of the atom is actually empty
space: If the atom was the size of your classroom, then the nucleus
would be the size of a grain of sand in the center of the room.
4.1 Studying Atoms: The History
A. Democritus: all matter consisted of extremely small particles that
could not be divided.

Atomos: a Greek word which means “uncut” or “indivisible.
B. Aristotle: thought that all substance
were built up from only four elements:
earth, air fire, and water.
Info
C. Dalton’s Atomic Theory

All matter is made up of individual particles called atoms,
which cannot be divided.

All elements are composed of atoms

All atoms of the same element have
the same mass, and atoms of
different elements have different
masses.
Dalton’s model of the
atom. He thought
atoms were tiny, hard
spheres.

Compounds contain atoms of more
than one element.

In a particular compound, atoms of different elements always
combine in the same way.
D. Thomson’s Model of the Atom

Provided the first evidence that atoms are made of
even smaller particles. (The “plum pudding”
model)

The Thomson model of the atom. The atom is a
positive sphere with negative electrons embedded
in it. Thomson discovered the electron.
E. Rutherford’s Atomic Theory

All of an atom’s positive charge is concentrated in its nucleus.

Protons and neutrons cluster together in the atom’s center,
called the nucleus.

Used high-speed, lightweight atoms called alpha particles
(generated by radioactive material), to bombard very thin
pieces of gold foil. Most of the alpha particles passed through
the foil and hit a screen behind it. But surprisingly, some of
them bounced back. They must have hit areas of the foil with
greater density!
4.2 The Structure of an Atom
Properties of Subatomic Particles
What do you
find when you
break apart an
atom?
How does one
kind of atom
differ from
another?
How are protons,
neutrons, and
electrons arranged
within an atom?

Atoms and molecules are called the building blocks of matter
because if you attempt to break down an atom, you no longer
have gold or water or any other recognizable substance. If
broken apart, almost all atoms contain three smaller particles
called protons, neutrons, and electrons. Because these particles
are even smaller than an atom, they are called subatomic
particles. These three types of particles are arranged in an atom
as shown to the right.

The number of protons distinguishes an atom of one element
from the atom of another element.

All atoms of the same element will
have the same number of protons, and
atoms of different elements will have
different numbers of protons.

Protons, electrons, and neutrons can
be distinguished by mass, charge, and
location in an atom.

The proton is positive, the electron is
negative, and the neutron is
electrically neutral.

Protons and neutrons have about the same mass. Each is about
2,000 times the mass of an electron. Since protons and neutrons
exist in the nucleus, almost all the mass of an atom is
concentrated there.
Properties of Subatomic Particles
Subatomic
particles have
charge and mass.
Particle
Symbol
Relative
Charge
Relative
Mass
Electron
e-
1-
1/1836
Proton
P+
1+
1
Neutron
N
0
1
Actual
Mass
(g) (info)
9.11 x
10-28
1.674 x
10-24
1.675 x
10-24
Location
Outside
Nucleus
Nucleus
Nucleus
What is the atomic
number?

Atomic Number: the number of
protons in an atom of that element.

Because the number of protons in an
atom remains the same during physical
and chemical changes, we can refer to
each element by the number of protons
its atoms contain. This unique number
is called the atomic number.

Atomic numbers start at 1, with the element hydrogen, and go
up by ones until 111, the element unununium. The heaviest
elements have been created in a laboratory and have not been
seen in nature.

Atoms of different elements have different numbers of
protons.

In addition to the atomic number, every atomic nucleus can be
described by its mass number. The mass number is equal to
the total number of protons plus neutrons in the nucleus of an
atom. Recall that atoms of the same element have the same
number of protons. Atoms of the same element can have
different numbers of neutrons.

Mass Number: the sum of the protons and neutrons in the
nucleus of that atom.
Can you change
the number of
protons?
What is this
element?
What is its mass
number?
What does the
atomic mass tell
you?
Number of neutrons = Mass number – Atomic number
Example: finding the number of neutrons
Example: How many neutrons are present in an atom of carbon
that has a mass number of 14?
Solution: The mass number is the number of protons plus the
number of neutrons.
(1) You are asked for the number of neutrons.
(2) You are given that it is carbon-14. Carbon has 6 protons.
(3) The relationship is n + p = mass number.
(4) Solve for n
n= mass number -p
(5) Plug in numbers and get answer.
n = 14 - 6 = 8
There are 8 neutrons in a carbon-14 nucleus.
Info

Isotopes: atoms of the same element that have
different numbers of neutrons and different mass
numbers.

Isotopes are atoms of the same element that have
different numbers of neutrons.

Many elements have atoms with different numbers of neutrons.
These different forms of the same element are called isotopes.

Isotopes of an element have the same atomic number but
different mass numbers because they have different numbers of
neutrons.

Atoms are electrically neutral. Neutral atoms will have the
same number of protons and electrons.
4.3 Modern Atomic Theory
Bohr’ Model of the Atom

Energy Levels: the possible
energies that electrons in an atom
can have.

The electron in an atom can move
from one energy level to another
when the atom gains or loses
energy.
Electrons are
found in the
electron cloud.
The Bohr model of the
atom. Electrons move
around the nucleus in
fixed orbits.
Evidence for Energy Levels

When electrons move back to lower energy levels they emit
energy.

Each energy level can hold up to a specific number of
electrons. Sometimes, when energy is added to an atom,
electrons can absorb enough energy to “jump” to a higher
energy level. When they fall back to their normal energy level,
they release light in a characteristic frequency.

Some of that energy is released as visible light.

Because no two elements have the same set of energy levels,
different elements emit different colors of light.
Electron Cloud Model

An electron cloud is a visual model of the most likely locations
for electrons in an atom.

The electrons move in the space around the nucleus. No one is
able to say exactly where an electron is at any one time. A
useful analogy is that electrons buzz around the nucleus much
like bees around a hive.
Atomic Orbitals

Orbital: a region of space around the nucleus where an
electron is likely to be found.

An electron cloud is a good approximation of how electrons
behave in their orbitals.

The farther away from the nucleus the electron is found, the
higher its energy state.

Lower energy: one orbital

Higher energy: more than one orbital
Energy Levels, Orbitals, & Electrons
Energy Level
Info
1
2
3
4
# of Orbitals
1
4
9
16
Electron Configurations

The most stable electron
configuration is the one in which
the electrons are in orbitals with
the lowest possible energies.

Ground State: When all the
electron in an atom have the lowest
possible energies.
Maximum # of
Electrons
2
8
18
32
1. Draw a pictorial model of an atom that has 5 protons, 5 neutrons,
and 5 electrons. Label the charge of each subatomic particle. What
element is this?
2. Two atoms are placed next to each other. Atom A has 6 protons, 6
neutrons, and 5 electrons. Atom B has 6 protons, 7 neutrons, and 6
electrons. Are atoms A and B different elements? How do you know?
5. For each of the nuclei shown below, do the following:
a. Name the element.
b. Give the mass number.
c. Show the isotope notation.