Atomic - Molecular
Theory of Matter
The Atomic - Molecular
Theory of Matter states
that all matter is
composed of small, fast
moving particles called
atoms. These atoms can
join together to form
molecules.
This theory is really
thousands of individual
theories that provide
evidence for the whole
theory.

History of Atom
 All atoms share the same basic structure
 During past 200 years, scientists have
proposed different models

Where did it all begin?
The word “atom”
comes from the
Greek word “atomos”
which means
indivisible.
The idea that all
matter is made up of
atoms was first
proposed by the
Greek philosopher
Democritus in the 5th
century B.C.

Dalton’s Model
 Based on experiments, Dalton developed
a theory of structure of matter
 4 main concepts:
 All matter is composed of tiny, indivisible
particles called atoms
 Atoms of each element are exactly alike and
have the same mass
 An atom of one element cannot be changed
into an atom of a different element.
 Atoms of different elements can join to form
compounds.

Dalton’s Model = “eight ball”
Dalton thought that atoms were like
smooth, hard balls that could not be
broken into smaller pieces.

Thomson’s Model

Thomson’s Model
 Also knew that atoms were electrically
neutral
 Must contain enough positive charge to
balance negative charge of electrons
 Thompson proposed a model where
electrons were stuck into a positively
charged sphere
 Like chocolate chips in cookie dough

Thomson’s Model = chocolate chip cookie

 By early 1900s,
scientists knew that
positive charge of atom
comes from subatomic
particles called protons
 1911—Rutherford begins to
test theory
 His experiments led him to
believe that protons are
concentrated in a small area
at center of atom
 Called this area the
nucleus

Rutherford’s Model = peach
 Rutherford’s model describes an atom as
mostly empty space, with a center nucleus
that contains nearly all the mass
 Like the pit in a peach

James Chadwick
Chadwick discovered a
third type of subatomic
particle, which he named
the neutron.
Neutrons always reside in
the nucleus of atoms, and
they are about the same
size as protons. Neutrons
do not have any electrical
charge; they are

Neils Bohr’s Model
 Modified Rutherford’s model in
1913
 Proposed that each electron
has a certain amount of energy
 Helped electron move around
nucleus
 Electrons move around
nucleus in region called
energy levels
 Energy levels surround
nucleus in rings, like layers of
onion

Bohr’s Model = planets

 Model accepted today
 Electrons dart around in an energy level
 Rapid, random motion creates a “cloud” of
negative charge around nucleus
 Electron cloud gives atom its size and
shape

Electron Cloud Model

Proton, Electron and Neutron
Atoms are the basic units of matter.
The atoms are made up of three
particles: protons, neutrons and
electrons. Nucleus, the center of the
atom is composed of protons and
neutrons. It was discovered by Ernest
Rutherford in 1911

Protons
Protons (p+) are positively charged
particles found within the nucleus of an
atom. Ernest Rutherford discovered protons
in his cathode-ray tubes experiment. The
number of protons of an atom is called the
atomic number. Hydrogen for example has
only one proton thus, its atomic number is 1.
The number of protons of an element is
equal to the number of electrons.

Electrons
Electrons (e- ) are negatively charged
particles found outside the nucleus of the atom, in
the electron orbits or levels. Compared to protons
and neutrons, electrons are many times smaller.
J.J. Thomson discovered the electron in 1897. The
number of electrons of an element is equal to the
number of protons which is the atomic number of
an element. Oxygen for example has an atomic
number of 8, thus, its number of protons and
electron is also 8.

Neutrons
Neutrons (n0) are neutral particles which
means that they have no electric charge. Neutrons
help make up the nucleus of the atom. Ernest
Rutherford theorized the existence of neutrons in
1920 and discovered by James Chadwick in 1932.
The number of neutron of an element is equal to
the mass number minus the number of protons of
the element. Sodium for example, has a mass
number of 23 and an atomic number of 11. To get
the number of neutron of Sodium, subtract the
atomic number 11 to the mass number 23, which
equals to 12.

How many protons, electrons,
and neutrons are in an atom?

Step 1
Use your periodic table to find an
element
http://education.jlab.org/qa/pen_number.html

Step 2
The number of Protons (+ charge)
The atomic number = # protons
Kr has 36 protons

Step 3
The number of Electrons (- charge)
Remember atoms have no overall charge.
Atoms must have an equal # of protons and
electrons
#protons=#electrons
Kr has 36 electrons

Step 4
The number of Neutrons (no charge)
Mass # (atomic weight rounded to nearest
whole #)
Neutrons = Mass # - Protons
Neutrons = 84 – 36
Neutrons = 48

Summary
Number of Protons = Atomic #
Number of Electrons = Protons
Number of Neutrons =Mass # - Protons

Problems
1. Find the protons (p+), electrons (e-),
neutrons (n) of Oxygen
2. Find the p+, e-, n of Xenon (Xe)
3. Find the p+, e-, n of Zirconium (Zr)

Oxygen (O)
p+, e-, n
Number of Protons = Atomic #
Number of Electrons = Protons
Number of Neutrons = Mass # - Protons
Protons = 8
Electrons = 8
Neutrons = 16-8 = 8

Xenon (Xe)
p+, e-, n
Number of Protons = Atomic #
Number of Electrons = Protons
Number of Neutrons = Mass # - Protons
Protons = 54
Electrons = 54
Neutrons = 131-54 = 77

Zirconium (Zr)
p+, e-, n
Number of Protons = Atomic #
Number of Electrons = Protons
Number of Neutrons = Mass # - Protons
Protons = 40
Electrons = 40
Neutrons = 91-40 = 51