The document summarizes the development of atomic theory over thousands of years through the proposals of different atomic models by scientists like Democritus, Dalton, Thomson, Rutherford, Bohr, and others. It describes how atomic models have evolved from early concepts of atoms as indivisible spheres to the current understanding of atoms having a small, dense nucleus surrounded by an electron cloud.

2. 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.

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

4. 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.

5. 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.

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

7. Thomson’s Model
 End of 1800s
 Thomson discovered that atoms were not
simple, solid spheres
 Atoms contained subatomic particles
 Very small, negatively charged
 Called them electrons

8. 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

9. Thomson’s Model = chocolate chip cookie

10. Rutherford’s Model
 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

11. 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

12. 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

13. Bohr’s Model = planets
 Has been called planetary model
 Energy levels occupied by electrons are like
orbits of planets at different distances from
the sun (nucleus)

14. Electron Cloud Model
 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

15. Electron Cloud Model

16. Modern Atomic Model
 In 1932, Chadwick discovered another
particle in the nucleus of an atom. This
new particle is called a neutron.
 Neutrons have no electrical charge.
 According to this theory,
“At the center of the atom is a tiny,
massive nucleus containing protons and
neutrons. Surrounding the nucleus is a
cloudlike region of moving electrons.”