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.
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Introduction to atomic theory ppt
1.
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
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
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.”