Download - Introduction to atomic theory ppt
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 structureDuring 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 atomsAtoms of each element are exactly alike and
have the same massAn 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
End of 1800sThomson discovered that atoms were not
simple, solid spheresAtoms contained subatomic particles
Very small, negatively charged Called them electrons
Thomson’s Model
Also knew that atoms were electrically neutralMust contain enough positive charge to
balance negative charge of electronsThompson proposed a model where
electrons were stuck into a positively charged sphereLike chocolate chips in cookie dough
Thomson’s Model = chocolate chip cookie
Rutherford’s Model
By early 1900s, scientists knew that positive charge of atom comes from subatomic particles called protons
1911—Rutherford begins to test theoryHis experiments led him to believe that
protons are concentrated in a small area at center of atomCalled 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 massLike the pit in a peach
Bohr’s Model
Modified Rutherford’s model in 1913Proposed that each electron has a certain
amount of energyHelped 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
Has been called planetary modelEnergy levels occupied by electrons are like
orbits of planets at different distances from the sun (nucleus)
Electron Cloud Model
Model accepted todayElectrons dart around in an energy levelRapid, random motion creates a “cloud” of
negative charge around nucleusElectron cloud gives atom its size and
shape
Electron Cloud Model
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.”