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.”