The History of Atomic Theory

DemocritusDemocritus

Greek philosopher 2400 years ago The Atom

Could matter be divided into smaller and smaller pieces forever?

Or was there a limit to the number of times a piece of matter could be divided?

AtomosAtomos

Matter could NOT be divided into smaller and smaller pieces forever

This piece would be indivisible.

“atomos” =“not to be cut.”

AtomosAtomos Atoms= small, hard

particles that were all made of the same material

Different shapes and sizes.

Atoms were infinite in number, always moving and capable of joining together.

AristotleAristotle

Same time periodDenounced the

idea of atomosEarth, Water, Air,

and Fire

AlchemistsAlchemists

Middle Ages Worked on

turning ordinary metals into gold

Derived modern chem

John DaltonJohn Dalton

English chemistEarly 1800’sRevisited the

particle theoryLaid the

groundwork for Modern theory

Father of the Atom

Dalton’s TheoryDalton’s Theory1. All matter small

particles a.k.a atoms

2. Atoms of the same element are exactly alike. Atoms of different elements are different.

3. Atoms cannot be divided, created, nor destroyed

Dalton’s TheoryDalton’s Theory4. Atoms of different

elements can combine to form compounds

5. In chemical reactions, atoms are combined, separated, or rearranged

Known as the “Billiard Ball Model”

J. J. ThomsonJ. J. Thomson

English scientist1897The electronThe first to

believe an atom is made of even smaller particles.

Thomson ModelThomson Model

Atoms were made from a positively charged substance with negatively charged electrons scattered

A.k.a: the “Plum Pudding Model”

Thomson Model: Cathode RayThomson Model: Cathode Ray

Thomson studied the passage of an electric current through a gas.

As the current passed through the gas, it gave off rays of negatively charged particles.

Robert A. MillikanRobert A. Millikan

American physicist 1909 Determined the mass

of an electron relative to the atom

Millikan’s: Oil Drop ExperimentMillikan’s: Oil Drop Experiment

Mist of fine oil droplets are dropped into the chamber

Some of the droplets were negatively charged

No electricity electrons fell through: gravity is only acting force

Millikan’s Oil Drop ExperimentMillikan’s Oil Drop Experiment

Voltage adjusted so down force = up force

Oil drop suspended between plates

The charge on the droplet was calculated

Mass of electron = charge of electron: charge-to-mass ratio of electron

Mass = 1/2000 of the Hydrogen atom

Ernest B. RutherfordErnest B. Rutherford

English physicist English physicist 19081908Gold Foil Gold Foil

ExperimentExperiment

Gold Foil Experiment: ResultsGold Foil Experiment: Results Most of the positively

charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all.

Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid.

What the Gold Foil Experiment ProvesWhat the Gold Foil Experiment Proves

Niels BohrNiels Bohr

Danish scientist1913Solar System Model

Bohr ModelBohr Model According to Bohr’s atomic

model, electrons move in definite orbits around the nucleus, much like planets circle the sun.

These orbits are located at certain distances from the nucleus based on relative energies.

1932

Neutron: There was unaccounted mass so there had to be another subatomic particle

Isotopes: Atoms of the same element (identical protons) but different numbers of neutrons and therefore different masses

James ChadwickJames Chadwick

Electron Cloud Model

The Electron Cloud ModelThe Electron Cloud Model Heisenburg Uncertainty Principle: cannot

know speed and location of electron at same time

Electrons in organized orbitals Electrons travel so fast that they seem to

form a “cloud” around the nucleus Location of electron depends on its energy Light spectrum

Electron Cloud:Electron Cloud: Electrons: low NRG are closest to nucleus Electrons: high NRG are further from nucleus

Historical Models Historical Models

Indivisible Electron Nucleus Orbit Electron Cloud

Greek X

Dalton X

Thomson X

Rutherford X X

Bohr X X X

Wave X X X