
460 BC - Greek philosopher proposes the existence of the atom

He pounded materials until he made them into smaller and smaller parts

He called them
which is Greek for “indivisible”.


His Theory:
All atoms:

Are small hard particles

Are made of a single material formed into different shapes and sizes

Are always moving, and they form different materials by joining together

John Dalton

1803 - British chemist; elements combine in specific proportions to form compounds
Solid Sphere Model or
Bowling Ball Model
Proposed by John Dalton

John Dalton

His Theory:

All substances are made of atoms that cannot be created, divided, or destroyed.

Atoms join with other atoms to make new substances.

Atoms of the same element are exactly alike, and atoms of different elements are different in mass and size.

J.J. Thomson

1897 - English chemist and physicist; discovered 1 st subatomic particles
Plum Pudding Model or
Raisin Bun Model
Proposed by J.J. Thomson

J.J. Thomson

His Theory:

Atoms contain negatively charged particles called electrons and positively charged matter.

Created a model to describe the atom as a sphere filled with positive matter with negative particles mixed in

Referred to it as the plum pudding model

Ernest Rutherford

1912 - New Zealand physicist discovered the nucleus
Nuclear Model
Proposed by Ernest
Rutherford

Ernest Rutherford

His Theory:

Small, dense, positively charged particle present in nucleus called a proton

Electrons travel around the nucleus, but their exact places cannot be described.

Niels Bohr

1913 - Danish physicist; discovered energy levels
Bohr Model or Planetary
Model
Proposed by Niels Bohr

Niels Bohr

His Theory:

Electrons travel around the nucleus in definite paths and fixed distances.

Electrons can jump from one level to a path in another level.

Erwin Shrodinger

1924 - Austrian physicist; developed the electron cloud model
Electron Cloud Model
Proposed by Erwin
Schrodinger

Erwin Shrodinger

His Theory:

The exact path of electrons cannot be predicted.

The region referred to as the electron cloud, is an area where electrons can likely be found.

James Chadwick

1932 - English physicist; discovered neutrons

His Theory:

Neutrons have no electrical charge.


Neutrons have a mass nearly equal to the mass of a proton.
Unit of measurement for subatomic particles is the atomic mass unit ( amu ).

Modern Theory of the Atom

Atoms are composed of three main subatomic particles : the electron , proton , and neutron .

Most of the mass of the atom is concentrated in the nucleus of the atom.

Modern Theory of the Atom

The protons and neutrons are located within the nucleus , while the electrons exist outside of the nucleus.

In stable atoms, the number of protons is equal to the number of electrons .

Modern Theory of the Atom

The type of atom is determined by the number of protons it has.

The number of protons in an atom is equal to the atomic number .

Modern Theory of the Atom

The sum of the number of protons and neutrons in a particular atom is called the atomic mass .

Valence electrons are the outermost electrons.