The Developmen

t of the Atomic Theory

By: Group 2 (Cappuccino)

Democritus 460 BC

• Greek philosopher proposes the existence of the atom.

• 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

Aristotle • He did not think there was a limit to the numberof times matter could be divided.

• He thought that all substances were built up from only four elements.

• Earth Fire• Water Air

John Dalton 1766-1844• British chemist• 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.

Dalton’s Model

•Solid Sphere Model or

Bowling Ball Model

•Proposed by John Dalton

J.J. Thomson 1856-1940

• English chemist and physicist; discovered 1st subatomic particles.• 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

J.J. Thomson’s Model

• Plum Pudding Model or

Raisin Bun Model

•Proposed by J.J. Thomson

Ernest Rutherford 1871-1937

•New Zealand physicist discovered the nucleus.•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.

Rutherford’s Model

•Nuclear Model

•Proposed by Ernest

Rutherford

Neils Bohr 1913

•Danish physicist; discovered energy levels.

•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.

Bohr’s Model• In Bohr’s model,

electrons move with constant speed in fixed orbits around the nucleus, like planets around a sun.

• Bohr proposed that electrons move in paths at certain distances around the nucleus.

• Electrons can jump from a path on one level to a path on another level.

Erwin Shrodinger 1924

• Austrian physicist; developed the electron cloud model.

• 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.

Erwin Shrodinger’s Model

•Electron Cloud Model

•Proposed by Erwin

Schrodinger

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.

• 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.

• 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.

• The sum of the number of protons and neutrons in a particular atom is called the atomic mass.

• Valence electrons are the outermost electrons.

Thank you..!!