development of the atomic theory. democritus 460 bc - greek philosopher proposes the existence of...

Development of the Atomic

Theory

Democritus 460 BC - Greek philosopher proposes the

existence of the atom ; calls it Atomos meaning indivisible.

His theory: all atoms are small hard particles made of a single material formed into different

shapes and sizes always moving, and that they form different

materials by joining together

Democritus

• Democritus model was a small sphere that was invisible

John Dalton 1803 - British chemist; elements

combined in specific proportions to form compounds.

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 (elements)

John Dalton

• Dalton’s model was a solid sphere that was invisible

Edward Frankland 1852 - English chemist developed the

valence theory

His theory: every atom has a fixed number of bonds

(chemical links) that it can form for the atom to be stable, all of these

bonds must be used.

J.J. Thomson 1897 - English chemist and physicist

discovered 1st subatomic particles

His theory: 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 model as the plum pudding model

J.J. Thomson

• Thomson’s model was a sphere filled with positive matter and negative particles.

Ernest Rutherford 1912 - 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

Ernest Rutherford

• Rutherford’s model had a nucleus of protons surrounded by electrons.

Niels Bohr 1913 - Danish physicist discovered

energy levels

His theory: electrons travel around the nucleus in

definite paths and fixed distances; orbits electrons can jump from one level to a

path in another level

Niels Bohr

• Bohr’s model had a nucleus surrounded by electrons orbits.

Erwin Shrodinger 1924 - Austrian physicist developed

the electron cloud model

His theory: electrons exact path cannot be

predicted regions, referred to as the electron

cloud, are areas where electrons can likely be found.

Erwin Shrodinger

• Shrodinger's model had a nucleus surrounded by an electrons cloud.

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)

James Chadwick

• Chadwick's model had a complete nucleus with protons and neutrons.

Modern Theory of the Atom Atoms are composed of three main

subatomic particles: the electron, the proton, and the 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.

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

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

Valence electrons are the outermost electrons and are where bonding takes place

Any Questions?