Atomic Theory

The history of atomic theory has evolved over thousands of years. Several new ideas, concepts and models have been put forth and have contributed to our understanding of the structure of an atom.

Democritus- believed that matter was made up of atoms. Had a theory that atoms remain unchanged but move about in space to form macroscopic objects.

Aristotle- His theory was that atoms were made up of 4 elements; earth, wind, fire and water. When these elements combine they created dryness, wetness, coldness and hotness.

Antoine Lavoisier- accidentally discovered oxygen and created an incomplete table of the elements. Created early version of Law of 1) Definite Proportions 2) Conservation of Mass

Joseph Proust- created the Law of Definite Proportions-a given compound always has the same elements in the same proportions by mass.

John Dalton- he proposed Dalton's Atomic Theory
1. elements are made up of atoms
2.atoms of same elements are identical
3.atoms of different elements can be told apart by atomic weight
4.atoms of different elements can combine in a reaction to form chemical compounds in fixed ratios.

JJ Thomson-proved the existence of the electron, proposed the raisin bun model of the atom.

 Ernest Rutherford- discovered the positively charged, central, dense nucleus.
Did this using his gold foil experiment in which he fired radioactive particles through thin foil; most passed through but some particles were reflected.

Niels Bohr- proposed the planetary model of the atom
-Electrons exist in energy levels surrounding the nucleus.
-studied gaseous samples of atoms

The Atom today:
It is the smallest particle of an element and cannot be broken down.
It contains 3 subatomic particles- the proton(+), the electron(-) and the neutron (0).
The protons and the neutrons occupy the nucleus and electrons exist in levels around the nucleus.