Atoms and subatomic particles

Online Introductory Chemistry

Atom theory and subatomic particles

Dr. Walt Volland revised July 5 , 2010

An atom of an element is the simplest particle that displays the properties of the element. We cannot see individual atoms with the naked eye. Visit the following site to see scanning tunneling microscope images of atoms. http://www.almaden.ibm.com/vis/stm/catalogue.html

Atomic theory has four assumptions:

1. Atoms make up all matter.

2. The atoms of one element are different from the atoms of another element.

3. Atoms combine in definite ratios to make compounds.

4. Combinations of atoms in compounds can change only when a chemical reaction happens. This means reactions alter atom combinations, but the identity of the atoms themselves remain the same.

Atomic theory history:

Atomic theory dates back more than two thousand years. Versions of the atomic theory can be traced to the Greeks in 300 BC.

A somewhat modern theory was put forward by an English school teacher, John Dalton in 1808. This Dalton theory described how atoms interacted to form compounds, but never even considered the possibility of subatomic particles. The first of the subatomic particles, the negatively-charged electron, was discovered by J. J. Thomson in 1899. click here for about disovery Both Thomson and Ernst Rutherford contributed to the identification of the positively-charged proton.

In 1912 Rutherford proposed the existence of a nucleus in each atom. The model for the atom we use has a massive nucleus made up of protons and neutrons. The neutrons and protons are more than a 1000 times more massive than electrons. In neutral atoms the positive charge of the protons in the nucleus is cancelled by the negative charge on electrons moving in space outside the nucleus.

The modern view of the atom proposes that there are 3 subatomic particles. This "modern" view was finalized in 1932, when the neutron, the last of the three subatomic particles, was identified. It is more than 70 years old. Today scientists have identified many other particles within atoms, but the three simple subatomic particles--the electron, the proton, and the neutron--are still used to explain many properties of atoms. The properties of these three simple subatomic particles are summarized on the next page.