1 Simple molecules [molecule: A molecule is a collection of two or more atoms held together by chemical bonds. 2  It is the smallest part of a substance that displays the properties of the substance.
Eon has a number of meanings. In Astronomy, an eon refers to 1 billion (109) years. But it also refers to a very long, unspecified period of time, or specific geologic stages of the Earth.
There are 4 Eons on the geological time scale in total. Era - 12 total, spanning several hundred million years each. Period - a timespan longer than an Epoch, but shorter than an Era. Epoch - spans tens of millions of years each. All of these time spans relate events recorded in the stratigraphy to a certain point (or span) in the history of the Earth.