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.
The Geologic Time Scale and a Brief History of Life on Earth. The Geologic Time Scale is divided into four major units: Eons, Eras, Periods and. Epochs. An Eon is the longest division of geologic time, so long in fact that there have only. been four Eons. Collectively the first three eons are called the Precambrian, that stretch of.