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 Phanerozoic eon is the interval of geologic time spaning from the appearance of abundant, macroscopic, hard-shelled fossils, roughly 542 million years ago (mya), to the present time. Preceding the Phanerozoic eon is the Proterozoic eon, with the boundary between the two eons being determined by the sudden appearance in the fossil record of a majority of the skeleton-bearing phyla of animals.