A perfect circle is symbolic of something that is whole, complete, ideal and eternal; a circle has no ending and no beginning, making it synonymous with cyclical ideas and processes. For example, a circular wedding ring is used as a symbol of everlasting love.
Cultures throughout time have used the circle to represent people or ideas considered to be flawless. Halos were placed on the heads of important religious figures, such as Jesus and Mary, to symbolize their purity and wholeness.
A snake that is eating itself appears to be a perfect circle. This is symbolic of the never-ending cycle of destruction and rebirth that occurs on Earth, an association that caused this symbol to become synonymous with chemical reactions.