A molecule is a group of two or more atoms of multiple elements, or of a single element, held together by attractive forces known as chemical bonds. Molecules as components of matter are common, constituting most of the oceans and the atmosphere as well as most organic substances (including proteins, DNA, RNA, sugars, carbohydrates, fats and vitamins). However, the majority of familiar solid substances are made partly or completely of crystals or ionic compounds rather than molecules, including the minerals that make up rocks, clay, metals and the molten interior of the earth.