When the atoms are of different elements, the substance formed is called a compound . For example, when two hydrogen atoms join together with an oxygen atom, water is formed - H 2 O. Chemically bonding occurs when two particles can exchange or combine their outer electrons in such a way that is energetically favorable . An energetically favorable state can be seen as analogous to the way a dropped rock has a natural tendency to fall to the floor. When the bond occurs, the atoms become a compound . Molecules in turn can join up in regular patterns, and these are what we know as crystals .
Molecules can get very complicated.
Some proteins contain thousands of atoms per molecule, largely carbon, oxygen, hydrogen, and nitrogen. When writing names of elements, a chemist usually uses abbreviations, since they are quicker to write than the names. The abbreviations that we use are called symbols. The symbols for all known elements are given in the Periodic Table of the Elements . A chemical formula is one or more symbols that represents a substance.