H20 is the chemical formula for water. The numbers behind the letters indicate the numbers of each type of atom present in the molecule. There are two atoms of hydrogen and one atom of oxygen in every molecule of water. Water can also be expressed as "HOH."
Covalent bonding must occur for the atoms of hydrogen and oxygen to join together to form water. In a covalent bond, atoms share electrons. This is the opposite of ionic bonding, in which electrons are transferred between atoms. Water freezes at 32 degrees Fahrenheit and boils at 212 degrees Fahrenheit. Water is less dense in its solid form than its liquid form.