Water molecules contain two hydrogen atoms (pictured in green) bonded to one oxygen atom (blue). The large oxygen atom has a stronger attraction for electrons than the small hydrogen atoms. Because the oxygen atom has a stronger pull on the negative bonding electrons the oxygen atom has a slightly negative charge and hydrogen atom a positive charge. This unequal sharing of electrons is called a polar bond or dipole. This causes the hydrogen atoms in one water molecule to be attracted to the oxygen atom in another water molecule. This attraction is known as hydrogen bonding. Water can change from a liquid to a solid or a gas and back to a liquid, but its molecular structure always stays the same.