Atoms of the same or different elements can react with one another to form completely new substances called compounds. When two or more atoms bond chemically, the new compound will have different chemical and physical properties than the original atoms.

Here's an example. Sodium is a soft, silvery metal. Sodium can react with chlorine which is a yellowish, poisonous gas. The result is sodium chloride (NaCl). You know it as table salt. Sodium chloride is neither a metal or poisonous gas.

Source: Chemical bonding, Vision Learning