Two isotopes, forms of elements that are chemically identical but have heavier atoms because of excess neutrons in the nucleus, are common in nature: Oxygen-16 and Oxygen-18 (where the number reflects the mass of the atom).
Two isotopes, formsof elements that are chemically identical but have heavieratoms because of excess neutrons in the nucleus, arecommon in nature: Oxygen-16 and Oxygen-18 (where thenumber reflects the mass of the atom).