Isotopes and Radioactivity

Dalton believed that every atom of an element was identical, but we now know that all elements have varieties called isotopes,2 which differ from each other only in number of neutrons and therefore in atomic mass. Most hydrogen atoms, for example, have only one proton; this isotope is symbolized 1H. Hydrogen has two other isotopes: deuterium (2H) with one proton and one neutron, and tritium (3H) with one proton and two neutrons (fig. 2.2). Over 99% of carbon atoms have an atomic mass of 12 (6p+, 6n0) and are called carbon-12 (12C), but a small percentage of carbon atoms are 13C, with seven neutrons, and 14C, with eight. All isotopes of a given element behave the same chemically. Deuterium (2H), for example, reacts with oxygen the same way 1H does to produce water.

The atomic weight of an element accounts for the fact that an element is a mixture of isotopes. If all carbon were

2iso = same + top = place (same position in the periodic table)

Tritium (3H) (1p+, 2n0, 1e-)


= Proton


= Neutron

= Electron

12C, the atomic weight of carbon would be the same as its atomic mass, 12.000. But since a sample of carbon also contains small amounts of the heavier isotopes 13C and 14C, the atomic weight is slightly higher, 12.011.

Although different isotopes of an element exhibit identical chemical behavior, they differ in physical behavior. Many of them are unstable and decay (break down) to more stable isotopes by giving off radiation. Unstable isotopes are therefore called radioisotopes, and the process of decay is called radioactivity (see insight 2.1). Every element has at least one radioisotope. Oxygen, for example, has three stable isotopes and five radioisotopes. All of us contain radioisotopes such as 14C and 40K—that is, we are all mildly radioactive!

Insight 2.1 Medical History

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