III.

Physical Properties

Pure hydrogen is a gas under normal conditions—that is, at room temperature and normal atmospheric pressure. Like most gaseous elements, hydrogen is diatomic, meaning its molecules contain two atoms. Molecular hydrogen is represented symbolically as H₂. Hydrogen gas is much lighter than air. At 0°C (32°F) and regular atmospheric pressure, hydrogen has a relative density of 0.090 grams/liter (g/L), whereas the relative density of ordinary air is 1.0 g/L. Hydrogen has such a small mass that it can escape Earth’s gravitational pull and fly off into space. As a result, it is not found in large amounts in the atmosphere. Hydrogen has a lower boiling point and freezing point than does any other substance except helium. Hydrogen boils at –252.8°C (-423.0°F) and freezes at –259.14°C (-434.45°F). Liquid hydrogen, first obtained by British chemist Sir James Dewar in 1898 (see Cryogenics), is colorless in small amounts but light blue in thick samples. Solid hydrogen is colorless.

Hydrogen exists in nature as three different isotopes. Isotopes are atoms of the same element that contain different numbers of neutrons, uncharged elementary particles, in their nuclei. The majority of hydrogen atoms have no (zero) neutrons in their nuclei. Scientists represent these hydrogen atoms with the symbol ¹H. Atoms of ¹H have just one proton in their nucleus and have an atomic mass of 1. This isotope, which accounts for 99.98 percent of hydrogen atoms, is sometimes called protium. About 0.02 percent of hydrogen atoms have one neutron and one proton in their nucleus. This isotope is called deuterium. Deuterium was the first isotope of any element that scientists discovered and isolated from a sample. It is used in a variety of scientific experiments. Deuterium is represented by the symbol ²H, or by the symbol D, and has an atomic mass of 2. The third isotope of hydrogen is called tritium (³H). This isotope has two neutrons and one proton in each atom’s nucleus, and it has an atomic mass of 3. Tritium accounts for fewer than one in 10,000 atoms of hydrogen. It is radioactive, meaning its nucleus can decay, or spontaneously change, into other particles (see Radioactivity). The half-life of a radioactive substance, such as tritium, is the length of time necessary for half of a sample of the substance to decay into other particles. Tritium has a half-life of 12.4 years. Scientists can make tritium in the laboratory in nuclear reactions. The names protium, deuterium, and tritium come from the Greek words for first, second, and third, respectively.