Читать книгу Geology and Mineralogy of Gemstones - Karl Wallace, David Turner R. - Страница 17

The total number of neutrons and protons defines the atomic mass of an atom. Because electrons are so small, they do not contribute much to the overall atomic mass of an atom. The weights of atoms are given in atomic mass units, or amu, where both protons and neutrons have an atomic mass approximately equal to 1 amu. Helium has two protons and almost always two neutrons; its atomic mass therefore is 4 amu.

In the periodic table of the elements, the mass of an element is not normally a round number but instead is defined to a few decimal places. For example, silver has an atomic number of 47 and an atomic mass of 107.868. This is not to say that silver has 47 protons and 107.868 neutrons. Rather, 107.868 represents the average atomic mass of a sample of gold that includes gold atoms with different numbers of neutrons. Material comprised of high atomic mass elements will generally be of higher density, such as in the case of the native metals in Table 1.1.