Читать книгу Geology and Mineralogy of Gemstones - Karl Wallace, David Turner R. - Страница 18

Protons and neutrons are roughly the same size and are located in the nucleus or core of an atom. Outside this nucleus are the electrons that orbit the atomic core in an unpredictable but organized electron cloud much larger than the size of the nucleus itself (Figure 1.1). Because the mass of neutrons and protons is so much greater than that of the electrons, nearly all the mass of an atom exists at its nucleus. Strong atomic forces keep the neutrons and protons tightly packed in a dense cluster.

Table 1.1 Atomic masses and physical properties of selected elements and their native metal mineral.

Element	Atomic number	Average atomic mass	Density (g/cm3) of native metal mineral
Copper, Cu	29	63.55	8.9
Silver, Ag	47	107.87	10.5
Gold, Au	79	196.97	19.3

Protons have a positive electrical charge, neutrons have no electrical charge, and electrons have a negative electrical charge. The sum of their charges denotes overall ionic or atomic charge. In a basic atom of a given element with all of its allotted electrons, an atom is neutral. This means that all of the negative charges of the electrons are balanced by all of the positive charges of the protons.

Some atoms are prone to gaining electrons from outside sources, which results in them having a net negative charge. Other atoms are prone to losing electrons, resulting in a net positive charge. The resulting charge, positive or negative, is called the valence state (or valence charge) of an atom. Charged atoms are called ions; specifically, positively charged ions are called cations while negative ones are called anions (Figure 1.2). The exchange (gain or loss) of electrons almost always occurs within the outermost portion of the electron cloud.

Figure 1.1 An atomic model of the element helium (He), with two protons, two neutrons, and two electrons.

The electron cloud of an ion can be estimated to be in the shape of a sphere and its size is defined by the distance from the center of the nucleus to the limit of the cloud. This is called the ionic radius, which is measured in units called Angstroms or Å. An Angstrom unit is very short – it is equal to one tenth of a nanometer. Note that a nanometer is 0.000000001 meter or 10^–9 meter!