Читать книгу Geology and Mineralogy of Gemstones - Karl Wallace, David Turner R. - Страница 23

Chemical bonds are attractive forces between atoms. In a simplistic view, they form when outer electrons from two or more atoms interact, resulting in their atoms becoming “joined”. The two main types of bonding seen in nature are ionic and covalent.

Ionic bonding occurs between two atoms, one with a strong tendency to gain electrons (the anion) and the other with a strong tendency to lose electrons (the cation). Here, the cation can be thought of as having “donated” electrons (therefore becoming positively charged with less electrons than it started with) to the anion (which then becomes negatively charged with the extra electron). Normal table salt (NaCl) is a good example of ionic bonding where sodium, which usually has a valence of +1, combines with chlorine, which usually has a valence of −1, in a one‐to‐one ratio. This is a simple case and in reality most compounds are more complicated than this. In the mineral world, most bonding that occurs is ionic bonding, where electrons are donated from cation to anion.

Covalent bonding occurs when atoms “share” valence (or outermost) electrons between them. Covalent bonding is much more common in organic compounds (those that form living matter). However, in the gem world, this type of bonding is best observed in diamond. Diamond is a compound made up of carbon (C). The carbon atoms share electrons between them in a tight 3D network forming “molecules” of interconnected carbon atoms. These covalent bonds are very strong and give diamond its hardness and strength.

Another type of bonding that is less common in nature, but commonly studied by scientists, is metallic bonding. This is the type of bonding that, not surprisingly, is typical in native metals such as silver, gold, and copper. Valence electrons in metallically bonded compounds are shared throughout the entire material (not simply between two atoms) and are “free” to move about.

Van der Waals bonding is another type of bonding found in nature but seldom in minerals. This form of bonding is quite weak and when present often defines cleavage planes, such as in the mineral graphite.