Chemical Bonds and Physical Properties

Chemical bonds are the electrical forces of attraction that hold atoms or ions together to form molecules. Different types of chemical bonds and their varying intensity are directly responsible for some of the physical properties of minerals such as hardness, melting and boiling points, solubility, and conductivity. Chemical bonds also influence such other properties as crystal symmetry and cleavage. Stronger bonds between atoms make them more difficult to separate and, in general, stronger chemical bonds result in greater hardness, higher melting and boiling points, and smaller coefficients of expansion. There are four principal types of chemical bonds found in minerals: ionic, covalent, metallic, and van der Waals.

An ionic bond is the result of the electrostatic attraction between two oppositely charged ions. Ionic bonds exist because some elements tend to capture or lose one or more electrons resulting in a net positive or negative charge. These are called ions. An ion that bears a positive charge is a cation. One with a negative charge is an anion. Ions may carry a single charge, such as Na⁺ and Cl⁻, or may have multiple charges, such as Ca²⁺ or Fe³⁺. Oppositely charged ions tend to attract one another because the cation can transfer electrons to the anion, allowing each ion to achieve better stability. For example, Na⁺ and Cl⁻ readily combine to form NaCl, halite (salt). Most minerals are held together by some form of ionic bond.

In order for an ionically bonded solid to melt, some of the bonds, but not all of them, must be broken. For boiling to occur, all of the bonds must be broken. As a result, ionic bonds produce moderate to high melting and boiling points. Ionic bonds are moderate in strength and so result in moderately hard minerals. The electrical conductivity is generally low and minerals with ionic bonds tend to dissolve better in water. In addition, because the charge on ions is evenly distributed around the surface of the atom, or nondirectional, a cation tends to evenly distribute as many anions as possible over its entire surface area. This often yields a high degree of crystal symmetry in minerals. Halite (salt) and fluorite are two common ionically bonded minerals.

Covalent bonds are different from ionic bonds in that electrons are shared between atoms of similar charge as opposed to electrons being donated by a cation to an anion. Covalent bonds form when the electron clouds of separate atoms draw near and overlap, enabling electrons to be shared. In covalent bonds, each participating atom usually contributes electrons, resulting in a strong bond. Covalent bonds are common between atoms and ions of the same element such the noble gasses.

Minerals with covalent bonds tend to be hard and insoluble. Diamond is one example. Covalent bonds produce high melting and boiling points and low conductivities. The forces that bind the atoms tend to be localized in the vicinity of the shared electrons and so are highly directional. This often yields a lower degree of crystal symmetry.

As the name suggests, metallic bonds are found in pure metallic minerals. Metallic bonds form when an atom of a metallic element, which usually contains loosely held electrons in the outer shell, shares these electrons with closely packed atoms of the same element. The shared electrons pass freely among all the metal atoms. The result may be described as a weak covalent bond. It is different from the true covalent bond, however, in that there are too few electrons to be shared continuously by all atoms simultaneously. The electrons are extremely mobile as they roam within the lattice of positive metal ions. The mobility of the electrons results in the high thermal and electrical conductivity of metals. The weakness of the bonds results in the lower hardness, low melting and boiling points, and high ductility so often observed in metallic minerals such as gold and copper.

Van der Waals bonds arise from very minor charge polarities that can develop on molecules that are already bonded together. For example, the directional characteristic of covalent bonds can produce a weak negative charge where the electron clouds overlap with a corresponding weak positive charge opposite the area of overlap. These dipoles may attract each other to form a very weak chemical bond known as Van der Waals. Van der Waals bonds are not common in minerals, but when present result in low hardness and easily cleaved zones. Graphite owes its greasy feel to the Van der Waals forces that link sheets of covalently bonded carbon atoms, allowing them to easily slip apart.

Most minerals are held together by a combination of chemical bonds. Often, distinct molecular units, consisting of strongly bonded atoms, are linked by weaker bonds, as in the graphite example above. Micas, which cleave perfectly into sheets, are another example. They are composed of covalent-bonded silica tetrahedral sheets joined together by ionic bonds. The ionic bonds tend to break first, separating into the more robust sheets. A single chemical bond in a mineral may also display the properties of more than one bond type. A common example is the silica tetrahedron, which consists of one silicon atom, Si⁺⁴, surrounded by four oxygen atoms, O⁻². The bond that binds silicon and oxygen together arises out of an ionic attraction, but it also involves overlapping electron clouds and subsequent sharing of electrons, so it is part covalent as well.

Atomic Structure; Chemical Elements; Crystals and Crystallography