Lecture 2 - Atoms and Bonding In Minerals

Atoms, Bonding, Crustal Composition, Minerals

Matter is composed of atoms, which consist of protrons, neutrons, and electrons.

The number of protons in the nucleus determines what the element is, and is called the atomic number.

Atoms can also be described by atomic mass number, the sum of neutrons and protons, which varies for a given element depending on the number of neutrons (usually stated as an average).

Elements are those groups of atoms with the same number of protons and electrons, and identical chemical properties; isotopes of an element have differing numbers of neutrons and thus differing mass number.

Elements can also be described by atomic weights, the weight in grams of a mole of that element the atomic weight of silicon is 28.08 grams, whereas the atomic weight of Fe and Mg are 55.847 and 24.305 grams, respectively. Elements also have atomic radii, which varies depending on what the element is bonded to.

Some isotopes are radioactive Ñ having an unstable combination of protrons, electrons, and neutrons. Such elements undergo spontaneous decay to reach a stable balance of subatomic particles.

Individual atoms can be electrically neutral Ñ if the number of protons equals the number of electrons. Ions are atoms with an electrical charge, resulting when the number of electrons orbiting the nucleus is not sufficient to balance the number of protons in the nucleus.

The electrical character and reactivity of elements is determined by the number and distribution of electrons in orbitals.

Noble gases are chemically inert elements with a full complement of eight electrons in their outermost orbitals, i.e., their outermost orbital is filled; chemically reactive elements bond together to obtain a noble-gas configuration of electrons. In a noble gas configuration atoms are electrically neutral and their outermost energy level is satisfied.

All charged ions will react to give up or take on electrons to reach charge balance.

Atoms may be joined by either ionic, covalent, metallic, or van der Waals ("weak") bonds.

Ionic bonding occurs as electrons are swapped between the valence shell of one atom and transferred to another. Ionic bonding is favored when bonded atoms have different valencies, and are close to a noble gas configuration.

Covalent bonds form as electrons are shared among atoms; covalent bonding is favored by atoms of similar valence.

Metallic bonding occurs when electrons are not bound with any particular nucleus, but are able to move in a flux through an atomic structure.

Van der Waals, or 'weak,' bonding arises from a polar attraction of atoms, comparable to a magnetic attraction, or the tension that holds a drop of water together, without sharing of electrons.

The physical properties of minerals (luster, hardness, cleavage, melting point, color, specific gravity, soluability, etc.) are determined by the type of atomic bond(s) in the mineral and the chemical composition.

Characteristics of minerals in which a particular type of atomic bond predominates:

*Note that diamond and graphite are both forms, or polymorphs, of native carbon. In the case of diamond, carbon atoms are linked by strong, covalent bonds in three-dimensions, forming the hardest material known. Carbon atoms are linked in sheets in graphite, a two-dimensional arrangnement of covalent bonds, and each sheet is weakly held together by Van der Waal's bonds.

Just what is a Mineral?

Minerals are naturally occurring, inorganic solids with fixed compositions and a repeating internal structure.

Note each point of the definition:

Minerals are found in nature. Although many minerals can be grown synthetically, in order for something to be a mineral, it must be found in nature.

Minerals are inorganic - they do not contain polymerized hydrocarbons. However, minerals may result from organic processes, as the activities of many bacteria, plants, and animals (including us) results in the growth and development of certain minerals. For example, CaCO3 in the shell of a clam is a mineral, and the skeletal materials of clams, other molluscs, corals, and other organisms can accumulate in extensive deposits in organic reefs - limestones are the indurated remnants of such reefs.

Mineral compositions are fixed, meaning that they can be expressed in terms of a formula. NaCl is the formula for the mineral halite; all halite, or common 'salt' crystals, have sodium and chlorine in a 1:1 proportion. Minerals are electrically neutral, meaning that the total charge of cations and anions are sufficient to balance. 'Fixed' does not, however, imply that the composition of minerals cannot vary. In fact, some minerals permit extensive substitutions of certain cations or anions. For example, Fe and Mg substitute freely for one another, as you might expect from their similar charge (+2) and size. Olivine is a common mineral in which Fe and Mg substitute in varying proportions, as indicated by its formula: (Fe,Mg)₂SiO₄.

Minerals are crystalline. This means there is a regular, repeating internal arrangement of atoms in the structure of a mineral. Another way to phrase this arrangement is to say that mineral structures are 'periodic' - meaning that there is a regular interval and relationship between the repeat of every atom in the structure.