Sandro Lanfranco
Department of Environmental Science, Junior College, University of
Malta, Msida
Overview
Most atoms are not isolated entities but have a tendency to combine
with other atoms to form molecules. Molecules may be composed of only two
atoms (termed diatomic molecules; examples include Oxygen gas O2,
Nitrogen gas N2 and Hydrogen gas H2) or from much
larger numbers (e.g. buckminsterfullerene C60). Interactions
between different atoms are directed by the valence electrons (electrons
in the outermost shell of the atom) of the atoms involved.
Chemical bonding
All atoms seek to fill their outermost electron shell to capacity.
This basic principle is termed the Octet Rule, since the maximum capacity
of the outer shell is generally taken to be eight electrons. Only six chemical
elements are characterised by a full outer shell (the noble elements: Helium,
Neon, Argon, Krypton, Xenon and Radon). All other atoms are required to
fill their outermost shell by interacting with other elements. Interaction
with other elements involves the formation of a chemical bond.
Ionic bonding
Atoms that have few electrons in their outer shell generally satisfy
the octet rule by transferring these electrons to another atoms. This process
is generally characteristic of atoms of metals (elements situated on the
left-hand side of the Periodic Table of Elements). Once these atoms lose
their valence electrons they are termed ions. Whilst atoms are electrically
neutral, ions possess an electric charge. Atoms that lost their outer electrons
acquire a positive charge for every electron that they transfer. The electrons
transferred by metals are generally accepted by atoms of the non-metallic
elements. Every electron received adds a negative charge to these atoms.
Example: An atom of sodium (Na) has eleven electrons arranged in a configuration of 2-8-1. The outer electron is transferred to another atom and sodium becomes an ion carrying a single positive charge; Na+. An atom of calcium has twenty electrons arranged in a configuration of 2-8-8-2. The two outer electrons are transferred to another atom or atoms and calcium becomes an ion carrying a double positive charge; Ca2+ or Ca++.
Example: an atom of chlorine has seventeen electron arranged in a configuration of 2-8-7. It only requires one more electron in order to satisfy the octet rule. Obtaining that single electron adds a negative charge and the chlorine atom now becomes a chloride ion; Cl-.
A pair of atoms that have exchanged electrons in this way experience
an attractive force which binds them together. In this way, a sodium ion
(Na+) and a chloride ion (Cl-) are mutually attracted
to form sodium chloride (common salt, NaCl). The bond between these two
ions is called an ionic bond and the resultant compound is an ionic compound.
Covalent compounds
Other atoms fill their outer shell by sharing, rather than transferring
or receiving, electrons. Compounds that form in this way are termed covalent
compounds and the bond is called a covalent bond. Examples of covalent
compounds include carbon dioxide (CO2) and water (H2O).
Chemistry of compounds of carbon (organic chemistry)
The study of the carbon atom is a separate and well-defined branch
of chemistry. Carbon is an important molecule since it forms the basis
of all living organisms and of all our fossil fuels. An extremely large
number of organic compounds (chemicals containing carbon) have been characterised,
some of the more significant being hydrocarbons, amino acids and carbohydrates.
Hydrocarbons are compounds that are exclusively composed of carbon atoms and hydrogen atoms. The simplest hydrocarbon is methane (CH4). Others include ethane (C2H6), propane (C3H8) and butane (C4H10). The smaller hydrocarbons are gases at room temperature and pressure. They are used as fuels.
Amino acids are the building blocks of proteins. They are composed of carbon, hydrogen, oxygen, nitrogen as well as, in some cases, phosphorous and sulphur. There are twenty different kinds of natural amino acids (including glycine, valine, threonine, cysteine, methionine). These form the structural materials of living organisms.
Carbohydrates are also called sugars. They are molecules containing atoms of carbon, hydrogen and oxygen. One of the most significant carbohydrates is glucose (C6H12O6). Others include fructose, galactose, sucrose, lactose and maltose. Some carbohydrates may attain very large sizes. One such example is starch, which is a polymer of molecules of glucose.
Chemical changes
When two chemicals interact, the atoms making up their molecules may
rearrange to form a product which is different from either. This process
is called a chemical reaction. The chemicals which start the process are
termed reactants whilst the resultant chemicals are the products
Example:
Important chemical reactions
One of the most important types of chemical reaction in the context
of this course is combustion (burning; also called oxidation). This
type of reaction occurs when oxygen gas is added to the chemical that is
being burnt.
Example: Combustion of coal.
Coal is principally made up of carbon, but also contains sulphur as
an impurity. Therefore, when a lump of coal is burnt, both carbon as well
as sulphur are being burnt. The chemical reactions are as follows:
C + O2 --> CO2 (carbon dioxide)
S + O2 --> SO2 (sulphur dioxide)