Introduction to molecules and compounds
What is a molecule?
A molecule is a chemical unit capable of independent existence and can consist of one or more atoms. Caution must be exercised in using the words 'atom' and 'molecule'. Only a few atoms are capable of independent existence, therefore only a few atoms are molecules. Refer to Image 1
A single helium atom is capable of independent existence so helium is a molecule as well as an atom. Molecules containing only one atom are monatomic molecules.
A single oxygen atom, on the other hand, cannot exist independently. The minimum number of oxygen atoms that can exist independently is two, so the molecule for oxygen is represented by the using the symbol 'O2'. Molecules containing two atoms are diatomic molecules. Refer to Image 2
What is a compound?
Molecules do not need to be of the same element. Molecules containing two or more different elements are compounds. A compound is a substance containing two or more elements chemically bonded together. It is important to note that all compounds are molecules, but not all molecules are compounds. The examples of molecules given above are not compounds because they contain only one element.
Water is an example of a molecular compound, represented by 'H2O', which indicates two atoms of hydrogen bonded with one atom of oxygen. Refer to Image 3
Types of compounds
There are two main types of compounds: ionic and covalent. From studying the periodic table, it will be known that each element has a valency - that is, the capacity to lose or gain electrons depending on the number of electrons on their outer shell. Some elements readily lose electrons, while others readily gain electrons to achieve stability (eight electrons) on their respective outer shells.
Ionic bonding occurs when two or more elements chemically react to form a new substance and there is a respective loss and gain of electrons in the process. When an atom donates one or more electrons to the compound it becomes a cation (positive ion) because it possesses more protons than electrons. When an atom gains one or more electrons in forming a compound it becomes an anion (negative ion) because it has more electrons than protons. The positive-negative charge between the atoms holds the compound together.
Magnesium oxide is an example of ionic bonding. Magnesium has two electrons on its outer shell, oxygen has six. Eager to get rid of its two excess electrons, magnesium attracts the oxygen, which needs those two electrons to fill its outer shell, thus forming magnesium oxide. In this reaction, the magnesium is the cation because it loses two electrons and has a positive charge, whereas the oxygen is the anion because it gains two electrons and has a negative charge. See Animation
In an ionic bond, the elements occur in ratio to one another, rather than as individual molecules. Ionic substances tend to form crystals, which have millions of atoms from each element in a particular ratio.
Covalent bonding occurs when atoms share (rather than lose or gain) electrons to stabilise their respective outer shells. Non-metals tend to share electrons because they have a high electronegativity, meaning they do not readily lose electrons in order to complete their outer shell. Electronegativity is the ability of the element to gain electrons; the halogens in group VII are the most electronegative atoms.
Water is a covalent compound containing two non-metals (hydrogen and oxygen) sharing electrons to stabilise their respective outer shells. Refer to Image 4
In a molecule where the atoms come from the same element (e.g. O2), the electrons are equidistant from each atom's nucleus. Different elements have different electronegativity so, in a molecular compound, the shared electrons sit slightly closer to the most electronegative atom. This polar covalent bond gives each element of the compound a slightly positive or slightly negative charge. Refer to Images 5 and 6
