Our entire universe is made up of combinations of elements the smallest unit of which is the atom. Each atom consists of a central nucleus, made up of protons, which is positively charged electrically. The number of protons in an atom determines what the element is. In the case of hydrogen, for example, there is just one proton, whereas carbon has 6 protons, sodium 11, calcium 20 and uranium 92.

Spinning around the central core of protons - in orbit, so to speak - are precisely the same number of negatively charged electrons, or, in the case of hydrogen, one electron. Electrons are much smaller than protons; by weight it actually takes 1835 electrons to equal one proton. Despite this imbalance in weight, the negative electrical charge of one electron is just as powerful as the positive electrical charge of one proton. Thus in a stable, electrically neutral atom, the electrical charge of the proton(s) is precisely in balance with that of the electron(s).

Electrons (much like people) have a natural tendency to find partners so that they can become paired. When this does not happen, as during radiation or certain chemical processes, a 'free radical' or a molecule (combination of atoms) with an unpaired electron may evolve (see below for more on these miniscule troublemakers).

The electrons which spin in orbit around the protons are arranged in such a way as to form layers, or 'shells' each such shell being placed just a little further out than the previous one from the nucleus. The arrangement of electrons in each 'shell', or layer (in most atoms) is always the same, with two electrons in the inner shell and eight electrons in the next. In some instances atoms, instead of having their own complement of negatively charged electrons (which provide electrical balance and harmony to the positive electrical charge of the protons) actually share electrons with other atoms.

This is the way molecules (combinations of atoms) are formed, allowing, for example, the two gases hydrogen and oxygen to combine together to form water. Some atoms find that they can gain or lose anything from one to three electrons when necessary, thus turning themselves into ions. An ion is simply an atom or group of atoms which has lost or gained one or more of these orbiting electrons, for one of a number of reasons, and which has therefore in the process become capable of conducting electricity.

Technically, a cation is an ion with a positive electrical charge (this happens to an ion which has lost one or more electrons) and an anion is an ion which is negatively charged (this happens to an ion which has gained one or more electrons).

If we look at what happens when this process occurs between sodium and chlorine it should become clearer.

The process of crystallization of salt
The sodium atom (Na) has 11 protons at its core and therefore (since the number of protons equals the number of electrons) requires 11 electrons to balance it electrically.

As in other atoms the first 'shell' contains two electrons, and the second 'shell', eight. Simple arithmetic tells us that there is one more electron needing a home in the case of sodium, and this is found in orbit, on its own, in the third 'shell' This so-called 'valency', electron is therefore available to attach itself easily to any passing atom which might be in need of an extra electron to complete its own electrical balance.