Atomic Theory

The elements are arranged in the several Periods of the Periodic Table according to the number of shells in their electronic configurations. So, for example, the elements lithium (2 1), beryllium (2 2),boron (2 3) all have two orbitals or shells of electrons in their atoms, so they are all found in Period 2 of the Periodic Table.

STABILITY OF AN ATOM

As discussed earlier, the atoms of all elements are neutral. This is because they have an equal number of protons (positively charged particles) and electrons (negatively charged particles). But not all atoms are stable.

For an atom to be stable it has to have a full outer electron shell. So, if it has only one orbital, this will be full if it contains two or a doublet of electrons. For the rest of the orbitals these are assumed to be full if they contain eight or an octet of electrons.

Atoms of the elements helium (2), neon (2 and argon (2 8 8), already have two or eight electrons in their outermost shell. They are therefore said to be stable atoms. All of these elements are found in group 8, sometimes also called group 0 of the Periodic Table. Elements in this group are very satisfied with this electronic configuration and would not like to change it. Therefore they do not react with any other element. They are called inert or noble, meaning, do not react, elements.

The elements in each of the other seven groups in the Periodic Table are not stable, as they do not have a doublet or an octet of electrons. In order to become stable they combine or chemically react with other elements to form compounds. They could in fact become stable, either by gaining, losing or sharing electrons.

ELECTROVALENT BONDING

In order to obtain a doublet or octet of electrons some atoms could gain or lose electrons.

The maximum number of electrons an atom could gain is two.

The maximum number of electrons an atom could lose is three.

Consider an atom of sodium whose atomic number is 11 and therefore an electronic configuration of 2 8 1. Being in group 1 of the Periodic Table and therefore having one electron in its outermost shell, a sodium atom is definitely unstable. In order to become stable it needs to lose the electron present in the last shell. Then it will end up with eight electrons in its outermost shell.