We all know how electrons in an atom are arranged in shells/orbitals. Valence electrons are those electrons which are present in the outermost orbit of the atom. From the Bohr-bury scheme, we can say that the outermost shell can contain a maximum of 8 electrons. Only a little chemical activity is observed when the outermost shell is completely filled. We can also say that it’s combining capacity becomes zero. Noble gases have a completely filled outermost shell and that’s why they are least reactive. Other element’s reactivity depends upon their ability to attain the noble gas configuration. In this section, we shall learn more about valency of an atom.
If the outermost shell has 8 electrons then the element is said to have a complete octet. By gaining, sharing and losing the electrons the atoms complete their outermost orbital and make an octet.
The capacity of an atom is described by the total number of electrons lost, gained or shared to complete its octet and it also determines the valency of the atom.
How to determine Valency:
As we know, the number of electrons in the outermost shell of hydrogen is 1, and in magnesium, it is 2. Therefore the valency of hydrogen is 1 as it can easily lose 1 electron and become stable. On the other hand, that of magnesium is 2 as it can lose 2 electrons easily and also attain the stability.
Furthermore, it is not only determined when an atom loses an electron. For example, fluorine has 7 electrons in its outermost orbital. It is hard to lose 7 electrons and so it completes its octet by gaining 1 electron. Since it gains 1 electron, its valency is 1. In the periodic table, the elements in the same group have the same valency. For example, all the elements in group 8 have 8 electrons and completely filled orbitals, that is why the valency of all the elements in this group is zero.
How Valency is different from oxidation number:
Valency is different from oxidation number, and it has NO SIGN. Thus, the valency of nitrogen is 3, whereas it can have oxidation numbers from -3 to +5. Oxidation number is the hypothetical charge of an atom in a molecule or ion, and it is a measure of its apparent capacity to gain or lose electrons within that species.
Let us look at the valency of the first 20 elements of the periodic table.