Valency

 

In our previous blog we have learnt how the electrons in an atom are arranged in different shells/orbits. Today we are going to talk about the valency or combining capacity of an atom.

The electrons present in the outermost shell of an atom are known as the valence electrons. From the Bohr-Bury scheme, we also know that the outermost shell of an atom can accommodate a maximum number of 8 electrons. It was observed that the atoms of elements, having a completely filled outermost shell show little chemical activity. In other words, atoms having completely filled outermost shell have no combining capacity or their valency is zero. These elements are known as inert elements. Like the helium atom has two electrons in its outermost shell and all other elements have atoms with eight electrons in the outermost shell.

The combining capacity of the atoms to other elements, or their tendency to react and form molecules with atoms of the same or different elements are known as valency. This could be explained as, an atom attempt to attain a fully-filled outermost shell. An outermost-shell, which had eight electrons was said to possess an octet. Atoms only react to achieve their octet, so in order to achieve an octet in the outermost shell they start sharing, gaining or losing electrons. The number of electrons shared, gained or lost to achieve the octet of electrons in the outermost shell, are directly proportional to the combining capacity of the element, that is, the valency. For example, hydrogen, lithium, sodium atoms contain one electron each in their outermost shell, therefore each one of them can lose one electron. So, they are said to have valency of one.

If the number of electrons in the outermost shell of an atom is close to its full capacity, then valency is determined in a different way. For example, the fluorine atom has 7 electrons in the outermost shell, and its valency could be 7. But it is easier for fluorine to gain one electron instead of losing seven electrons. Hence, its valency is determined by subtracting seven electrons from the octet and this gives you a valency of one for fluorine.

Fore more details refer to Beyond Reaction: Fundamental Chemistry