Valency

Valency:

The electrons present in the outermost shell of an atom are known as the valence electrons.

The outermost shell of an electron can accommodate a maximum of 8 electrons

It was observed that the atoms of elements, completely filled with 8 electrons in the outermost shell show little chemical activity.

In other words, their combining capacity or valency is zero.

The valency of the same group of the element present in the periodic table is the same. If we consider group 8 in the periodic table, all the elements of group 8 have completely filled outermost orbit and have attained octet arrangement. So, the elements of group 8 have zero valencies. The valency of any element can be determined primarily by 3 different methods:

1) The Octet Rule

If we cannot use the periodic table to determine valency then the octet rule is followed. This rule states that atoms of an element or chemicals have a tendency to obtain 8 electrons in their outermost orbit either by gaining or losing electrons in whatever form of compound it is present. An atom can have a maximum of 8 electrons in its outermost orbit. The presence of 8 electrons in the outermost shell indicates stability of an atom.

An atom tends to lose electron if it has one to four electrons in its outermost orbit. When an atom donates these free electrons it has positive valency. An atom will gain electrons if it has four to seven electrons in its outermost orbit. In such cases, it is easier to accept electron rather than donating it. Therefore, we determine the valency by subtracting the numbers of electrons from 8. All noble gases have 8 electrons in its outermost orbit except helium. Helium has 2 electrons in its outermost orbit.

2) Using the Periodic Table

In this method, valency is calculated by referring to the periodic table chart. For example, all the metals, be it hydrogen, lithium, sodium and so on, present in column 1 have valency +1. Similarly, all the elements present in column 17 have valency -1 such as fluorine, chlorine, and so on. All the noble gases are arranged in column 18. These elements are inert and have valency 0.

However, there is an exception to this method of valency determination. Certain elements like copper, iron, and gold have multiple active shells. This exception is usually noticed in transitional metals from column 3 through 10. It is also observed in heavier elements from column 11 through 14, lanthanides (57-71), and actinides (89-103).

3) On the Basis of the Chemical Formulae

This method is based on the octet rule. The valencies of many transitional elements or radicals can be determined in a particular compound by observing how it chemically unites with elements of known valency. In this case, the octet rule is followed where the elements and radicals combine and try to attain eight electrons in the outermost shell in order to become stable.

For instance, consider the compound NaCl. We know that the valency of sodium (Na) is +1 and Chlorine (Cl) is -1. Both sodium and chlorine have to gain one electron and lose one electron respectively to achieve stable outermost orbit. Therefore, sodium donates an electron and chlorine accept the same electron. This is how the valency is determined. It is the classic example of ionic reaction as well.