Question

How to find out valency?

Answer 1

Hint: Valency is the capacity of an atom to bond with another atom or molecule. Valence electrons are the electrons of the outermost shell of an atom. The valence electrons participate in any chemical reaction.

Complete step by step answer:
The methods to find valency are as follows-
1. Octet rule-
The Bohr-buy scheme states that the outermost orbit of an atom can have a maximum eight electrons. The tendency of an atom to obtain eight electrons in its outermost orbit by gaining or losing electrons is called octet rule. The atom whose octet is complete is most stable. If the number of electrons in the outermost shell is \[4\] or less then the valency of the atom is equal to this number.
If the number of electrons is more than \[4\] then, it will be easier for the atom to accept electrons than donating it. So we can find valency by this formula-
$ \Rightarrow $ Valency=$8$$ - $ the number of valence electrons
But for this it is necessary to know the atomic number of an atom so the valence electrons can be known.
2. Using periodic table-
In periodic table elements are arranged in such a way that the valency of an atom can be easily determined on seeing periodic table charts. For example- all the alkali metals placed in I column have valency $ + 1$ , All the elements of column $17$ which include the halogens have $ - 1$ valency. The elements of column $18$ which are known as noble gas have zero valency and hence are inert gases.

Note:
The valency of an element can also be determined from chemical formula. It is achieved by observing how elements form bonds and combine with each other. It is based on the octet rule. For example-
In $NaCl$, sodium gains one electron and chlorine loses one electron to become stable. So the valency of sodium is $ + 1$ and chlorine has valency $ - 1$ .