Question

Valency of magnesium and oxygen in MgO is__________________?

              a) one and one
             b) two and two
             c) one and two
             d) two and one

Answer 1

- Hint: Here, the word valency means the number of electrons in the valence shell (outermost shell) or the number of electrons required to fill that shell completely. Magnesium belongs to the group 2 and oxygen belongs to the group 16 of the modern periodic table.

Complete step-by-step solution -
The electrons present in the outermost shell of an atom are called valence electrons, and their numbers determine the valency of the atom.
To calculate the number of valence electrons, apply the following formulas-
Valency of a metal = no. of electrons present in the outermost shell
Valency of a non-metal = eight minus no. of electrons present in the outermost shell.
Now,
We will calculate the no. of valence electrons of magnesium and oxygen by using electronic configuration of magnesium and oxygen as follows:
      Mg [\[1{{s}^{2}},2{{s}^ {2}}, \text{}2{{p}^ {6}}, \text{}3{{s}^ {2}} \]]
       O [\[1{{s}^{2}},2{{s}^ {2}}, \text{}2{{p}^ {6}} \]]
From the above configuration we conclude that Mg has 2 valence electrons and oxygen have 6 valence electrons.
Therefore, valency of Mg is 2. And valency of oxygen is 8-6=2. (since oxygen is non-metal)
Hence, in the given compound both Mg and O has valency 2.
Therefore option (b) is correct.

 Additional information: Valency is also known as the oxidation state of an element i.e. the charges present on metal and non-metal. Charges are the result of donation or transfer of electrons from one metal to another.

Note: Remember for non-metal valency is not the number of valence electrons in the outermost shell, we often make this mistake so keep in mind. For non- metals the valency is equal to the number of valence electrons subtracted from 8.
An element can have variable valency depending on the type of molecule it forms.