Question

Define valency by taking an example of nitrogen and boron.

Answer 1

Hint
Before dealing with the valency of an electron we have to know many basic terms and their role in study of the atoms. We should know about the orbits, their energy, the shells, how a shell is filled with electrons and a lot more.

Complete step by step answer
An atom has a proton electron and neutron inside it.
The proton neutron forms the nucleus the electron revolves around the nucleus the electron has a negative charge and a proton has a positive charge and the neutron has no charge.
The electrons on the atom revolve around the nucleus in an elliptical path called orbits.
These orbits are associated with definite energies and are called energy shells or levels which are designated as K, L, M, N ……etc.
The atom can have more than 1 shell according to the number of electrons it have. The electrons in the outermost shell may contain free electron or it may be completely filled
The maximum number of electrons the shells K, L, M, N...... can have is $2,8,8,18.......$
As long as an electron stays in a particular orbit, it does not lose or gain energy i.e. Energy of an electron in a particular path remains constant and is known as stationary states.
The capacity of an atom to get accompanied with another atom is known as its valency. That capacity of atom to get combined with another atom is determined by the electrons in the outer orbit of an atom. The number of bonds that an atom can form with another atom is expressed by the valency of the element.
The electrons which are free and ready to make bonds with another electron in another atom is known as the valence electrons. The number of valence electrons in which an atom has is the valency of the atom.
The electrons first occupy the orbitals of lowest-energy available to them and then enter into higher energy orbitals only after the lower energy orbitals are filled.
For example
-Nitrogen
The atomic number of nitrogen is $7$. The K shell is filled with $2$ electrons. The M shell is filled with $5$ electrons. The capacity of M shell is $8$ but in nitrogen atoms only 5 electrons are there in M shell, these $5$ electrons in the outermost shell are known as valence electrons. Valency of nitrogen is $5$.
-Boron
The atomic number of boron is 5. The K shell is filled with $2$ electrons.
The M shell is filled with $3$ electrons. The capacity of M shell is $8$ but in boron atoms only 3 electrons are there in M shell, these $3$ electrons in the outermost shell are known as valence electrons. Valency of boron is $3$.

Note
The vacancy is just the count of electrons in the outermost shell; it cannot be negative i.e. it cannot have negative value.