Define valence shell and valence electrons.

Hint: We know that when the bonds are formed between two atoms then the electrons are involved. The number of valence electrons in sodium (Na) is 1, valence electrons in magnesium (Mg) are 2, valence electrons in oxygen (8) are 6, etc.

Complete answer:
Bonding is a major characteristic of an atom, in which valence term is used. We know that the bonds are formed between two atoms to form a molecule or compound. The bonds are formed between two atoms then the electrons are involved. Based on the electrons in their last shell is used in the bond formation. So the last shell is known as the valence shell and the electrons in the valence are known as valence electrons. These valence electrons are responsible for the formation of a bond.
For example, the number of valence electrons in sodium (Na) is 1, valence electrons in magnesium (Mg) are 2, valence electrons in oxygen (8) are 6, etc. This can be explained as:
The atomic number of sodium (Na) is 11, so 11 electrons are present in sodium, therefore, the electronic configuration is
$1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{1}}$
The last shell is 3 and the electron in this shell is 1, so the valence electron is 1.
The atomic number of Magnesium (Mg) is 12, so 12 electrons are present in magnesium, therefore, the electronic configuration is
$1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}$
The last shell is 3 and the electron in this shell is 2, so the valence electrons are 2.
The atomic number of Oxygen (O) is 8, so 8 electrons are present in oxygen, therefore, the electronic configuration is
$1{{s}^{2}}2{{s}^{2}}2{{p}^{4}}$
The last shell is 2 and the electron in this shell is 6, so the valence electrons are 6.

Note:
All the subshells of the shell must be considered, like if the last shell is 3, and in the electronic configuration the electrons are present in 3s and 3p, so valence electrons will be equal to the addition of electrons in 3s and 3p.