Question

How do you calculate the number of valence electrons in a molecule?

Answer 1

For calculating the number of valence electrons present in a molecule, we have to add all the valence electrons present in each atom which takes part in the formation of the molecule.

Complete answer:
As we know that valence electrons are those electrons which are present in the outermost shell of the atom.
-So for calculating the total number of electrons present in a molecule we have to know about the atomic number of each atom which constituent the molecule, because atomic number (${\text{Z}}$) of an atom is equal to the number of protons or number of electrons present in it and their relation is shown as:
Atomic Number (${\text{Z}}$) = No. of electrons = No. of protons
-And when we know about the number of electrons present in an atom we will write the electronic configuration and calculate the number of valence electrons in an atom.
For example if we have to calculate number of valence electrons present the water (${{\text{H}}_{\text{2}}}{\text{O}}$) molecule then we will follow the following process:
-Number of valence electrons in the hydrogen atom is one as its atomic number is one.
-Number of valence electrons in the oxygen atom is six (${\text{1}}{{\text{s}}^{\text{2}}}{\text{,2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{4}}}$) as its atomic number is eight.
-So the total number of valence electrons in a water molecule is the sum of the valence electrons present in hydrogen and oxygen atoms i.e. $1 + 1 + 6 = 8$.

Note:
Here some of you may do wrong calculations if you count all the electrons present in an atom, always keep in mind that here we have to count only outermost shell electrons.