Solution:
The molecular formula of sodium formate is HCOONa. The ionized form will have Na^{+} and HCOO^{-} ions.
The lone pair of electrons or non-bonding electrons on an atom are electrons not participated in bonding. The electrons involved in bonding are bonding electrons. The formal charge on an atom can be calculated from the following formula:
FC=VE-NE-\frac{BE}{2}
Here, VE stands for valence electrons, NE for non-bonding electrons and BE for bonding electrons
Calculate formal charge on each atom as follows:
In sodium atom, number of valence electrons is 1, bonding electrons are 0 and non-bonding electron are also zero. Thus,
FC=VE-NE-\frac{BE}{2}=1-0-\frac{0}{2}=1-0=1
In oxygen near to sodium atom, number of valence electrons are 6, bonding electrons are 2 and non-bonding electrons are 6 thus,
FC=VE-NE-\frac{BE}{2}=6-6-\frac{2}{2}=6-6-1=-1
In carbon atom, number of valence electrons are 4, bonding electrons are 8 and non-bonding electrons are 0 thus,
FC=VE-NE-\frac{BE}{2}=4-0-\frac{8}{2}=0
In hydrogen atom, number of valence electron is 1, bonding electron are 2 and non-bonding electrons is 0 thus,
FC=VE-NE-\frac{BE}{2}=1-0-\frac{2}{2}=0
In other oxygen atom, number of valence electrons are 6, bonding electrons are 4 and non-bonding electrons are 4 thus,
FC=VE-NE-\frac{BE}{2}=6-4-\frac{4}{2}=0
Thus, formal charge on sodium, oxygen near to sodium atom, carbon, hydrogen and other oxygen atom is +1, -1, 0, 0 and 0 respectively.
The structure of sodium formate showing lone pairs and formal charges on atoms is in the attachment.
