Lewis dot structures shows how the bonding occurs in simple molecules and polyatomic ions in terms of shared e- pairs.Basic steps for writing lewis dot stuctures are as follows:-
1.Add the valence electrons of the combining atoms which gives the total no. of electrons for writing the structure.
2.For polyatomic anions,add one e- to the total no. for each unit negative charge.
For ex- CO₃^2- (carbonate anion) the total no. of electrons will be
Total e- = 4e- from C + 3 * 6e- from each O atom + 2* 1e- for each negative charge
= 4 + 3*6 + 2* 1
= 24 electrons
3. For polyatomic cation subtract one e- for each unit positive charge.
4. Write the skeleton structure by placing least electronegative atom in the centre and more electronegative atoms on the terminal positions.
5.Distribute the electrons properly as shared pairs in proportion to the total bonds.
6.Now remaining electrons are utilised either for multiple bonding or for indicating the lone pairs.