How to Draw Dot-Cross Diagram

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1	Determine the central atom and surrounding atoms. Central atom is commonly the: - Element with the lesser no. of atoms - First element in chemical formula (except H) - Least electronegative element
2	For polyatomic ions: Anion: add the e– to the most electro-ve atom (one e– per atom) Cation: remove the e– from the least electro-ve element Ignore this step for neutral molecules
3	Determine no. of bonds each surrounding atom can form: no. of valence e– on atom [n(e)] no. of bonds the atom can form [n(b)] 0 – 4 n(e) 5 – 8 8 – n(e) For Hydrogen: n(b) = 0 for n(e) = 0 or 2
4	Draw all the bonds to the central atom. For atoms with 8 e– (or 2 for H): form dative bond from surrounding atom to central atom For atoms with 0 e–: form dative bond from central atom to surrounding atom
5	Assign remaining e– on central atom as lone pairs. If central atom is in Period 2 and no. of e– around the atom is > 8, convert a double bond to a dative bond from the central atom to surrounding atom.

 

Examples:

 

SO₂

1	Central atom: S Surrounding atoms: O
2	N.A.
3
4
5	N.A. (S is in P3; can expand beyond octet)

 

HCN

1	Central atom: C Surrounding atoms: H and N
2	N.A.
3
4
5	N.A.

 

NH₃

1	Central atom: N Surrounding atoms: H
2	N.A.
3
4
5	N has 5 e– and 3 bonds > it has 2 remaining e–:

 

NO₃^–

1	Central atom: N Surrounding atoms: O
2	Add one e– to one O atom.
3
4
5	N has 10e– > convert one double bond to dative bond from N to O:

 

AlH₄^–

1	Central atom: Al Surrounding atoms: H
2	Add one e– to one H atom.
3
4
5	N.A.

 

NH₄⁺

1	Central atom: N Surrounding atoms: H
2	Remove one e– from one H atom.
3
4
5	N.A.