How to Draw Dot-Cross Diagram
Electrochemistry

How to Draw Dot-Cross Diagram


← Back to Chemical Bonding

 

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.




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