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Covalent Structures

Following the four previous rules we can build a more complex structure like that of the sulfite ion.

As the molecule has two negative charges, we must account for two more electrons than the 24 provided by the atoms.

1. The central atom is chosen. Which one is less electronegative? Sulfur or oxygen?

2a. The valence electrons (outermost) of all atoms are counted (1 x S = 6, 3 x O = 18, total 24 electrons or 12 pairs)

2b. 2 more electrons are added for the two negative charges: total 26 electrons or 13 pairs

3. Bonds are formed between the central atom and the peripheral ones.

4. The "excess" electrons are placed on the atoms as unshared pairs (starting with the most electronegative ones).

Lewis structure of the sulfite ion, SO₃₂⁻, step by step

Charges are assigned to atoms based on the electrons they own.

The electrons owned by each atom are its unshared pairs and half of the shared ones.

Formal charges in the sulfite ion structure with single bonds: +1 on sulfur and −1 on each oxygen

Oxygen does not accept more than eight electrons. Sulfur could because it has an empty "3d" orbital. Electrons can be rearranged to remove as many charges as possible.

Sulfite ion with an S=O double bond: rearranging the electrons avoids two charges

Could you write the Lewis structure for carbon dioxide (CO2) and the cyanide ion (CN−)?