To draw the Lewis structure for CN (cyanide ion), follow these steps:
- Count the total number of valence electrons: Carbon (C) has 4 valence electrons, and Nitrogen (N) has 5 valence electrons. Since CN is an ion with a negative charge, add one extra electron. Total valence electrons = 4 (C) + 5 (N) + 1 (charge) = 10 electrons.
- Draw the skeletal structure: Place the Carbon atom next to the Nitrogen atom. Carbon is less electronegative than Nitrogen, so it will be the central atom.
- Distribute the electrons: Place a triple bond between Carbon and Nitrogen. This uses 6 electrons (3 pairs). The remaining 4 electrons will be placed as lone pairs on the Nitrogen atom.
- Check the octet rule: Carbon has 4 electrons (from the triple bond) and Nitrogen has 8 electrons (6 from the triple bond and 2 from the lone pair). Both atoms satisfy the octet rule.
To determine the formal charge of each atom:
- Formal Charge = (Valence Electrons) – (Non-bonding Electrons) – (Bonding Electrons / 2)
- For Carbon: Formal Charge = 4 (valence) – 0 (non-bonding) – 6/2 (bonding) = 4 – 0 – 3 = +1
- For Nitrogen: Formal Charge = 5 (valence) – 2 (non-bonding) – 6/2 (bonding) = 5 – 2 – 3 = 0
Thus, the formal charge on Carbon is +1, and the formal charge on Nitrogen is 0.