What is the formal charge of C in CN-

What is the formal charge of C in CN- ?

The Correct Answer and Explanation is:

The formal charge on carbon in the cyanide ion (CN-) is -1.

To understand how this is determined, we need to follow the steps for calculating formal charges:

1. Determine the number of valence electrons for each atom:
   • Carbon (C) is in group 14 of the periodic table, so it has 4 valence electrons.
   • Nitrogen (N) is in group 15, so it has 5 valence electrons.
   • The cyanide ion (CN-) carries an additional electron, giving it a total of 10 electrons (4 from C + 5 from N + 1 extra electron).
2. Determine how many electrons are assigned to each atom in the molecule:
   In the cyanide ion, there is a triple bond between carbon and nitrogen. This means there are 6 shared electrons between the two atoms. Carbon typically forms four bonds, and here it is bonded to nitrogen with a triple bond, which means it is sharing three pairs of electrons. The remaining 2 electrons belong to nitrogen, as the lone pair.
3. Calculate the formal charge:
   The formula for calculating formal charge is: Formal Charge=(Valence Electrons)−(Non-Bonding Electrons)−Bonding Electrons2\text{Formal Charge} = (\text{Valence Electrons}) – (\text{Non-Bonding Electrons}) – \frac{\text{Bonding Electrons}}{2}Formal Charge=(Valence Electrons)−(Non-Bonding Electrons)−2Bonding Electrons​ For carbon:
   • Valence electrons = 4
   • Non-bonding electrons = 0 (carbon has no lone pairs)
   • Bonding electrons = 6 (from the triple bond)
     Using the formula:
   Formal Charge of C=4−0−62=4−3=+1\text{Formal Charge of C} = 4 – 0 – \frac{6}{2} = 4 – 3 = +1Formal Charge of C=4−0−26​=4−3=+1 However, we must subtract the extra electron from the cyanide ion (since it carries a negative charge), so the formal charge on carbon becomes -1.

Thus, the formal charge on carbon in the cyanide ion (CN-) is -1.