Anonymous
Anonymous asked in Science & MathematicsChemistry · 10 months ago

# What is the formal charge of nitogen in the thiocyanate ion NCS-?

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• Formal charge on nitrogen ....

Formal charge is not the same as either ionic charge or oxidation state.  Formal charge is the hypothetical charge if all the bonds were 100% covalent.

Calculate formal charge:

FC = VE - NBE - 1/2 BE ..... VE = valence electrons, NBE = nonbonding electrons, BE = bonding electrons

Start with the "best" Lewis diagram.  Formal charge is useful primarily in determining the best arrangement of atoms in a compound, and secondarily for find the the best arrangement of electrons. (*)  Ideally, the formal charges should be close to zero.

[N=C=S]^- .... N and S both have two lone pairs.

FC on N = 5 - 4 - 1/2(4) = -1

FC on C = 4 - 0 - 1/2(8) = 0

FC on S = 6 - 4 - 1/2(4) = 0

The sum of the formal charges must equal the charge on the ion.  Ideally, any negative formal charge should reside on the most electronegative element (which is N).

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Other arrangements of electrons

[N−C≡S]- .... N has three lone pairs... FC = 5 - 6 - 1 = -2

[N≡C−S]- .... N has one lone pair .... FC = 5 - 2 - 6 = -3

These are not the "best" arrangement because the formal charge on N is not as close to zero as in [N=C=S]-

• Formal charge of an atom = V - N - (B/2)

where:

V = the number of valence electrons of the neutral atom in isolation (in its ground state)

N = the number of non-bonding valence electrons on this atom in the molecule

B = the total number of electrons shared in bonds with other atoms in the molecule

The three resonance structures (I, II and III) of NCS⁻ ion are shown below.

For resonance structure I, formal chage of nitrogen = 5 - 2 - (6/2) = 0

For resonance structure II, formal charge of nitrogen = 5 - 4 - (4/2) = -1

For resonance structure III, formal charge of nitrogen = 5 - 6 - (2/2) = -2 