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Anonymous asked in Science & MathematicsChemistry · 3 months ago

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

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• 3 months ago

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]-

• 3 months ago

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