Question

formal charge at carbonium ion is
(1) =0
2=+1
3=-1
4=-2
​

Answer 1

Answer:

Need to figure out if an atom is negative, positive, or neutral? Here’s the formula for figuring out the “formal charge” of an atom:

Formal charge = [# of valence electrons] – [electrons in lone pairs + 1/2 the number of bonding electrons]

This formula explicitly spells out the relationship between the number of bonding electrons and their relationship to how many are formally “owned” by the atom.

For example, applying this to BH4 (top left corner in the image below) we get:

The number of valence electrons for boron is 3.

The number of non-bonded electrons is zero.

The total number of bonding electrons around boron is 8 (full octet). One half of this is 4.

So formal charge = 3 – (0 + 4) = 3 – 4 = –1

There is a slightly easier way to do this, however.

Since a chemical bond has two electrons, the “number of bonding electrons divided by 2” is by definition equal to the number of bonds surrounding the atom. So we can instead use this shortcut formula:

Formal Charge = [# of valence electrons on atom] – [non-bonded electrons + number of bonds].

Applying this again to BH4 (top left corner).

The number of valence electrons for boron is 3.

The number of non-bonded electrons is zero.

The number of bonds around boron is 4.

So formal charge = 3 – (0 + 4) = 3 – 4 = –1

The formal charge of B in BH4 is negative 1.

Let’s apply it to :CH3 (one to the right from BH4)

The number of valence electrons for carbon is 4

The number of non-bonded electrons is two (it has a lone pair)

The number of bonds around carbon is 3.

So formal charge = 4 – (2 +3) = 4 – 5 = –1

The formal charge of C in :CH3 is negative 1.

Same formal charge as BH4!

Let’s do one last example. Let’s do CH3+ (with no lone pairs on carbon). It’s the orange one on the bottom row.

The number of valence electrons for carbon is 4

The number of non-bonded electrons is zero

The number of bonds around carbon is 3.

So formal charge = 4 – (0 +3) = 4 – 3 = +1

You can apply this formula to any atom you care to name.

Here is a chart for some simple molecules along the series B C N O . I hope beryllium and fluorine aren’t too offended that I skipped them, but they’re really not that interesting for the purposes of this table.

Explanation:

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