NH2− Lewis Structure
Amide ion (azanide) · Ammonia minus a proton. An extremely strong base, used in organic synthesis.
The NH2- Lewis structure has 8 valence electrons - the 1− charge adds one, drawn as two bonds to hydrogen, with two lone pairs in total, all inside square brackets. The central N is sp3 hybridized, which makes it a bent ion with a bond angle of 104.5 degrees. The N carries a 1− formal charge, and that is the ion's 1− charge. The charge belongs to the ion as a whole, which is what the brackets around the drawing mean.
- 8 valence e−
- bent
- ≈104.5° bond angle
- sp3
- 1− charge
NH2− molecular geometry
Bent, from 4 electron domains on the central N: 2 bonded atoms and 2 lone pairs (AX2E2). The electron geometry is tetrahedral; the shape is what is left when the lone pairs are not counted as atoms. Wedges point toward you, hashed wedges away. Every shape in the VSEPR chart →
Calculated properties
| Total valence electrons | 8 |
|---|---|
| Bonding electrons | 4 (2 shared pairs) |
| Nonbonding electrons | 4 (2 lone pairs) |
| Lone pairs on N | 2 |
| Electron domains (steric number) | 4 |
| VSEPR notation | AX2E2 |
| Electron geometry | tetrahedral |
| Molecular geometry | bent |
| Bond angle | ≈104.5° |
| Hybridization | sp3 |
| Charge | 1− |
| Charge distribution | localised |
| Formal charges | N: -1 |
| Resonance structures | none |
| Molar mass | 16.023 g/mol |
N Nitrogen 5 valence e− H Hydrogen 2 atoms, 1 valence e− each
How to draw the NH2− Lewis structure
Step through it. The bar shows how much of the 8-electron budget is spent at each stage - it never grows, which is the whole constraint.
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Skeleton. Join every connected pair with one bond. 2 bonds spends 4 electrons.
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Centre. Everything still unplaced goes on N, which now has 8 electrons around it.
The reasoning at each step
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Count the valence electrons
Add up the outer-shell electrons every atom brings, then account for the charge. A 1- ion has gained one electron, so it is added to the budget. Skip this and every count that follows will be wrong.
5 (N) + 2 x 1 (H) + 1 (1- charge) = 8 valence electrons -
Choose the central atom
N is the least electronegative atom here, so it takes the centre. Hydrogen is never central: it can only form one bond.
-
Join everything with single bonds first
Every connection starts as one shared pair. Draw them all before worrying about double bonds - the arithmetic in the next step is what tells you where the double bonds have to go.
2 bonds x 2 = 4 electrons used, 4 left -
Work out how far short you are
Add up what every atom still needs to fill its shell - eight electrons for most atoms, two for hydrogen - minus what its single bonds already give it. Compare that with the electrons you have left. The difference decides everything that follows.
needs 4, has 4, exactly enough -
Fill in the lone pairs
Every electron not in a bond sits as a lone pair on an atom. Each atom takes exactly what it needs to finish its shell - there is no choice left at this point.
N: 2 (4 electrons) -
Check the central atom
N ends up with a full octet. Every electron in the budget is now placed.
N: 4 bonding + 4 nonbonding = 8 -
Check the formal charges
Formal charge is valence electrons, minus lone-pair electrons, minus the number of bonds. They are not real charges, and they are not the ion's charge either - but they must add up to it. Adding these up gives 1-, which is the check that the structure is complete.
N: 5 - 4 - 2 = -1 -
Work out the shape
Count the electron domains on N: 2 bonded groups plus 2 lone pairs. A double or triple bond still counts as one domain, because it points in one direction. That gives tetrahedral electron geometry; ignore the lone pairs and the atoms themselves sit in a bent arrangement. Two lone pairs repel more strongly than bonding pairs, closing the angle well below 109.5 degrees.
steric number 4 -> sp3 -> bent, bond angle 104.5 -
Decide whether it is polar
The N-H bonds are polar, and the lone pairs on the central atom bend the molecule out of shape, so the dipoles do not cancel.
dipoles do not cancel -> polar
Where the 1− charge sits
The N carries a 1− formal charge, and that is the ion's 1− charge. The charge belongs to the ion as a whole, which is what the brackets around the drawing mean.
People also ask whether NH2− is polar. Strictly it is neither: polar and nonpolar sort neutral molecules by whether their bond dipoles cancel. A 1− ion is pulled towards any nearby charge whatever its shape, which is why its salts dissolve in water regardless of geometry. The table below is still worth reading - it shows how unevenly each bond holds its electrons, which is what decides whether the charge ends up spread or concentrated.
| Bond | Electronegativity difference | Character |
|---|---|---|
| N–H | 0.84 | polar covalent |
Common questions
How many valence electrons does NH2- have?
NH2- has 8 valence electrons. The atoms bring 7 between them, and the 1− charge adds one. 4 of them are in bonds and 4 sit in lone pairs.
Why does NH2- have a 1− charge?
The N carries a 1− formal charge, and that is the ion's 1− charge. The charge belongs to the ion as a whole, which is what the brackets around the drawing mean. The brackets around the drawing are part of the answer: they say the charge belongs to the whole ion rather than to whichever atom happens to be nearest.
What is the molecular geometry of NH2-?
NH2- is a bent ion. The central N has 2 bonded groups and two lone pairs, a steric number of 4, which gives tetrahedral electron geometry and a bent ion.
What is the bond angle in NH2-?
The bond angle in NH2- is 104.5 degrees. Two lone pairs repel more strongly than bonding pairs, closing the angle well below 109.5 degrees.
What is the hybridization of NH2-?
The central N in NH2- is sp3 hybridized. Steric number 4 means 4 orbitals have to be mixed, which is exactly what sp3 gives you.
Is NH2- polar or nonpolar?
Neither, strictly. Polar and nonpolar describe neutral molecules, by asking whether their bond dipoles cancel out. NH2- carries a net 1− charge, so it is pulled towards any nearby charge whatever its shape - which is why ionic compounds dissolve in water regardless of geometry. The question the shape does answer is whether the charge is spread evenly: it is not, because the N-H bonds are polar, and the lone pairs on the central atom bend the molecule out of shape, so the dipoles do not cancel.