NH2- Lewis Structure, Molecular Geometry, Polarity & Hybridization

The chemical name of NH2- or H2N- is Azanide. It is also known as amide ion or ammonia ion or monoamide or amide. NH2- is the conjugate base of ammonia and it is not stable so that it is generally found in the form of Hydrazine (NH2-NH2).

Moreover, it mostly exists with organic compounds with structures like RNH- and NR2 where nitrogen is bonded with corresponding carbon atoms.

In this article, we will discuss NH2- molecular geometry and Lewis structure of NH2- along with its shape, bond angle, polarity, hybridization, and other chemical and molecular properties. So, if you want to clear each and every doubt with a simple explanation, be continue...

Contents

• NH2- Lewis Structure
• NH2- Molecular Geometry & Shape
• NH2- Bond Angle
• NH2- Hybridization
• NH2- Acid or Base
• NH2- Polarity (Polar or Nonpolar)

NH2- Lewis Structure

NH2- has a total of 8 valence electrons which are surrounded on the H-N-H structure. Both hydrogen atoms need only two electrons while nitrogen needs 8 electrons to complete duplet and octet respectively.Here both hydrogen atoms share a one-one electron with the nitrogen atom to form two single bonds (H-N) which can also represent by simply placing two dots for a single bond as shown in the above figure. The whole structure is surrounded by a negative sign because NH2- is an ion with negative charge.

Steps to be followed for drawing NH2- Lewis structure

1. Find out the total number of valence electrons

Here the amide ion is made up of two different atoms: Nitrogen (N) and Hydrogen (H) so first, we have to figure out the valence electrons of these two atoms separately.

Valence electrons of hydrogen atom = 1

Valence electron of nitrogen atom = 5

Here in this molecule, we have one nitrogen atom and two hydrogen atom and one negative charge ion. Now,

Total number of valence electrons: 5 + 1*2 + 1 = 8.

(we have to add +1 electrons in total valence electrons because NH2- has one negative sign on it.)

2. Determine the total number of valence electrons pairs

As we know the total number of valence electrons are equal to the addition of sigma bonds, pi bonds, and lone pair present at the valence shells. But it can be simply calculated by just dividing the total number of valence electrons by two.

For NH2-, total valence electrons are 8 (as calculated in step 1), so total electrons pairs are 8/2= 4.

3. Find out the central atom

Finding the central atom while drawing a Lewis structure is the most tricky part but as described in how to draw a Lewis structure guide, there is a simple trick for selecting the central atom which is obviously save of extra time and energy.

The nitrogen atom is in the least number so simply it will be the central atom surrounded by two hydrogen atoms from either side.

4. Drawing a simple skeleton

As we have already known the central atom so we can easily draw a simple structure placing a nitrogen atom in the center surrounded by two hydrogens from either side. The skeleton looks like this,

5. Put the lone pairs of electrons on atoms

It is time to put lone pairs of electrons on atoms. For this, you have to make sure every atom except the central atom must have 8 electrons to follow the octet rule (hydrogen is an exception because it follows duplet rules).

Here we have to place two lone pairs of electrons on nitrogen atom so that they have 8 electrons. No need to placed a lone pair of electrons on H atoms as all the hydrogen atoms have two electrons and hydrogen is happy with this.

We used all 8 valence electrons and all atoms are stable as nitrogen has 8 electrons and each hydrogen atoms have two electrons for achieving octet and duplet respectively.

6. Identifying formal charge on the atom

The formal charge on each atom can be calculated as,

Formal charge (F.C) = Valence electrons (V) – Lone pair of electrons (L) – Bond pair of electrons (B)/2.

From this, we get one negative charge on the ions. Finally, this is our NH2- Lewis structure diagram.

NH2- Molecular Geometry & Shape

NH2- has two pairs of bonding and two pairs of non-bonding electrons participated in the formation of a molecule. The central nitrogen atom has two pairs of non-bonding electrons cause repulsion on both bonding pairs which pushes the bonds closer to each other. So, NH2- has a bent (angular) molecular geometry.

I) Electron Domain (ED) Geometry

From the above Lewis dot structure, NH2- has four regions of electron density around the central nitrogen atom i.e. 2 bond pairs and 2 lone pairs.

These electrons are arranged in a tetrahedral shape with a structure like H-N-H. As we already familiar with electron-electron repulsion, both lone pairs and bond pairs of electrons repel each other.

But the repulsive force of lone pair of electrons is higher than bond pairs which cause it bends like V shape as the repulsive force of lone pairs of electrons overtake the repulsive force of bond pairs.

II) VSEPR Shape

According to VSEPR theory, there is a total of 8 valence electrons in which N contributes 5 electrons, Two H contribute 2 electrons, and one negative charge contributes 1 electron.

From the Lewis structure of Nitrogen, we have two-two electrons bonding regions which show it has 2 lone pairs of electrons.

Also, two pairs of electrons participate in the two H-N formations that show there are a total of 4 pairs of electrons pairs present.

Hence the NH2- ion has a bent V shape, based on the arrangement of the tetrahedral geometry, where the bond angle lesser than ideal 109.5°.

NH2- Bond Angle

As the arrangement of NH2- is more likely to be H-N-H but due to the lone pairs and bond pairs repulsion, it acquires bent V-shape geometry. The repulsion of lone pairs of electrons is more than bond pairs.

And there are two lone pairs of electrons present on the N atom which strongly repel the bond pairs. Thus, both N-H bond pairs come closer to each other and occupy less space than two non-bonding lone pairs of electrons. That’s why NH2- has a bond angle of 104.5°, not 107° or 109.5°.

Ammonia (NH3) which has only 1 pair of non-bonding lone pairs electrons which have comparatively lower repulsive force and bond angle is around 107°.

But in the case of NH2-, there are two pairs of non-bonding electrons presence on the nitrogen atom which exerted higher repulsion, as a result, NH2- has a bond angle 104.5° same as water (H2O) bond angle.

NH2- Hybridization

NH2- has an sp3 hybridization type. The central Nitrogen atom has four regions which are responsible for identifying hybridization. Here N has two unbonded electrons pairs and two sigma bonds. And due to these four regions around the central nitrogen atom, NH2- has sp3 hybridization.

In NH2-, there are three 2p orbitals and one 2s orbital. These orbitals collectively combined to form four different sp3 hybrid orbitals. In which two of the four sp3 hybrid orbitals are used to form bonds hydrogen and the remaining two orbitals are used to hold two lone pairs of electrons.

We can also find out hybridization with the help of a simple formula. The calculation is showing below,

Hybridization = ½ [V.E + M - C + A]

Where,

V.E = Total no. of valence electrons presence on the molecule

M = Total number of monoatomic atoms bonded to the central atom

C = Cation charge (positive charge)

A= Anion charge (negative charge)

Now, on the above hybridization formula, we have to put corresponding values to achieve NH2- hybridization.

Hybridization of NH2- = ½ [ 5+2+1] = 4

= sp3

NH2- Acid or Base

NH2- is a strong base because it is unstable with its negative charge in a solution so that it wants to take the edge off with a negative charge by accepting a proton and acting as a base. According to the Bronsted-Lowry acid-base definition, molecules that accept protons are bases and those which are donated protons are acids. So, it is considered as a Bronsted base.

NH2- is a conjugate base of ammonia. It is a very strong base as NH3 itself also acts as a very weak base and we know that the conjugated bases of weak bases are incredibly strong and vice-versa.

Here are some of the example reactions which show NH2- is a base.

a) Double Displacement (Metathesis): When ammonia reacts with water it gives NH2- ion as base and H3O (hydronium ion) as acid as shown below,

NH3 + H2O → NH2- (conjugated base) + H3O+

b) Dissociation: When ammonia undergoes dissociation, it forms NH2- as the conjugated base and H+ as conjugated acid.

NH3 (ammonia) → NH2- (conjugated base) + H+ (conjugated acid)

NH2- Polarity (Polar or Nonpolar)

Now it’s time to know polarity, it means to figure out whether NH2- is a polar or nonpolar molecule. As we know the polarity is all about charges on the entire molecule which is induced due to differences in electronegativity values of atoms of molecules.

NH2- is a polar molecule because of the electronegativity difference between nitrogen (3.04) and hydrogen (2.2). Due to this difference in electronegativity, there is formation of a partial positive charge on the hydrogen atom and negative charge on the nitrogen atom.

The molecular geometry of NH2- is also not symmetrical due to lone pairs of electrons as mentioned earlier. And if the geometry of a molecule is not symmetrical, its resultant of pole charges is also do not cancel by each other.

As we know the net dipole moment is simply a product of induced charge and distance between the bonded atoms. And NH2- has induced charge because geometry is not symmetrical which indicates it has a net dipole moment, as a result, NH2- is considered as a polar molecule.

Its polarity can also be figured out by the Pauli scale which says if the E.N difference between two atoms is between 0.4 to 2.0, the formed bond will be polar. The E.N difference of N-H is 0.84 which clearly within the range of Pauli polar molecules range.

Summary (Conclusion)

NH2- is an incredibly strong conjugate base of NH3. It has a total of 8 valence electrons which are participated in the formation of the Lewis dot structure whereas there are 2 bonding pairs and 2 lone pairs of electrons within the molecule. Due to the presence of two lone pairs of electrons that repel bond pairs N-H, it acquires a bent V-shape molecular shape with a bond angle of 104.5°.

NH2- is a polar molecule due to the higher electronegativity difference between nitrogen and hydrogen atoms. As there are a total of four pairs of regions (two bond pairs and two lone pairs) for the electrons are attached with central nitrogen atom which makes it sp3 hybridization.

References

[1] https://en.wikipedia.org/wiki/Azanide[2] https://www.quora.com/Is-NH2-an-acid-or-base