The chemical name of NH2- or H2N- is Azanide. It is alsoknown 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 ofHydrazine (NH2-NH2).
Moreover, it mostly exists with organic compounds withstructures like RNH- and NR2 where nitrogen is bonded with corresponding carbonatoms.
In this article, we will discuss NH2- moleculargeometry and Lewis structure of NH2- along with its shape, bond angle, polarity,hybridization, and other chemical and molecular properties. So, if you want toclear each and every doubt with a simple explanation, be continue...
Contents
- NH2- Lewis Structure
- 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 simplyplacing two dots for a single bond as shown in the above figure. The wholestructure is surrounded by a negative sign because NH2- is an ion with negativecharge.
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 ofthese 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 twohydrogen atom and one negative charge ion. Now,
Total number ofvalence electrons: 5 + 1*2 + 1 = 8.
(we have to add +1 electrons in total valence electrons becauseNH2- 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 tothe addition of sigma bonds, pi bonds, and lone pair present at the valenceshells. But it can be simply calculated by just dividing the total number ofvalence electrons by two.
For NH2-, total valence electrons are 8 (as calculated instep 1), so total electrons pairs are 8/2= 4.
3. Find out the central atom
Finding the central atom while drawing a Lewis structure isthe 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 saveof extra time and energy.
The nitrogen atom is in the least number so simply it willbe 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 easilydraw a simple structure placing a nitrogen atom in the center surrounded by two hydrogensfrom 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 tofollow the octet rule (hydrogen is an exception because it follows duplet rules).
Here we have to place two lone pairs of electrons onnitrogen atom so that they have 8 electrons. No need to placed a lone pair ofelectrons on H atoms as all the hydrogen atoms have two electrons and hydrogenis 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 electronsfor 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 ofelectrons (L) – Bond pair of electrons (B)/2.
From this, we get one negative charge on the ions. Finally, thisis our NH2- Lewis structure diagram.
NH2- Molecular Geometry & Shape
NH2- has two pairs of bonding and two pairs of non-bondingelectrons participated in the formation of a molecule. The central nitrogen atom hastwo pairs of non-bonding electrons cause repulsion on both bonding pairs which pushesthe bonds closer to each other. So, NH2- has a bent (angular) moleculargeometry.
I) Electron Domain (ED) Geometry
From the above Lewis dot structure, NH2- has four regions ofelectron density around the central nitrogen atom i.e. 2 bond pairs and 2 lonepairs.
These electrons are arranged in a tetrahedral shape with a structure like H-N-H. As we already familiar with electron-electron repulsion, bothlone pairs and bond pairs of electrons repel each other.
But the repulsive force of lone pair of electrons is higherthan bond pairs which cause it bends like V shape as the repulsive force of lonepairs of electrons overtake the repulsive force of bond pairs.
II) VSEPR Shape
According to VSEPR theory, there is a total of 8 valenceelectrons in which N contributes 5 electrons, Two H contribute 2 electrons, andone negative charge contributes 1 electron.
From the Lewis structure of Nitrogen, we have two-two electronsbonding regions which show it has 2 lone pairs of electrons.
Also, two pairs of electrons participate in the two H-Nformations that showthere are a total of 4 pairs of electrons pairs present.
Hence the NH2- ion has a bent V shape, based on the arrangement ofthe tetrahedral geometry, where the bond angle lesser than ideal 109.5°.
NH2- Bond Angle
And there are two lone pairs of electrons present on the N atomwhich strongly repel the bond pairs. Thus, both N-H bond pairs come closer toeach other and occupy less space than two non-bonding lone pairs of electrons. That’swhy NH2- has a bond angle of 104.5°, not 107°or 109.5°.
Ammonia (NH3) which has only 1 pair of non-bonding lonepairs electrons which have comparatively lower repulsive force and bond angle isaround 107°.
But in the case of NH2-, there are two pairs of non-bondingelectrons presence on the nitrogen atom which exerted higher repulsion, as aresult, NH2- has a bond angle 104.5°same as water (H2O) bond angle.
NH2- Hybridization
NH2- has an sp3 hybridization type. The central Nitrogen atomhas four regions which are responsible for identifying hybridization. Here Nhas two unbonded electrons pairs and two sigma bonds. And due to these four regionsaround the central nitrogen atom, NH2- has sp3 hybridization.
In NH2-, there are three 2p orbitals and one 2s orbital. Theseorbitals collectively combined to form four different sp3 hybrid orbitals. In whichtwo 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 simpleformula. The calculation is showing below,
Hybridization = ½ [V.E + M - C + A]
Where,
V.E = Total no. of valence electrons presence on themolecule
M = Total number of monoatomic atoms bonded to the centralatom
C = Cation charge (positive charge)
A= Anion charge (negative charge)
Now, on the above hybridization formula, we have to putcorresponding 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 negativecharge in a solution so that it wants to take the edge off with a negative chargeby 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 baseas NH3 itself also acts as a very weak base and we know that the conjugated basesof weak bases are incredibly strong and vice-versa.
Here are some of the example reactions which show NH2- is abase.
a) Double Displacement (Metathesis): When ammonia reacts withwater 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, itforms 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 whetherNH2- is a polar or nonpolar molecule. As we know the polarity is all aboutcharges on the entire molecule which is induced due to differences inelectronegativity values of atoms of molecules.
NH2- is a polar molecule because of the electronegativitydifference between nitrogen (3.04) and hydrogen (2.2). Due to this differencein electronegativity, there is formation of a partial positive charge on the hydrogenatom and negative charge on the nitrogen atom.
The molecular geometry of NH2- is also not symmetrical dueto lone pairs of electrons as mentioned earlier. And if the geometry of a moleculeis not symmetrical, its resultant of pole charges is also do not cancel by eachother.
As we know the netdipole moment is simply a product of induced charge and distance between thebonded atoms. And NH2- has induced charge because geometry is not symmetricalwhich indicates it has a net dipole moment, as a result, NH2- is considered asa polar molecule.
Its polarity can also be figured out by the Pauli scale whichsays if the E.N difference between two atoms is between 0.4 to 2.0, the formed bondwill be polar. The E.N difference of N-H is 0.84 which clearly within the rangeof Pauli polar molecules range.
Summary (Conclusion)
NH2- is an incredibly strong conjugate base of NH3. It hasa total of 8 valence electrons which are participated in the formation of the Lewis dotstructure whereas there are 2 bonding pairs and 2 lone pairs of electrons withinthe molecule. Due to the presence of two lone pairs of electrons that repel bondpairs 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 electronegativitydifference between nitrogen and hydrogen atoms. As there are a total of fourpairs of regions (two bond pairs and two lone pairs) for the electrons are attachedwith central nitrogen atom which makes it sp3 hybridization.
