Azanide [NH2]- ion Lewis dot structure, molecular geometry or shape, electron geometry, bond angle, hybridization, formal charges, polar vs non-polar
NH2– represents the amide anion. The IUPAC name for this molecular ion is Azanide. You must be familiar with ammonia (NH3) which is a weak base. In this article, you will learn about NH2– a conjugate base of ammonia.
If you are wondering how can there be a conjugate base of a chemical substance which is a base itself, then we would like to inform you that it is quite possible. Self-ionization and/or deprotonation of NH3 under special pH conditions results in the formation of the Azanide [NH2]– ion.
We will discuss in the proceeding sections, everything you need to know about the Azanide (NH2–) ion including its Lewis structure, molecular geometry or shape, electron geometry, bond angle, hybridization, formal charges, polarity, etc.
The Lewis structure of the azanide [NH2]– ion consists of a nitrogen (N) atom at the center. It is bonded to two atoms of hydrogen (H) at the sides. There are a total of 4 electron density regions around the central N-atom in [NH2]– lewis structure. The 4 electron density regions comprise 2 bond pairs and 2 lone pairs present on the central N-atom.
Drawing the Lewis dot structure of azanide [NH2]– ion is a super easy task if you follow the simple guidelines given below.
Steps for drawing the Lewis dot structure of [NH2]–
1. Count the total valence electrons in [NH2]–
The very first step while drawing the Lewis structure of [NH2]– is to find the total valence electrons present in the concerned elemental atoms.
As NH2 – consists of atoms from two different elements i.e., nitrogen (N) and hydrogen (H) so you just need to look for these elements in the Periodic Table. Nitrogen (N) is present in Group V A of the Periodic Table so it has a total of 5 valence electrons. On the other hand, hydrogen (H) lies at the top of the Periodic Table containing a single valence electron only.
The [NH2]– ion consists of 1 N-atom, 2 H-atoms, and a negative (-1) charge which means 1 extra valence electron. Therefore, the valence electrons in the Lewis structure of [NH2]– = 1(5) + 2(1) + 1 = 8 valence electrons.
2. Choose the central atom
In the second step of drawing the Lewis structure of a molecule or a molecular ion, we need to place the least electronegative atom at the center.
As electronegativity refers to the ability of an elemental atom to attract a shared pair of electrons from a covalent chemical bond therefore the least electronegative atom is the one that is most likely to share its electrons with other atoms.
However, in the case of NH2 –, there are only two types of atoms involved. Out of the two elemental atoms, hydrogen is undoubtedly less electronegative than nitrogen but it cannot be chosen as the central atom. A hydrogen (H) atom can accommodate only 2 electrons so it can form a bond with a single adjacent atom only. This denotes that H is always placed as an outer atom in a Lewis structure.
Consequently, nitrogen (N) is chosen as the central atom in the Lewis structure of azanide [NH2]– while the 2 H-atoms are placed in its surroundings, as shown below.
3. Connect outer atoms with the central atom
Now we need to connect the outer atoms with the central atom of the Lewis structure using single straight lines. As hydrogen atoms are the outer atoms while the nitrogen atom is the central atom in NH2– Lewis structure so both the H-atoms are joined to the central N-atom via straight lines, as shown in the diagram below.
Each straight line represents a single covalent bond i.e., a bond pair containing 2 electrons. There are a total of 2 single bonds in the above diagram.
As 2(2) = 4, that means 4 valence electrons are already consumed out of the 8 initially available.
4. Complete the duplet of outer atoms
Each H-atom needs a total of 2 valence electrons in order to achieve a stable duplet electronic configuration.
The Lewis structure obtained till this step already shows 2 electrons around each H-atom. This means each H-atom already has a complete duplet and we do not need to make any changes regarding H-atoms in this structure.
5. Complete the octet of the central atom
Total valence electrons available – electrons used till step 4 = 8 – 4 = 4 valence electrons.
These 4 valence electrons are consequently placed as 2 lone pairs on the central N-atom in NH2– Lewis structure, as shown below.
The diagram above illustrates that in addition to a complete duplet of the outer H-atoms, the central N-atom also has a complete octet with 2 single bonds and 2 lone pairs around it.
The final step is to check the stability of the Lewis structure obtained in this step. Let us do that using the formal charge concept.
6. Check the stability of the NH2– Lewis structure using the formal charge concept
The less the formal charge on the atoms of a molecule, the better the stability of its Lewis structure.
The formal charge can be calculated using the formula given below.
Non-bonding electrons = no lone pairs = 0 electrons
Formal charge = 1-0-2/2 = 1-0-1 = 1-1 = 0
The above calculation shows that a zero formal charge is present on each H-atom in the NH2– Lewis structure. However, a -1 formal charge is present on the more electronegative N-atom which is equivalent to the charge present on the azanide ion overall.
Therefore, the above Lewis structure is enclosed in square brackets and a -1 charge is placed at the top right corner, as shown below. This final Lewis structure obtained is thus the correct and most stable Lewis representation of the azanide (NH2–) ion.
What are the electron and molecular geometry of NH2-?
The azanide [NH2]– ion has tetrahedral electron geometry. However, the molecular geometry or shape of the ion is bent or V-shaped. It is due to the 2 lone pairs of electrons on the central N-atom in NH2– that the molecular ion adopts a different shape from its ideal electronic geometry.
Molecular geometry of [NH2]–
The azanide [NH2]– molecular ion has a bent shape or molecular geometry. There are 2 lone pairs of electrons on the central N-atom in [NH2]–. These lone pairs lead to lone pair-lone pair and lone pair-bond pair electronic repulsions in addition to the N-H bond pair-bond pair repulsive effect.
The valence bond theory (VBT) states that lone pair-lone pair repulsions > lone pair-bond pair repulsions > bond pair-bond pair repulsions.
This strong repulsive effect distorts the shape and geometry of NH2–. The N-H bonds are pushed farthest away from the 2 lone pairs at the centre so the molecular ion adopts a bent, angular or V-shape.
An important point to remember is that the molecular geometry or shape of a molecule or molecular ion is strongly influenced by the different number of lone pairs and bond pairs around the central atom.
Contrarily, the ideal electronic geometry only depends on the total number of electron density regions around the central atom no matter whether it’s a bond pair or a lone pair.
Let’s see how this concept applies to the electron geometry of the azanide [NH2]– ion.
Electron geometry of [NH2]–
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, the ideal electron geometry of a molecule or a molecular ion containing a total of 4 electron density regions around the central atom is tetrahedral.
In NH2– ion, there are 2 single bonds and 2 lone pairs around the central nitrogen atom which makes a total of 2+2 = 4 electron density regions. Thus, its electron geometry is tetrahedral.
A shortcut to finding the electron and the molecular geometry of a molecule or a molecular ion is by using the AXN method.
AXN is a simple formula to represent the number of atoms bonded to the central atom in a molecule and the number of lone pairs present on it.
It is used to predict the geometry or shape of a molecule using the VSEPR concept.
AXN notation for [NH2] – molecular ion
A in the AXN formula represents the central atom. In the [NH2] – ion, nitrogen is present at the center so A=N.
X denotes the atoms bonded to the central atom. In [NH2] –, two hydrogens (H) atoms are bonded to the central N so X=2.
N stands for the lone pairs present on the central atom. As per the Lewis structure of [NH2] –, there are two lone pairs of electrons on the central nitrogen so N=2.
Hence, the AXN generic formula for the [NH2] – ion is AX2N2.
Now, you may have a look at the VSEPR chart below.
The VSEPR chart above shows that the ideal electron geometry of a molecule or molecular ion with AX2N2 generic formula is tetrahedral while its molecular geometry or shape is bent or V-shaped, as we already noted down for the azanide [NH2]– ion.
Hybridization of [NH2]–
The central nitrogen atom has sp3 hybridization in the [NH2]– ion.
The electronic configuration of a nitrogen (N) atom is 1s2 2s2 2p3.
During chemical bonding, the 2s atomic orbital of nitrogen hybridizes with three half-filled 2p orbitals to yield four sp3 hybrid orbitals. Each sp3 hybrid orbital has a 25 % s-character and a 75% p-character.
Two sp3 hybrid orbitals contain paired electrons. These paired electrons are situated as two lone pairs on the central N-atom in NH2–. The other two sp3 hybrid orbitals contain a single electron each.
These sp3 hybrid orbitals overlap with the s-orbital of hydrogen on each side of the azanide ion to form two N-H sigma (σ) bonds by sp3-s overlap.
You may also keep in mind that the sixth electron in the sp3 hybrid orbital of nitrogen is the extra electron gained while NH2– formation.
A shortcut to finding the hybridization present in a molecule or a molecular ion is by using its steric number against the table given below. The steric number of central N in [NH2]– is 4 so it has sp3 hybridization.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The [NH2]– bond angle
The ideal H-N-H bond angle in a tetrahedral molecule or molecular ion is 109.5°. However, the lone pairs of electrons present on the central N-atom lead to lone pair-lone pair and lone pair-bond pair repulsions. The N-H bonds are thus pushed away and the H-N-H bond angle decreases by approx. 5° i.e., 109.5° -5° = 104.5°.
A specific electronegativity difference of 0.84 units exists between the bonded nitrogen (E.N = 3.04) and hydrogen (E.N = 2.20) atoms in each N-H bond in [NH2]–. Nitrogen being more electronegative attracts the shared electron cloud from each N-H bond.
Consequently, the central N-atom gains a partial negative (δ–) charge while each outer H-atom attains a partial positive (δ+) charge. Each N-H bond thus possesses a particular dipole moment value (symbol µ) in NH2–.
The asymmetrical bent shape of the ion further endorses the polarity effect. The electron cloud stays non-uniformly distributed overall. Thus NH2– ion is polar in nature (net µ > 0).
The correct Lewis dot structure of NH2– is shown below.
It consists of a total of 8 valence electrons i.e., 8/2 = 4 electron pairs.
Out of the 4 electron pairs, there are 2 bond pairs and 2 lone pairs.
Both the lone pairs are present on the central N-atom.
The central N-atom also carries a -1 formal charge.
What is the geometry and shape of NH2– according to the VSEPR concept?
According to the VSEPR concept, the AXN generic formula for NH2– is AX2N2. 2 H-atoms are bonded to the central N-atom and there are 2 lone pairs of electrons on this central atom in NH2–.
According to this formula, the NH2– ion has a bent shape and molecular geometry as opposed to its ideal electron pair geometry i.e., tetrahedral.
How is the shape of NH2– different from NH4+?
NH2– represents an anion called the azanide or amide ion. It has a bent shape. There are 2 lone pairs of electrons situated on the central N-atom in NH2–.
The lone pair-lone pair and lone pair-bond pair repulsions make the anion adopt a different molecular geometry or shape from its ideal electron pair geometry i.e., tetrahedral.
In contrast to that, NH4+ is a cation known as the ammonium ion. There is no lone pair of electrons on the central N-atom in NH4+ therefore it maintains a shape identical to its ideal electronic geometry i.e., tetrahedral.
How is the shape of NH2– different from NH3?
Both NH2– and NH3 have an electronegative nitrogen atom at the center which is surrounded by less electronegative hydrogen atoms.
There are a total of 4 electron density regions around the central N-atom in both NH2– and NH3. Therefore, both have a tetrahedral electron geometry.
However, it is due to the difference in the number of lone pairs on the central N-atom that the shape of azanide, NH2– (a molecular ion) differs from that of ammonia (NH3) (a neutral molecule).
There are 2 lone pairs on the central N-atom in NH2– so it has a bent shape and molecular geometry. The H-N-H bond angle is 104.5°, reduced from the ideal bond angle of 109.5°.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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