Hybridization of Ammonia

Elements that are unstable, usually combine with other elements either by losing, gaining, or sharing electrons. This is said to be chemical bonding. Usually, elements combine with others only with the help of valence electrons. Valence electrons are those that are present in the outermost shell of an atom. These are far from the nucleus and can easily bond with other elements.

Hybridization: 

Hybridization is the phenomenon of intermixing of atomic orbitals of the central atom that differ slightly in energy and their re-distribution to an equal number of hybrid orbitals of equal shape and energy. Hybridization occurs in the central atom only. In a covalent molecule, the element that has high valency is considered the central atom.

Purpose of hybridization:

Hybridization occurs in covalent molecules and between the atomic orbitals of the central atom. Hybridization does not occur in all covalent molecules but occurs in heteroatomic molecules like methane, Ammonia, etc. Hybridization occurs in the central atom, where the electrons in the valence shell exist in orbitals of different energy and shape.

Importance of Hybridization:

Hybridization occurs in the central atom to get orbitals having equal energy, and the overlapping of orbitals with equal energy is effective. Hybridization helps in knowing the types of bonds formed between the two elements. The types of bonds formed between the covalently bonded atoms can be of two types: sigma bond and pi bond.

Sigma bond: Sigma bond is a covalent bond formed by the sharing of electrons present in the orbitals. The type of overlapping observed between the orbitals is head-on overlapping or inter-axial overlapping. This type of overlapping leads to a strong overlapping of orbitals, and hence, a stronger bond which is a stable bond with minimum energy.

Pi bond: Pi bond is also a covalent bond formed by the sharing of electrons present in the orbitals. The type of overlapping observed between the orbitals is sideways overlapping or parallel overlapping. Sideways overlapping leads to a weaker bond with high energy and less stability.

Sigma bond is a stronger bond and can exist independently, but the pi bond is weaker and cannot exist independently.

Hybridization of Ammonia molecule:

Ammonia molecule has the molecular formula NH3. In ammonia molecules, the central atom is Nitrogen that belongs to the VA group or 15th group or pnicogens or the Nitrogen family. Ammonia is a slightly basic compound, and Nitrogen is a non-metal. And, down the group, basicity increases. As it belongs to the VA group, Nitrogen has 5 electrons in its valence shell and has valency equal to three; and in a few compounds, it may extend to four. Nitrogen uses only three electrons to form bonding, and this is called the combining capacity(valency). Nitrogen forms three bonds with other elements, commonly with Hydrogen, Oxygen, halogen, etc.

Nitrogen (N) – atomic number – 7

Its electronic configuration is 1s2 2s2 2p3 – ground state configuration

The second shell is the valence shell in Nitrogen, where electrons are present in 2s and 2p orbitals – 2s with 2 electrons and 2p with three electrons which are 2px1 2py1 2pz1. When Nitrogen has to form bonds, it uses all the four orbitals in the valence shell. But the orbitals have different shapes and energy, so it undergoes hybridization.

Ammonia molecule is obtained by the sp3 hybridization of N, where Nitrogen uses all the four orbitals (2s2 2px1 2py1 2pz1) in hybridization. All four atomic orbitals intermix and result in an equal number of hybrid orbitals. Those are sp3 hybrid orbitals, and Nitrogen is said to be undergoing sp3 hybridization.

In an ammonia molecule, out of the four sp3 hybrid orbitals formed, three hybrid orbitals are half-filled or with a single electron, and the fourth hybrid orbital has a pair of electrons. Three orbitals with a single electron align themselves in a head-on direction with the s orbitals of 3 hydrogen atoms to form sigma bonds. Three sigma bonds are observed between N and H atoms, represented as N-H bonds. Each line between N and H indicates the bonding and the number of electrons shared between them, which is two.

After the ammonia molecule is formed, we observe that there are three bond pairs around N, and on Nitrogen, there is one lone pair. The bond between N and H is the sigma bond formed by the overlapping of s orbitals of H with sp3 orbitals of Nitrogen.

In Ammonia, the sum of lone pair and bond pair is 1+3 = 4

If the number of hybridizations is four, then the hybridization of the central atom is sp3. The expected bond angle is 109.28, and the geometry is tetrahedral.

In contrast to this, Ammonia shows trigonal pyramidal geometry and <109 bond angle.

Reason:

In the case of ammonia molecules, around N, there are three bonds between N and H (N-H). N has four sp3 hybrid orbitals in which three are with unpaired electrons, and one orbital is with lone pair electrons. So, there are three bond pairs and one lone pair, which are placed close to one another. There exists bond pair-bond pair repulsions as well as lone pair-bond pair repulsions. Compared to bond pair-bond pair repulsions, lone pair-bond pairs are stronger and push the bond pairs far from the lone pair, which results in the decrease in bond angle to 107 and a deviation in the geometry to trigonal pyramidal shape.

Conclusion:

Hybridization leads to the intermixing of atomic orbitals of the central atom that differ slightly in energy to redistribute their energies to get an equal number of hybrid orbitals of equal shape and energy. In ammonia, the nitrogen atom, which is the central atom undergoes sp3 hybridization forming three sigma bonds with the three H atoms. Because of the presence of lone pair-bond pair repulsion, the ammonia molecule shows a deviation from the expected bond angle and geometry.

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