Question

\(\mathrm{PCH}_{5}\) exists but \(\mathrm{NCl}_{5}\) does not exist because (1) Nitrogen has no vacant \(2 \mathrm{~d}\) -orbitals. (2) NCls is unstable. (3) Nitrogen atom is much smaller than p. (4) Nitrogen is highly inert.

Short answer

Nitrogen has no vacant 2d orbitals.

Step-by-step solution

- Understanding the question

Determine why \(\text{PCl}_{5}\) exists while \(\text{NCl}_{5}\) does not.

- Analyze the electronic configuration

Phosphorus (P) has the electronic configuration \[ [Ne] 3s^2 3p^3 3d^0 \], which includes a vacant 3d orbital. Nitrogen (N), however, has the electronic configuration \[ 1s^2 2s^2 2p^3 \], which lacks any vacant d orbitals.

- Utilize available orbitals

Phosphorus can use its vacant 3d orbitals to form \(\text{PCl}_{5}\) with sp^3d hybridization. Nitrogen cannot achieve a similar expansion in its valence shell because it lacks these available d orbitals in the second period.

- Compare sizes and stability

While the other reasons, such as the smaller atomic radius of nitrogen, could contribute to instability, the primary reason is the lack of vacant 2d orbitals in nitrogen.

- Conclusion

From the options provided, the first one is the most accurate: \(\text{Nitrogen has no vacant } 2d \text{ orbitals}\).

Key concepts

electronic configuration

The electronic configuration provides a roadmap of where electrons are located in an atom.
Think of it as the address of electrons in various energy levels and sublevels. For example, Phosphorus (P) has the electronic configuration \[ [Ne] 3s^2 3p^3 3d^0 \], which means:

• '[Ne]' indicates that phosphorus has the same electron arrangement as neon in its core.
• '3s^2 3p^3' tells us there are 2 electrons in the 3s sublevel and 3 electrons in the 3p sublevel.
• The '3d^0' denotes the availability of empty 3d orbitals.

On the other hand, Nitrogen (N) has the configuration \[ 1s^2 2s^2 2p^3 \], where

• '1s^2' indicates 2 electrons in the 1s sublevel.
• '2s^2' shows 2 electrons in the 2s sublevel.
• '2p^3' tells us there are 3 electrons in the 2p sublevel.

Notice, nitrogen lacks any d orbitals because it's in the 2nd period of the periodic table.

vacant d orbitals

Vacant d orbitals play a crucial role in the ability of an atom to expand its valence shell.
In the case of phosphorus, the electronic configuration \[ 3s^2 3p^3 3d^0 \] shows it has d orbitals in the third energy level.

• These 3d orbitals are empty and can be used to form additional bonds.
• This ability allows phosphorus to exhibit sp^3d hybridization, forming compounds like \( \text{PCl}_5 \).

Nitrogen, with the configuration \[ 1s^2 2s^2 2p^3 \], does not have d orbitals in its second energy level.

• Without these vacant d orbitals, nitrogen cannot expand its valence shell in the same way, which is why \( \text{NCl}_5 \) does not exist.

sp3d hybridization

Hybridization is a process where atomic orbitals mix to form new hybrid orbitals suitable for the pairing of electrons.
In \( \text{PCl}_5 \), phosphorus undergoes sp^3d hybridization:

• Phosphorus uses one s orbital, three p orbitals, and one d orbital.
• This creates five equivalent sp^3d hybrid orbitals.
• These orbitals form a trigonal bipyramidal shape, accommodating five chlorine atoms.

The lack of d orbitals in nitrogen means it cannot undergo sp^3d hybridization.

• Therefore, nitrogen is limited to forming only up to four bonds (sp^3 hybridization).

This geometric limitation is a key reason why \( \text{NCl}_5 \) is non-existent.

valence shell expansion

Valence shell expansion refers to an atom's ability to have more than the usual eight electrons in its valence shell.
Atoms like phosphorus can expand their valence shell due to the availability of vacant d orbitals.

• With the configuration \[ 3s^2 3p^3 3d^0 \], phosphorus can accommodate additional electrons by using 3d orbitals.
• This allows phosphorus to form a pentavalent compound like \( \text{PCl}_5 \).

In contrast, nitrogen cannot expand its valence shell:

• It's limited to the 2nd energy level, which only has s and p orbitals.
• This constraint prohibits nitrogen from accommodating more than eight electrons, making \( \text{NCl}_5 \) structurally unfeasible.

Hence, phosphorus and nitrogen exhibit different chemical behaviors due to their ability (or inability) for valence shell expansion.