Problem 69 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Give reason(s) why elemental nitrogen exists as a diatomic molecule whereas elemental phosphorus as a tetraatomic molecule.

Step-by-Step Solution

Answer

Nitrogen forms a strong diatomic N₂ due to triple bonds; phosphorus forms tetraatomic P₄ due to single bonds and vacant d-orbitals.

Step 1: Understanding Nitrogen's Structure

Nitrogen, which has the atomic number 7, has a valence shell electron configuration of 2s²2p³. Due to this configuration, nitrogen atoms tend to share three electrons with another nitrogen atom to achieve a stable octet. This results in a triple bond, denoted as N≡N, forming a diatomic N₂ molecule known for its strong bond and non-reactive nature.

Step 2: Exploring Phosphorus's Configuration

Phosphorus, having the atomic number 15, has a valence shell electron configuration of 3s²3p³. Unlike nitrogen, phosphorus atoms prefer forming single bonds rather than triple bonds due to available vacant d-orbitals, which are capable of accommodating more electrons. This electronic arrangement allows phosphorus to form a tetraatomic structure (P₄), where each phosphorus shares a single pair of electrons with three other phosphorus atoms, forming a stable tetrahedral shape.

Step 3: Comparing Molecular Structures

The key difference between nitrogen and phosphorus bonding lies in orbital hybridization and the strength of bonds. Nitrogen forms a strong triple bond that stabilizes the diatomic molecule, while phosphorus, with available d-orbitals, forms weaker single bonds that stabilize the tetrahedral P₄ molecule. The difference in bond strength and electron availability leads to their distinct molecular forms.

Key Concepts

Electronic Configuration

Electronic configuration is the arrangement of electrons in an atom's orbitals. For nitrogen, its electronic configuration is

• 1s² 2s² 2p³

This shows that nitrogen has five electrons in its outer shell. These five electrons urge the atom to complete an octet, often resulting in sharing three electrons per atom to form elemental nitrogen as a diatomic molecule. This sharing leads to the formation of a strong triple bond, symbolized as N≡N.

In contrast, phosphorus has the electronic configuration

• 1s² 2s² 2p⁶ 3s² 3p³

It has more electrons that can participate in bonding, including vacant d-orbitals in its third shell. These orbitals allow phosphorus atoms to expand their valency beyond an octet, leading to the formation of the tetraatomic P₄ molecule with weaker single bonds.

Molecular Structure

Molecular structure refers to the specific arrangement of atoms within a molecule. For nitrogen, the molecular structure is simplistic yet robust due to the strong triple bond. The shared electrons create a strong, linear diatomic N₂ molecule. This linear structure is very stable, rendering nitrogen less reactive.

On the other hand, phosphorus opts for a more complex structure. The P₄ molecule forms a tetrahedral shape. Each phosphorus atom shares its electrons with three other phosphorus atoms, resulting in a 3-dimensional arrangement. This structure is a result of the ability of phosphorus to use its d-orbitals, which nitrogen lacks, to form stable bonds. This tetrahedral formation allows phosphorus atoms to achieve stability in a different way than nitrogen.

Valency

Valency is the number of available bonds an atom can form. It influences how atoms interact and combine to form molecules. Nitrogen displays a valency of three. This means it tends to form three covalent bonds, achieving an octet by sharing electrons, as seen in the formation of N₂.

Phosphorus, however, can exhibit a valency greater than three due to its ability to make use of d-orbitals. This greater flexibility in bonding allows phosphorus atoms to utilize five electrons for bonding when forming compounds like phosphorus pentachloride (PCl₅). However, in its elemental state as P₄, it mainly shows a valency of three, aligning somewhat with sharing single pairs of electrons with neighboring phosphorus atoms in a tetrahedral geometry.