Question

\(\mathrm{N}\) forms \(\mathrm{NCl}_{3}\) whereas \(\mathrm{P}\) can form both \(\mathrm{PCl}_{3}\) and \(\mathrm{PCl}_{5}\). Why? (a) \(\mathrm{N}\) atoms are larger than \(\mathrm{P}\) atoms in size (b) P has low lying \(3 \mathrm{~d}\) orbitals, which can be used for bonding but \(\mathrm{N}\) does not have \(3 \mathrm{~d}\) orbitals in its valence shell (c) \(\mathrm{P}\) is more reactive towards \(\mathrm{Cl}\) than \(\mathrm{N}\) (d) none of these

Short answer

(b) P has low lying 3d orbitals, which can be used for bonding but N does not.

Step-by-step solution

Understanding Valence Orbitals

Both nitrogen (N) and phosphorus (P) are in the same group of the periodic table, meaning they have the same number of valence electrons (five). Nitrogen uses its 2p orbitals for bonding, while phosphorus can use its 3p, as well as low-lying 3d orbitals for bonding.

Examining Valence Shell Expansion

Nitrogen, being in the second period, does not have 3d orbitals available in its valence shell, restricting its bonding to a maximum of three covalent bonds as seen in NCl3. Phosphorus, however, is in the third period and possesses vacant 3d orbitals, allowing it to expand its octet to form five covalent bonds as in PCl5.

Analyzing the Given Options

(a) Nitrogen atoms are not larger than phosphorus atoms as it is higher in the periodic table, making this incorrect. (b) Phosphorus has low-lying 3d orbitals, unlike nitrogen, allowing it to form more than three covalent bonds, making this correct. (c) Reactivity with chlorine does not directly explain the ability to form PCl5, making this incorrect. (d) As option (b) provides a valid explanation, it is not 'none of these.'

Key concepts

Valence Electrons

Valence electrons are the electrons that reside in the outermost shell of an atom. These electrons play a crucial role in chemical bonding. For elements in the same group of the periodic table, like nitrogen (N) and phosphorus (P), the number of valence electrons is the same. Both N and P have five valence electrons, as they are part of Group 15. This group is often referred to as the "pnictogens.”
Understanding valence electrons is essential because they determine how an atom can bond with others. In chemical reactions, atoms aim to achieve a full outer shell, similar to the electron configuration of noble gases. This often means either gaining, losing, or sharing electrons through bonds.
For nitrogen, this typically results in the formation of three covalent bonds, such as in nitrogen trichloride ( Cl_{3} ). Meanwhile, phosphorus can form up to five bonds as seen in phosphorus pentachloride ( PCl_{5} ) which is closely tied to another concept known as orbital hybridization.

Orbital Hybridization

Orbital hybridization is a concept used to describe the mixing of atomic orbitals to form new hybrid orbitals. These hybrid orbitals can accommodate more bonds, enabling atoms to make use of both their energy and space efficiently.
In the case of phosphorus, it is in the third period of the periodic table, meaning it has access to additional d orbitals that can be utilized for bonding, unlike nitrogen, which is limited to its 2p orbitals. This is why phosphorus can expand its valence shell by using its 3d orbitals to form PCl_{5} with five covalent bonds.
So when phosphorus goes through orbital hybridization, it effectively combines its s and p orbitals, and potentially its d orbitals too, to create new bonding conditions that allow for more complex molecular structures. This explains why phosphorus can engage in more versatile chemical bonding than nitrogen.

Periodic Table Groups

The periodic table is organized into columns known as groups. These groups are collections of elements that share similar properties and the same number of valence electrons. The behavior of an element in chemical reactions can largely be anticipated by its group.
Both nitrogen and phosphorus belong to Group 15. Elements in this group are known for their ability to form trivalent or even pentavalent compounds, as seen in the case of phosphorus. The alignment in a group often leads to shared characteristics, such as similar bonding behaviors and electron configurations.
The position of an element in its period (row) informs us about the available orbitals for bonding. Group affiliation is crucial when predicting how an element, like phosphorus, can expand its octet unlike nitrogen. This difference reflects the unique ability of phosphorus within its group to utilize low-lying 3d orbitals for forming additional bonds, showcasing the richness of the periodic table's organization in predicting elemental behavior.