Question

Among the following, the correct statement is (A) Between \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}, \mathrm{NH}_{3}\) is a better electron donor because the lone pair of electrons occupies spherical 's' orbital and is less directional (B) Between \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}, \mathrm{PH}_{3}\) is a better electron donor because the lone pair of electrons occupies \(\mathrm{sp}^{3}\) orbital and is more directional (C) Between \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}, \mathrm{NH}_{3}\) is a better electron donor because the lone pair of electrons occupies \(\mathrm{sp}^{3}\) orbital and is more directional (D) Between \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}, \mathrm{PH}_{3}\) is a better electron donor because the lone pair of electrons occupies spherical 's' orbital and is less directional

Short answer

(D) Between NH3 and PH3, PH3 is a better electron donor because the lone pair of electrons in the less electronegative phosphorus is more loosely held and thus is more available for donation.

Step-by-step solution

Identify the Electron Configurations of Nitrogen and Phosphorus

Consider the positions of nitrogen and phosphorus in the periodic table to determine the orbitals in which the lone pairs reside. Nitrogen is in the second period and has its valence electrons in the second shell, implying an electron configuration that ends in 2p. Phosphorus is in the third period and has its valence electrons in the third shell, indicating an electron configuration that ends in 3p.

Determine the Hybridization of NH3 and PH3

Both ammonia (NH3) and phosphine (PH3) have a trigonal pyramidal geometry, which is due to the hybridization of orbitals. In NH3, the nitrogen atom is sp3 hybridized, meaning that it has one s orbital and three p orbitals that hybridize to form four sp3 orbitals. Similarly, in PH3, the phosphorus atom is also sp3 hybridized.

Evaluate Directionality of the Sp3 Orbital

The sp3 orbitals are directional because they have a specific geometric arrangement in space that maximizes the separation between electron pairs according to VSEPR theory. These orbitals extend outward from the central atom to form a tetrahedral shape.

Compare Electron Donation Ability

The electron pair in an sp3 hybrid orbital is held more tightly near the nitrogen in NH3 than near the phosphorus in PH3. This is because nitrogen is more electronegative than phosphorus and thus exerts a stronger pull on the electron pair, making it less available for donation compared to the electron pair in PH3.

Conclusion

Since NH3 has the lone pair in an sp3 hybrid orbital and is more electronegative, it would hold the lone pair more tightly, making it less available for donation. PH3, on the other hand, because of lower electronegativity, would be a better electron donor. The correct statement among the given options is (D).

Key concepts

Chemical Hybridization

Chemical hybridization is a fundamental concept that explains the molecular shape and reactivity of molecules such as NH3 (ammonia) and PH3 (phosphine).

In simple terms, hybridization is the process through which atomic orbitals mix to form new hybrid orbitals. This mixing is not a physical one but rather a conceptual tool to understand the observed shapes of molecules. For instance, both nitrogen in NH3 and phosphorus in PH3 undergo sp3 hybridization. This process involves the combination of the s orbital with the three p orbitals to form four equivalent sp3 hybrid orbitals.

The tetrahedral arrangement of these sp3 orbitals allows for a trigonal pyramidal molecular geometry, with the lone pair of electrons occupying one of the hybrid orbitals. The reason behind the hybridization is the atom's attempt to minimize electron pair repulsions to achieve a lower energy, more stable configuration, which is explained by VSEPR theory.

VSEPR Theory

VSEPR (Valence Shell Electron Pair Repulsion) theory is a model that predicts the geometry of molecules based on the repulsion between electron pairs in the valence shell of the central atom.

The fundamental premise of VSEPR theory is that electron pairs—both bonding and nonbonding (lone pairs)—will arrange themselves around the central atom in a way that minimizes repulsion, thus achieving a maximal separation from one another. For NH3 and PH3, the VSEPR theory would predict a trigonal pyramidal geometry because there are three bonded pairs and one lone pair of electrons on the central atom.

As the electron pairs repel each other, the geometry provides insight into the directionality of electron pairs, a concept crucial for predicting the behavior of molecules during chemical reactions, such as electron donation.

Electron Pair Directionality

The directionality of electron pairs is influenced by the molecular geometry which is, in turn, determined by hybridization and VSEPR theory.

In sp3 hybridized molecules like NH3 and PH3, the electron pairs are directed toward the vertices of a tetrahedron. This spatial arrangement allows for the clear directionality of electron pairs, crucial in understanding how molecules interact with each other. For electron donation, the lone pair must be accessible to potential acceptors; its directionality will influence how effectively it can overlap with the orbital of another atom to form a bond. The more directional the orbital, the more effectively it can overlap and donate an electron pair.

Electron Negativity and Electron Donation

Electronegativity is a measure of an atom's ability to attract and hold onto electron pairs. When comparing NH3 and PH3, we observe that nitrogen is more electronegative than phosphorus, hence it has a stronger attraction for the shared electrons.

The difference in electronegativity impacts electron donation. Since nitrogen has a tighter grip on its electrons, the lone pair in NH3 is less available for donation than the lone pair in PH3. Phosphorus, being less electronegative, doesn't hold its lone pair as tightly, making PH3 a better electron donor. The concept is critical in predicting how molecules will behave in reactions, such as acid-base reactions where electron pair donation is a key step.