Rationalize why the lone pair hybrid orbitals for NH3 have less s-character than the lone pair in PH3, AsH3, and SbH3. You must use at least two of the following terms in your rationalization: principal quantum number, Zeff, radial maxima, primogenicity.
The Correct Answer and Explanation is:
Answer:
The lone pair on nitrogen in ammonia (NH₃) has less s-character than the lone pairs in phosphine (PH₃), arsine (AsH₃), and stibine (SbH₃) due to differences in effective nuclear charge (Z_eff) and principal quantum number (n). As we move down Group 15 from nitrogen to antimony, the principal quantum number increases (from n = 2 for N to n = 5 for Sb), which affects the shape, energy, and localization of the lone pair orbital. In NH₃, the lone pair resides in a hybrid orbital with more p-character, while in the heavier hydrides, the lone pair resides more in an s-type orbital. This trend is influenced by both the spatial extent of orbitals (via radial maxima) and the energetic cost of hybridization due to primogenicity.
Explanation:
In NH₃, nitrogen uses sp³ hybrid orbitals for bonding and accommodating its lone pair. The lone pair resides in one of these sp³ hybrids, and the molecule adopts a trigonal pyramidal shape. The nitrogen atom has a relatively small atomic radius, high Z_eff, and its valence electrons occupy the second shell (n = 2). The s and p orbitals in this shell are relatively close in energy, making hybridization energetically favorable. Consequently, nitrogen’s lone pair adopts an orbital with significant p-character, optimizing bonding and minimizing lone pair–bond pair repulsion.
However, in PH₃, AsH₃, and SbH₃, the central atoms are larger, with their valence electrons occupying higher principal quantum numbers (n = 3, 4, 5 respectively). These higher orbitals are more diffuse, with larger radial maxima, making orbital overlap less effective. Additionally, as primogenicity suggests, heavier elements tend to avoid hybridization because the energy cost of mixing distant s and p orbitals increases with atomic size. Therefore, the lone pairs in PH₃, AsH₃, and SbH₃ reside primarily in non-hybridized s orbitals with greater s-character.
Thus, nitrogen’s higher Z_eff and lower principal quantum number encourage hybridization and more p-character in the lone pair, while the heavier analogs favor more s-like lone pairs due to poor orbital overlap and energetic constraints.
