= Solution
\b[<Gallium phosphide> forms a tetrahedral network of <polar covalent bonds>, with electron density displaced toward <phosphorus>.] The alternating diamond-related structure is the <zinc blende structure>: each Ga has four P neighbours and each P has four Ga neighbours. A local <orbital hybridization> picture combines outward-pointing s,p-derived orbitals into four approximately $sp^3$ bond directions; in the crystal these interactions form extended occupied bonding bands.
Since $\chi_{\rm Allen}(\mathrm P)-\chi_{\rm Allen}(\mathrm{Ga})=2.25-1.76=0.49>0$, P attracts shared <electron density> more strongly. Write $\mathrm{Ga}^{\delta+}-\mathrm P^{\delta-}$ to indicate an ionic contribution superposed on <covalent bonds>. Formal <oxidation states> Ga($+3$) and P($-3$) are electron-assignment labels, not proof of completely transferred charges or a purely ionic crystal.
\b[Neither atom requires $3d$ hybrids for these four bonds.] The Ga $3d^{10}$ shell is filled and relatively core-like; the relevant Ga valence orbitals are $4s,4p$. P uses principally $3s,3p$; its empty $3d$ orbitals are too high in energy to be a necessary bonding set. Detailed band calculations may include d-state mixing or core–valence effects, but that does not turn the tetrahedral bonding into an expanded-valence d-hybrid construction.
Back to article page
