In the crystal lattice of sphalerite, zinc (Zn) forms four bonds with sulfur (S) atoms, resulting in a coordination number of four for zinc. This means that each zinc atom is surrounded by four sulfur atoms in the lattice.
While zinc typically has two valence electrons in its neutral state, in the context of bonding with sulfur in sphalerite, the zinc atoms undergo oxidation to form Zn2+ ions. When zinc loses two electrons, it achieves a stable electron configuration similar to that of a noble gas (argon).
In the crystal lattice of sphalerite, each Zn2+ ion is bonded to four S2- ions. The sulfur atoms in the lattice contribute two electrons each to form a bond with the zinc atom. This results in a total of eight electrons from the sulfur atoms (4 sulfur atoms × 2 electrons each), which combine with the two valence electrons originally from zinc to form a shared pair of electrons in each bond.
So, in the case of sphalerite's crystal lattice, the two additional electrons required to form a common pair with sulfur come from the sulfur atoms themselves.