In: Chemistry
draw the unit cell of a lead sulfide , in which there are sulfide anions in a face centered cubic lattice , and lead cations are in alternate tetrahedral holes. Derive the stoichiometry of the mineral and the charge of the lead cation.
In FCC, there is 1 aton at each of the 8 corners of the unit cell shared by 8 surrounding unit cells and 1 atom at each of the 6 faces shared by 2 surrounding unit cells.
Hence number of sulfide (S2-) anions per unit cell = 8 x (1/8) + 6 x (1/2) = 4 S2- anions.
In FCC there are a total of 8 tetrahedral holes per unit cell.
Since lead cations occupy alternate tetrahedral holes, number of lead cations per unit cell = 8/2 = 4 Pb cation.
Since there are equal number of cations and anions, the charge on both the cations and anions are same.
Hence charge of lead cation = +2 (answer)
Stoichiometry of the mineral = Pb4S4 or PbS (answer)
For structure take a 3d figure for the structure of PbS from internet.