Thallium chloride TlCl crystallises in either a simple cubic lattice or fcc lattice of `Cl^-`ions with `Tl^+` ions in voids. If the density of solid is `7 "g/cm"^3` and edge length of unit cell is `3.85xx10^(-8)` cm. What is the geometry of unit cell ? (At. number of Tl=204.37 & Cl=35.5)

Answer

Z = 1. Simple cubic

Explanation

Formula unit mass of `TlCl = 208.37 + 35.5 = 243.87`
`Z = ? `
Density = `(Z xx M)/(a^3 xx N_A)`
`7.00 = (Z xx 243.87)/((3.85 xx 10^(-8))^(3) xx(6.022 xx 10^(23)))`
or `Z = (7.00 xx (3.85 xx 10^(-8))^(3) xx (6.022 xx 10^(23)))/(243.87)`
`= 0.99 or 1`.
`:. ` A unit cell of TlCl contains one formula unit of TlCl.
`:.` It is a simple cubic arrangement.