Question

How many lithium atoms are present in a unit cell with edge length 3.5 Å and density 0.53 g cm^-3 (At. mass of Li= 6.94).

Options

• one atom

• four atoms

• two atoms

• six atoms

Answer 1

two atoms

Explanation:

`rho = "nM"/("N"_"A" "a"^3)`

n = `(rho "N"_"A" "a"^3)/"M"`

`= (0.53 xx 6.022 xx 10^23 xx (3.5 xx 10^-8)^3)/6..94`

`= (25.81 xx 0.53)/6.94`

`= 13.68/6.94`

= 1.97

∴ n = 2

For BCC crystal structure,

r = `sqrt3/4  "a" = 1.732/4 xx 500` pm 

= 0.433 × 500 pm

= 216.5 pm