Question

The density of chromium metal is 7.0 g cm  3 . If the unit cell is cubic with edge length of 289 pm . Calculate the number of atoms per unit cell.

Answer 1

Explanation:

The expression for density is as follows:

Density =d=

a

3

×N

A

n×M

GIven: d=7.2 g cm

−3

a=289 pm =289×10

−10

cm

Molecular weight of Cr =52 g mol

−1

n=

M

d×a

3

×N

A =

52

7.2×(289×10

−10

)

3

×6.023×10

23

n=

52

7.2×2.41×10

−23

×6.023×10

23

n=

52

7.2×2.41×6.023

=2

Since there are 2 Cr atoms in a unit cell, Cr crystallizes in bcc unit cell