The crystal AB (rock salt structure) has molecular weight 6.023 y u where, y is an arbitrary number and u is the unified atomic mass unit. If the minimum distance between cation and anion is `y^(1//3)` nm and the observed density is 20 `kg//m^(3)` then find the
(i) density in `kg//m^(3)` and
(ii) type of defect.

Explanation

In rock salt like crystal AB, there are four AB units per unit cell. Therefore, density (d) is
`d=(4xx6.023y)/(6.023xx10^(23)xx8yxx10^(-27))`
`[ :. a=2y^(1//3) nm =2y^(1//3) xx 10^(-9)m]`
`=5xx10^(3)g//m^(3)=5kg//m^(3)`
Since, observed density is greater than expected, theoretical density, so there must be some excess metal occupying interstitial spaces. This type of defect is known as metal excess defect.