The Wavelengths of `K_(alpha)` X - rays of two metals A and B are `4/(1875R) and 1/(675)R` respectively, where R is Rydberg constant . The number of elements lying between A and B according to their atomic numbers is

Options

A. 3
B. 6
C. 5
D. 4

Answer

D

Explanation

`1/lambda = R(Z-1)^(2)[(1)/(n_(1)^(2)) - (1)/(n_(2)^(2))]`
For wavelength of `K_(alpha), n_(1) = 1` to `n_(2) = 2`
`(1875R)/(4) = R(Z_(A) - 1)^(2) [1-1/4] = "…"(i)`
and `(675R)/(1) = R[Z_(B) - 1]^(2) [1-1/4]"……"(ii)`
By solving eq. (i) & (ii) we get
`Z_(A) = 26` and `Z_(B) = 31`
[Four elements lie between these two]