Q.

If λCu is the wavelength of Kα X-ray line of copper (atomic number 29) and λMo is the wavelength of the Kα X-ray line of molybdenum (atomic number 42), then the ratio λCuλMo is close to                                   [2014]

1 1.99  
2 2.14  
3 0.50  
4 0.48  

Ans.

(2)

∵  1λ=R(z-b2)(1nf2-1ni2)

For K-series, b=1 and for K∝ transition nf=2 to ni=1,

∴  1λ∝(z-1)2

∵  λCuλMo=(ZMo-1)2(ZCu-1)2=(42-129-1)2=(4128)2=2.14