Which element has a hydrogen like spectrum whose lines have wavelength one fourth of atomic hydrogen ?

Options

A. `He^(+)`
B. `Li^(2 +)`
C. `Be^(3 +)`
D. `B^(4 +)`

Answer

A

Explanation

(a) `(1)/(lamda_H) = R xx 1^2 ((1)/(n_1^2) -(1)/(n_2^2))` ....(i)
`(1)/(lamda_X) = RZ^2 ((1)/(n_1^2) -(1)/(n_2^2))` ....(ii)
Dividing equation (i) by equation (ii), we get
=`(lamda_x)/(lamda_H) = (1)/(Z^2)`
`(1)/(4) = (1)/(Z^2)`
`Z = 2 (He^+)`.