Question

The longest wavelength that a singly ionized Helium atom in its ground state will absorb is:

• A. $912\mathring{A}$
• B. $304\mathring{A}$
• C. $606\mathring{A}$
• D. $1216\mathring{A}$

Answer 1

The correct option is B $304\mathring{A}$

The longest wavelength, a singly ionized helium ion can absorb is for excitation into $n=2$.
Here, $Z=2,n_2=2,n_1=1$
Therefore, $\frac{1}{\lambda }=(1.097\times {10}^7)\left(4\right)(\frac{1}{1}-\frac{1}{4})=3.291\times {10}^7{m}^{-1}$
$\Rightarrow \lambda =\frac{1}{3.291\times {10}^7{m}^{-1}}=304\mathring{A}$