An electron in a hydrogen atom makes a transition from n = 2 to n =...

Let us assume that the electron in the& lithium-ion is knocked off from the $n^{th}$ energy level, then

$R\left({Z}_{H}\right)^{2}\left(\dfrac{1}{12} - \dfrac{1}{22}\right) = R\dfrac{\left({Z}_{Li}\right)^{2}}{n2}$

$R \left(1\right)^{2} \times \dfrac{3}{4} =$R \left(3\right)$^{2}\left[\dfrac{1}{n2}\right]$

$\dfrac{3}{4} = \dfrac{9}{n2}$

$n = \sqrt{12} \approx$3. 5$</p><p>Hence the least quantum number must be$4$.