Question

A hydrogen-like atom has one electron revolving round a stationary nucleus. The energy required to excite the electron from the 2nd to the 3rd orbit is $47.2eV$. The atomic number (Z) of the atom is:

• A. $3$
• B. $4$
• C. $5$
• D. $6$

Answer 1

Answer: (C)

$5$

$\Delta E=13.6eV\times z^2\times (1/2^2-1/3^2)$
According to question, $\Delta E=47.2eV$
$47.2eV=1.89eV\times z^2$

$\Rightarrow z^2=25$

$\Rightarrow z=5$.