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Physics · 2026 · 3 marks
CBSE 2026 · Region 4 · Set 2 · Q24
In Bohr model of hydrogen atom, calculate: (a) the radius of the innermost electron orbit, and (b) the speed of electron in this orbit.
In Bohr model of hydrogen atom, calculate :
(a)
the radius of the innermost electron orbit, and
(b)
the speed of electron in this orbit.
Marking-scheme solution
(a)
(b)
Calculating speed of electron in the same orbit.
(a)
The radius of innermost electron orbit is given by,
\[\mathrm{r}=-\frac{\mathrm{e}^{2}}{8 \pi \varepsilon_{0} \mathrm{E}} \quad-\ldots--(1)
\]
where $\displaystyle \mathrm{E}=$ Energy of electron in hydrogen atom in innermost orbit ( $\displaystyle \mathrm{n}=1$ ).
Using formula, $\displaystyle \mathrm{E}_{\mathrm{n}}=-\frac{13.6}{\mathrm{n}^{2}} \mathrm{eV}$
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CBSE Class 12 Physics past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.