An electron in a hydrogen atom is revolving in an orbit at a radial...

The kinetic energy $K$ of an electron in a hydrogen atom is given by the formula:
$K = \dfrac{1}{4\pi\varepsilon_0} \dfrac{e^2}{2r}$

Substituting the given values to find the energy in Joules:
$K = \dfrac{9 \times 10^9 \times (1.6 \times 10^{-19})^2}{2 \times 2.12 \times 10^{-10}}$

$K = \dfrac{9 \times 10^9 \times 2.56 \times 10^{-38}}{4.24 \times 10^{-10}}$

$K = \dfrac{23.04 \times 10^{-29}}{4.24 \times 10^{-10}} \approx 5.43 \times 10^{-19}$ J

To convert this energy into electron-volts (eV), divide by $1.6 \times 10^{-19}$ J/eV:
$K = \dfrac{5.43 \times 10^{-19}}{1.6 \times 10^{-19}} \approx 3.4$ eV

Note: The total energy is $-3.4$ eV, and the potential energy is $-6.8$ eV. Kinetic energy is always positive and equal in magnitude to the total energy.

Answer: $3.4$ eV