(b) The electron in a hydrogen atom, moves with constant acceleration, called centripetal acceleration, round the nucleus. Acceleration of electron $a = \frac{v^{2}}{r}$
Given, $v = 2.18 \times 10^{6}m/s$
$r = 0.528\ \mathring{\mathrm{A}} = 0.528 \times 10^{- 10}m$
$\therefore a = \frac{\left( 2.18 \times 10^{6} \right)^{2}}{0.528 \times 10^{- 10}} = 9 \times 10^{22}\ m/s^{2}$