An atom is modelled by a stationary central positive point charge $+Q$ surrounded by a spherical electron cloud (radius $R$, total charge of $-Q$ ) of uniform charge density. The electron cloud is displaced slightly from its equilibrium position and released. Assuming that the cloud maintains its shape, select the correct statement(s) from the following options.

• Correct answer
  A

  The frequency of oscillation of electron cloud around positive charge is proportional to $R^{-3 / 2}$.
• Correct answer
  B

  The force due to electron cloud on positive charge is inversely proportional to $R^3$.
• C

  The frequency of oscillation of electron cloud around positive charge is inversely proportional to $R$.
• D

  The motion of displaced electron cloud is periodic but not simple harmonic.

Hence, the correct answer is options: 'A', 'B'.