Assertion (A) : If the electrons in an atom were stationary, then they would fall into the nucleus.

Reason (R) : Electrostatic force of attraction acts between negatively charged electrons and positive nucleus.

Both A and R are true and R is the correct explanation of A.

The electrostatic force provides the necessary centripetal force for the electron to revolve in the atom. If electron were stationary, it would cause electrostatic force to be unbalanced and hence, attract the electron into the nucleus.