Both A and R are true, but R is not the correct explanation of A. The discovery of subatomic particles (electrons, protons, and neutrons) allowed scientists like Thomson, Rutherford, and Bohr to propose different atomic models, which fundamentally helped in understanding the atomic structure. Reason (R) is also true because, for an atom to be electrically neutral, the number of protons must be equal to the number of electrons. However, this numerical equality explains the electrical neutrality of an atom, not how the discovery of these particles aided in understanding the atom's structural arrangement.