E.1 Structure of the atom

In classical electromagnetism, any charged particle undergoing acceleration must emit electromagnetic radiation. An electron in a circular orbit is continuously accelerating (centripetal acceleration) toward the nucleus.

As the electron emits radiation, it loses kinetic energy. With less energy, it cannot maintain the same orbital radius, so it moves to a smaller orbit — this process is continuous and causes the electron to spiral inward.

Classical physics predicts the electron would crash into the nucleus in approximately $10^{-8}$ seconds. Since stable matter clearly exists, this classical prediction is fundamentally flawed, which motivated Bohr's new model.

The correct explanation is that an accelerating charged particle continuously emits electromagnetic radiation, losing energy, causing the electron to spiral inward.

The question asks for the classical physics reason why the Rutherford orbital electron would spiral into the nucleus — focusing on a fundamental electromagnetic principle.

This is incorrect — gravity between an electron and proton is negligibly weak compared to electrostatic forces. Also, the electrostatic force between the (negative) electron and (positive) nucleus is attractive, not repulsive.

Quantisation of angular momentum is a Bohr postulate — it is the solution to the problem, not the classical cause of the spiral. This option confuses the problem with its resolution.

Collisions between electrons and the nucleus are not the mechanism described in classical electromagnetism as causing the orbital decay. This option has no basis in the classical electromagnetic argument.

The remaining option — an accelerating charged particle continuously emits electromagnetic radiation, losing energy — is the correct classical electromagnetic principle that predicts orbital decay.

Bohr's first postulate states that electrons occupy stable, non-radiating orbits. In these special orbits, the classical rule that accelerating charges emit radiation is suspended.

The statement 'an electron in a stable orbit does not emit electromagnetic radiation' directly captures Bohr's first postulate, explaining why atoms are stable against the classical collapse predicted by Rutherford's model.

This is precisely what Bohr proposed to resolve the instability problem of the Rutherford model — electrons in allowed orbits simply do not radiate.

The question asks which statement is a genuine Bohr postulate — we need to identify the statement consistent with Bohr's model, not classical physics or incorrect claims.

'An electron emits radiation continuously as it orbits the nucleus' is the classical prediction that Bohr's model was designed to contradict. This is the failure of the Rutherford model, not a Bohr postulate.

Bohr's second postulate specifically restricts orbits to those where angular momentum is an integer multiple of $\hslash$. Arbitrary orbital radii are not allowed in the Bohr model.

There is no Bohr postulate involving the nucleus absorbing electrons. This option is a fabrication with no basis in the Bohr model or atomic physics.

'An electron in a stable orbit does not emit electromagnetic radiation' is the correct Bohr postulate — it is the key departure from classical physics that makes stable atoms possible in his model.