Why does a rubidium atom's electron occupy a 5s orbital instead of a 4d or 4f orbital?

The correct option highlights that 5s orbitals have lower energy than both 4d and 4f orbitals, which is the main reason a rubidium atom's electron occupies a 5s orbital. In the context of electron configuration, electrons fill orbitals according to their energy levels, following the Aufbau principle. Generally, for elements in the same period, you move to higher principal energy levels, and within those levels, different orbitals (s, p, d, f) have their own order of energy.

For rubidium, which is in period 5, the 5s orbital is filled before the 4d and 4f orbitals due to its lower energy. The 4d and 4f orbitals are considered higher in energy than the 5s despite being part of the fourth principal energy level because of the effective nuclear charge and electron-electron repulsions experienced in those orbitals. As a result, when building the electron configuration for rubidium, the electrons occupy the 5s orbital first, which is lower in energy compared to the 4d and 4f orbitals. Understanding these energy relationships and the principles of orbital filling is crucial in atomic theory and electron configuration.