The electronic configuration $1s^2, 2s^2 2p^5, 3s^1$ represents:

The energy of atomic orbitals in a given shell follows the order:

Identify the pair of orbitals having similar $(n + l)$ values from the following.

The quantum numbers $n$ and $l$ for four electrons are given as: $(1) \, n = 4, l = 1; \, (2) \, n = 4, l = 0; \, (3) \, n = 3, l = 2; \, (4) \, n = 3, l = 1$. Arrange them in increasing order of their energy.

The set of quantum numbers for the $19^{th}$ electron in chromium $(Cr)$ is:

Assertion $(A)$: Atoms with completely filled and half-filled subshells are stable.
Reason $(R)$: Completely filled and half-filled subshells have symmetrical distribution of electrons and have maximum exchange energy.
The correct answer is: