Which electronic transition is forbidden for a hydrogen-like atom?

The transition from 2p to 1s is considered forbidden for hydrogen-like atoms due to the selection rules that govern electronic transitions. These rules state that a transition can only occur if there is a change in the orbital angular momentum quantum number, which is denoted by the symbol Δl.

In the given transition, both the 2p and 1s orbitals have their respective angular momentum quantum numbers. The 2p orbital has ( l = 1 ) and the 1s orbital has ( l = 0 ). The transition involves moving from an orbital with ( l = 1 ) to one where ( l = 0), which satisfies the selection rule ( Δl = ±1). Therefore, this transition is allowed and can be expected to produce spectral lines under appropriate conditions.

However, for an electronic transition to be fully allowed, other selection rules, including changes in spin and parity, must also be satisfied. The transition from 2p to 1s involves not only a change in angular momentum but also a change in parity from odd to even. The rules indicate that an electronic transition must change its parity, and the transition from a p orbital to an s orbital does not satisfy this