# Hydrogen Electron Transitions (Allowed/Forbidden)

## Homework Statement

Determine if the electron transitions are "allowed" or "forbidden" using the transition rules:
i. 2p→2s
ii. 1s→2p
iii. 3d→2p
iv. 3d→1s
v. 4s→2p

## Homework Equations

Δl = ±1, Δml= 0, ±1
E = (-2.18*10-18eV)(1/n12 - 1/n22)

## The Attempt at a Solution

I know iv. is forbidden because Δl is greater than 1, but what about the others? If i. is allowed then the energy is 0eV, which I think is not possible because photos are emitted. May anyone please offer a brief explanation to why it is allowed?

Last edited:

Simon Bridge
Homework Helper
Just write out the angular momentum and energy (quantum number) changes for each transition, and, as you have been doing, check the numbers against the rules. It is usually easier to think in terms of eliminating the transitions that are not allowed.

Good observation that (i) violates conservation of energy.

A R
Just write out the angular momentum and energy (quantum number) changes for each transition, and, as you have been doing, check the numbers against the rules. It is usually easier to think in terms of eliminating the transitions that are not allowed.

Good observation that (i) violates conservation of energy.

Great, I will follow your steps! I did not know whether (i) is allowed to forbidden, but the explanation is sensible as I did not know it is possible for it to emit photons but with 0eV of energy. Thank you, Simon.