Final answer:
An electron absorbs energy when it transitions from the n=2 to the n=3 energy level in a hydrogen atom. This occurs because the electron is moving to a higher, less negative energy state, which requires energy in the form of a photon.
Step-by-step explanation:
When an electron is promoted from the n=2 to the n=3 energy level in a hydrogen atom, it absorbs energy. This is because energy levels in an atom are quantized, with higher energy levels having less negative energy values. Since moving to a higher energy level (from n=2 to n=3) is moving to a less negative energy state, the electron must absorb energy to do so. This absorbed energy typically comes in the form of a photon with a frequency that corresponds to the energy difference between the two levels.
In the case of a hydrogen atom, where the atomic number (Z) is 1, the energy required for this transition can be measured in electron-volts. An energy-level diagram can be used to visualize the transitions and to measure the energies involved, with transitions into higher energy states (excited states) requiring the absorption of a photon's energy.