What characterizes a metastable state in an atom?

A metastable state in an atom is characterized by having a longer lifespan than typical excited states. When an electron in an atom is excited to a higher energy level, it can return to the ground state by emitting a photon. However, in a metastable state, the transition to the ground state is not immediate due to specific quantum mechanical effects, such as selection rules that restrict certain transitions. This results in a relatively stable excited state that can last much longer than other excited states, which typically decay very quickly.

In contrast, the other choices describe characteristics that do not apply to metastable states. The option about having a short lifespan misrepresents the nature of a metastable state since these states are defined by their increased stability. Instant transitions are not characteristic of metastable states, as they are defined by their delayed decay. Additionally, while metastable states may influence energy absorption, this is not a defining feature of them. Rather, it is their prolonged existence that is the key characteristic.