Final answer:
An atom's electrons are in the ground state when they occupy the lowest possible energy levels. Excited states occur when electrons absorb energy and move to higher levels, a
Step-by-step explanation:
When we say that an atom's electrons are in the ground state, it means they are in the lowest energy state possible for that atom. This state is characterized by electrons occupying the innermost orbits around the nucleus, as described by the Bohr model.
The Bohr model likens these orbits to rungs on a ladder, where the electron can only exist on specific rungs, corresponding to quantized energy levels.
When an atom receives energy, such as from heat or light, the electrons can absorb this energy and move to higher energy levels, known as excited states.
These are unstable states, and the electrons eventually fall back to the ground state, releasing energy in the form of photons - this is how we observe atomic emission spectra.
The transition between energy levels adheres to the law of conservation of energy, ensuring that the amount of energy absorbed is equal to the energy emitted when the electron returns to its ground state.