An atom in an excited state is one whose

Analyze the question: We are asked what defines an atom in an excited state.

Concept: According to Bohr's model, electrons in an atom occupy discrete energy levels. The lowest level is the ground state, where the atom is most stable. When an atom absorbs energy (heat, light, or collision with another particle), one or more electrons jump from a lower energy level to a higher one. The atom is then said to be excited. This state is unstable, so the electron soon falls back to a lower level and emits the extra energy as a photon (light).

Checking each option:

• A. Potential energy is minimum: This describes the ground state, not the excited state. ✗
• B. Potential energy is maximum: An excited atom has more energy than the ground state, but "maximum" would mean the electron is completely removed. That is ionization, not excitation. ✗
• C. Electrons are in the conduction band: This is a solid-state (band theory) idea about materials, not the definition of an excited atom. ✗
• D. Electrons have moved to higher energy level: This is exactly what excitation means. ✓

Answer: The correct option is D.

Tip: Remember "Excited = Elevated." Absorb energy, and the electron goes up. Emit energy, and it comes back down. If you see "ground state," think lowest energy. If you see "excited state," think electron at a higher level.