An atom of oxygen is in an excited state. When an electron in this atom moves from the third shell to the second shell, energy is

(1) emitted by the nucleus

(2) emitted by the electron

(3) absorbed by the nucleus

(4) absorbed by the electron

The change in energy level was experienced by the electron that moved from a higher to a lower level. This does not affect the nucleus, so the nucleus itself does not change energy levels. It takes more energy to hold an electron at a higher than a lower energy level, so the change in energy levels means that some energy was released by the electron, and this is choice (2).

An atom of oxygen is in an excited state. When an electron in this atom moves from the third shell to the second shell, energy is emitted by the electron.