The energy represented by an electron in the band model of a substance.

The specific, quantized energy levels that an electron may have in an atom.

simply put, rings around the nucleus of an atom in which specific electrons are most probably located. Particular electrons orbit in their own ring, or shell, with exceptions when they are excited or used in bonds. Electrons always fill the lowest possible energy level to its maximum extent before beginning to orbit (fill) a higher (farther from the nucleus) energy level. The general rule is that two electrons may occupy the first energy level, and eight in several subsequent levels.