A quantum is a unit of energy in regards to atoms. More specifically, a quantum is an electron shell surrounding the nucleus of an atom. Larger atoms (those located further down the periodic table) have more electron shells, each of which is known as a quantum.
The valence electrons of lithium (Li) are in the 2nd quantum, while the valence electrons of sodium (Na) are in the 3rd quantum.
by GautamG March 08, 2006