The third energy level of an atom, referred to as the M shell, can hold a maximum of 18 electrons. The number of electrons that any particular element has in the third level depends on its location on the periodic table and can range from none to 18 electrons.

The maximum number of electrons on any given level can be determined by the formula two times n squared, where n is equal to the energy level. For the third energy level, the formula is two times three squared. The third energy level contains s, d and p orbitals.

The first element to have electrons in the third energy level is sodium, which is the first element in the third row of the periodic table, represented by the symbol Na. Sodium contains one electron in its 4s orbital. The element to its right, magnesium, contains two electrons in the 3s orbital. The next six elements, in the p-block, also contain two electrons in the 3s orbital and between one and six electrons in the 3p orbital. The next two elements at the beginning of row four also contain two electrons in the 3s orbital and six in the 3p orbital. The next ten elements, between scandium and zinc, contain electrons in the 3s, 3p and 3d orbitals, with the final element, zinc, containing a total of18 electrons in the third energy level.