(The horizontal rows correspond to the filling of a quantum shell of electrons).
With the development of modern quantum mechanical theories of electron configurations within atoms, it became apparent that each period (row) in the table corresponded to the filling of a quantum shell of electrons.
Stability of isotopes is affected by the ratio of protons to neutrons, and also by the presence of certain "magic numbers" of neutrons or protons that represent closed and filled quantum shells.
These quantum shells correspond to a set of energy levels within the shell model of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide.
These are electron shell numbers, or column numbers.
The electrons occupy a series of electron shells (numbered shell 1, shell 2, and so on).
The quantum number n labels the shell of the atom.
The number l, called the orbital quantum number, must be less than the principal quantum number n, which corresponds to a "shell" of electrons.
It required the introduction of a fourth quantum number, a highly counterintuitive move as the new quantum number represents nothing that can be visualised.
There is a magnetic quantum number also associated with the angular momentum of the quantum state.
The orbitals can be classified, first, by principal quantum number, and the orbitals have increasing energy as the principal quantum number increases from 1 to 2, 3, 4, etc. (The sets of orbitals defined by the principal quantum numbers 1, 2, 3, 4, etc., are often referred to as shells designated K, L, M, N, etc).
