We notice here that the first energy level, quantum number n = 1,
is complete at helium and there is only one orbital the Is (first
quantum level, s type orbital). When this is full (Is2), we may call it
the helium core. Filling of the quantum level begins at lithium;
at beryllium the 2s orbital is filled and the next added electron
must go into a 2p orbital. All three 2p orbitals have the same energy
in the absence of a magnetic or electric field and fill up singly at first—
elements boron to nitrogen—before the electrons kpair up'. (The effect
of pairing on the ionisation energy is further discussed on page 16.)
The n = 2 quantum level is completed at neon, and again we may
use "neon core' for short.