Final answer:
Sodium has one valence electron in its outermost shell. This is lost when sodium forms a cation, resulting in an electron configuration similar to the noble gas neon.
Step-by-step explanation:
The element sodium has an atomic structure that consists of 11 protons and 11 electrons when in its neutral state. Its electron configuration shows that sodium has one valence electron in its outermost shell. This outer electron is in the third energy level. When sodium becomes a cation (Na+), by losing its single valence electron its electron configuration resembles that of neon, having a full shell with 10 electrons shared across the first two energy levels and zero in the outermost third level.