When sodium (Na) achieves a noble-gas electronic configuration, it loses one electron from its outermost shell to attain a stable configuration similar to neon (Ne). The formula of the ion formed in this process is Na+.

Sodium has an atomic number of 11, meaning it has 11 electrons. Its electron configuration is 2, 8, 1. In order to achieve a noble-gas configuration, sodium must lose its one outermost electron, leaving it with a stable configuration of 2, 8.

The loss of an electron results in a positive charge on the atom, forming a positively charged ion known as a cation. Therefore, the formula of the ion formed when sodium achieves a noble-gas electronic configuration is Na+ (sodium ion).