Sodium loses an electron and attains a +1 charge because it has one more valence electron than the number of protons in its nucleus. The number of protons in an atom determines the number of electrons that the atom needs to have in order to be electrically neutral. Sodium has 11 protons, so it needs to have 11 electrons to be neutral. However, sodium's outermost electron is loosely held, and it can be easily removed from the atom. When sodium loses this outermost electron, it becomes positively charged because it now has one more proton than electron.

The reason why sodium loses its outermost electron is because it is not very strongly attracted to the nucleus. This is because the outermost electron is far from the nucleus, and the force of attraction between the nucleus and the electron decreases as the distance between them increases. In addition, the outermost electron is shielded from the nucleus by the other electrons in the atom. This shielding effect also reduces the force of attraction between the nucleus and the outermost electron.

As a result of these factors, sodium's outermost electron is weakly held and can be easily removed from the atom. When sodium loses this electron, it becomes positively charged because it now has one more proton than electron.