An atom of sodium and chloride, why these two constitute such a successful molecule? Talking yesterday about ions, the ability of atoms to not being electrically neutral, the answer is newly here.
An eight could be seen as another number, but atoms express a curious interest of having eight electrons in orbit concretely in the second shell / layer / energy level. This Wikipedia page shows how electrons are arranged in the elements of the periodic table, with the surprising pattern of 2n^2 and some singularities more to analyze.
So the sodium atom is an ion and the same for chloride, and with different sign. Chloride has a seven electrons in its last energy level and that creates a negatively charged ion or anion when it gains one. Sodium has a single one, so it loses it and then it's a cation. When you dissolve salt in water, the molecule of NaCl breaks into these two atoms creating more electric conductivity in water, a property that practically doesn't exist in its pure state.
From composite molecule to single atoms in great proportions as in the oceans. Although this process cannot continue forever because the maximum solubility of this ionic compound into water is about 350 grams per litter or a bit more in dependance of the temperature. This quantity is very rare, so the taste!