Here's why:

* Na is the chemical symbol for sodium.

* + indicates that the atom has a positive charge.

* Sodium, in its neutral state, has 11 protons and 11 electrons.

* When it loses one electron, it now has 11 protons and 10 electrons, resulting in a net positive charge of +1.

Therefore, Na⁺ represents a sodium ion, which is a sodium atom that has lost an electron and has a positive charge.