When an atom loses one electron, the charge on its ion is

The charge on its ion is positive.

This is because each electron brings a negative charge, meanwhile each protons bring a positive charge. Orginally, if it's an atom, the number of protons and electrons should be the same, so the charge of atom should be neutral, but once a electron is removed or added, it turns into an ion.

If electrons is removed, the charge of the ion would be positive, well depending on the number of electrons removed. One electron removed will make the charge of ion be +1, if 2, then it'll be +2. Same concept for adding electrons. The more electrons they add, the more negative charge it brings.