(a)  The number of protons is determined from the atomic number. The atomic number of sodium is 11, so there are 11 protons in Na⁺. It has a positive charge because it does not have enough electrons to balance the positive charges from the protons. A 1+ charge shows that there is one less electron than the number of protons, so its total number of electrons must be 10. The Na⁺ ion is a cation because it is positively charged.

(b)  The atomic number for oxygen is 8, so there are 8 protons. A negative charge results when there are more electrons than protons. Since the charge is 2–, there are two more electrons than protons, so there are 10 electrons. The O^2– ion is an anion because it is negatively charged.

(c)  The atomic number of chromium is 24, so there are 24 protons in Cr³⁺. A 3+ charge indicates that there are three fewer electrons than the number of protons, so there are 21 electrons. The Cr³⁺ ion is a cation because it is positively charged.