How many protons and electrons compose the following ions? Identify each as a cation or anion.
(a) Na+ (b) O2– (c) Cr3+
(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 O2– 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.