For an ion, which statement correctly describes the number of electrons?

• A. Number of electrons equals the number of protons
• B. Number of electrons equals the number of protons minus the ionic charge
• C. Number of electrons equals the number of protons plus the ionic charge
• D. Number of electrons equals the number of neutrons

Answer: (B)

Think of the ion’s charge as the difference between its protons and electrons. The charge on an ion equals the number of protons minus the number of electrons. So the number of electrons can be found by subtracting the ionic charge from the number of protons: electrons = protons − ionic charge. This explains both cations and anions: a positive charge means electrons are lost (fewer electrons than protons), while a negative charge means electrons are gained (more electrons than protons). For example, a Na+ ion has 11 protons and 10 electrons (11 − 1 = 10), and a Cl− ion has 17 protons and 18 electrons (17 − (−1) = 18). Neutrons don’t affect the electron count, so using neutrons would misrepresent the situation.