A metallic ion Z$^{2+}$ with an inert gas structure contains 18 electrons. How many protons are contained in this ion?

Analyze the question We have a metal ion Z²⁺ with 18 electrons (the inert gas structure of argon). We need the number of protons in the ion.

Concept

• A neutral atom has equal protons and electrons.
• A 2+ charge means the atom has lost 2 electrons, so protons exceed electrons by 2.
• Charge = protons − electrons

Solution Charge = protons − electrons

+2 = protons − 18

protons = 18 + 2 = 20

Check: The neutral atom has 20 electrons, which is calcium (atomic number 20), a metal. Ca loses 2 electrons to form Ca²⁺ with the configuration 2, 8, 8, the same as argon (an inert gas). This matches the question.

Answer: A (20)

Tip: For a positive ion, protons = electrons + charge. For a negative ion, protons = electrons − charge. Also remember that metals ions with inert gas structure like Ca²⁺, K⁺ and Mg²⁺ are common in WASSCE questions, so memorize the first 20 elements.