Successive Ionization Energies Identify Period 3 Elements Forming MgCl₂ Steps: - Examine successive ionization energy graphs for sharp increases indicating electron shell completions in Period 3 elements (Na to Ar). - Identify first element with jumps after 2 electrons (Group 2, like Mg) and second with jumps after 7 electrons (Group 17, like Cl). - Match to Period 3: Mg (atomic number 12) and Cl (17) fit the patterns without core electron jumps until higher energies. - Confirm compound: Mg²⁺ and 2Cl⁻ form MgCl₂, the only choice with both elements in Period 3 and correct formula. Why A is correct: - Mg and Cl are Period 3 elements; their ionization energies show characteristic jumps (Mg after 2nd IE for [Ne] core, Cl after 7th for [Ne] core), aligning with octet stability per ionization energy trends. Why the others are wrong: - B: Ca and Br are Period 4 elements, not Period 3. - C: NaS has incorrect formula (should be Na₂S); Na (Group 1) jumps after 1st IE, but S (Group 16) after 6th, mismatched for typical graphs. - D: …