Element X (atomic number 12) is magnesium, forming XCl₂ chloride

Steps:

• Atomic number 12 identifies X as magnesium (Mg) in the periodic table.
• Magnesium is in group 2, with 2 valence electrons.
• It loses 2 electrons to form Mg²⁺ ions.
• Chloride ions (Cl⁻) require two per Mg²⁺, yielding MgCl₂ or XCl₂.

Why C is correct:

• Group 2 metals achieve octet by +2 oxidation state, bonding with two Cl⁻ ions per the ionic compound formula rule.

Why the others are wrong:

• A: XCl fits group 1 elements with +1 charge, like NaCl.
• B: XCl₃ fits group 13 elements with +3 charge, like AlCl₃.
• D: XCl₄ fits group 14 elements with +4 oxidation, like SnCl₄.

Final answer: C