The successive ionization energies of an unknown main group element are given as: , , and . Based en — Periodic Table and Periodicity Chemistry Question
Question
The successive ionization energies of an unknown main group element $M$ are given as: $IE_1 = 170 \text{ kCal/mol}$, $IE_2 = 345 \text{ kCal/mol}$, and $IE_3 = 1840 \text{ kCal/mol}$. Based entirely on this ionization data, what will be the exact number of halogen atoms ($X$) bonded to $M$ in the most stable halide compound formed by this element?
💡 Solution & Explanation
There is an enormous jump in ionization energy between $IE_2$ and $IE_3$ ($345$ vs $1840 \text{ kCal/mol}$). This indicates that the third electron must be removed from a highly stable, deeply buried noble gas core. The element must have exactly 2 valence electrons (an Alkaline Earth Metal). Therefore, its stable oxidation state is $+2$, and it will form a halide with the formula $MX_2$, bonding with exactly 2 halogen atoms.