The ionization energies in kcal for mg and ca are mg ca 1st 176 141 2nd 346 247 3rd 1847 a) indicate the reasons for the relatively high 3rd ionization energy for magnesium. b) indicate the reasons for the differences in 1st ionization energies for mg and ca.

a) Answer is: the reasons for the relatively high 3rd ionization energy for magnesium is necause magnesium cation Mg²⁺ has stable electron configuration as noble gas neon and hardly lose electron.

Magnesium has atomic number 12, which means it has 12 protons and 12 electrons. It lost two electrons to form magnesium cation (Mg²⁺) with stable electron configuration like closest noble gas neon (Ne) with 10 electrons.

Electron configuration of magnesium ion: ₁₂Mg²⁺ 1s² 2s² 2p⁶.

b) Answer is: calcium has lower 1st ionization energy because valence electron are farther from nucleaus and interaction between protons and electrons are weaker.

Atomic number of magnesium (Mg) is 12, it means that it has 12 protons and 12 electrons.  

Electron configuration of magnesium atom: ₁₂Mg 1s² 2s² 2p⁶ 3s².  

Atomic number of calcium is 20, it means that it has 20 protons and 20 electrons, so atom of calcium is neutral.  

Electron configuration of calcium atom: ₂₀Ca 1s² 2s² 2p⁶ 3s² 3p⁶ 4s².

Magnesium (Mg) and calcium (Ca) are metals from 2. group of Periodic table of elements and have low ionisation energy and electronegativity, which means they easily lose valence electons (two valence electrons).

The ionization energy (Ei) is the minimum amount of energy required to remove the valence electron, when element lose electrons, oxidation number of element grows (oxidation process).