What occurs when a magnesium atom becomes a magnesium ion

Answer is: Electrons are lost and the oxidation number increases.

Magnesium (Mg) is metal from 2. group of Periodic table of elements and has low ionisation energy and electronegativity, which means it easily lose valence electons (two valence electrons).

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 atom: ₁₂Mg 1s² 2s² 2p⁶ 3s².  

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