CBSE Class 10 Science 2019 · Paper 31/3/2 · Question 3

Magnesium (atomic number $12$, configuration $2,8,2$) has two valence electrons, and oxygen (atomic number $8$, configuration $2,6$) needs two more electrons to complete its octet. The magnesium atom transfers its two valence electrons to the oxygen atom: $\text{Mg}\to {\text{Mg}}^{2+}+2e^{-}$ ($2,8,2\to 2,8$) $\text{O}+2e^{-}\to {\text{O}}^{2-}$ ($2,6\to 2,8$) In electron-dot form, the two electrons (×) of Mg move to the O atom with its six electrons (•), giving ${\text{Mg}}^{2+}$ and the oxide ion with a complete octet, $[\text{O}{]}^{2-}$. The oppositely charged ions are held together by strong electrostatic attraction, the ionic bond: ${\text{Mg}}^{2+}+{\text{O}}^{2-}\to {\text{Mg}}^{2+}{\text{O}}^{2-}$, that is, magnesium oxide ($\text{MgO}$). Both ions have the stable configuration $2,8$ of neon.