Question

The ion Q2+ has three complete shells of electrons.
What is Q?
A calcium
B magnesium
C oxygen
D sulfur

Answer 1

Observe that, the Ion $\sf{Q}$ loses its $\sf{2}$ electrons to form $\sf{{Q}^{2+}}$

We know that Calcium's atomic number is $\sf{20}$, So it's electronic configuration would be $\sf{2, \: 8, \: 8, \: 2}$

Hence, if Calcium loses its $\sf{2}$ electrons, it would complete it's duplet in the $\sf{{1}^{st}}$ shell, and it's octate in the $\sf{{2}^{nd}}$ and $\sf{{3}^{rd}}$ shell, forming $\sf{3}$ complete shells.

Therefore, $\sf{A) Calcium}$ is correct.