In aqueous solution, magnesium forms ${\mathrm{Mg}}^{2+}\text{ ions }$ and not ${\mathrm{Mg}}^{+}\text{ions }$because

Mg is an atom of magnesium element. It is electrically neutral and has two electrons in its outermost or valence shell. Mg+2 is an ion of magnesium atom which has lost 2 electrons from its valence shell and has attained an electronic configuration of noble gas that is Neon and hence become positively charged.

The formation of ${\mathrm{Mg}}^{2+}\text{ ion }$is favoured due to the larger hydration energy