When group 2a elements form ions, they

They lose two electrons.

Explanation:

When an atom loses are gain the electrons ions are formed.

There are two types of ions.

Anion

Cation

1 = Anion

It is formed when an atom gain the electrons. when atom gain electron negative charge is created on atom. For example.

X + e⁻ → X⁻

2= Cation

It is formed when an atom loses the electrons. when atom lose electron positive charge is created on atom. For example.

X → X⁺ + e⁻

There are two valance electrons in group two elements. They loses two electron to complete the octet and form cation with a charge of +2.

For example:

Mg → Mg⁺² + 2e⁻