2+ on an atom means that the atom needs 2 electrons to complete its octet.
As we know,
There are two types of elements present one is a Cation and the other is Anion.
Usually, when any of the elements are short 2 or more electrons, then in that case we place a plus sign in the superscript of the elements for denoting the deficiency of the electrons.
If any atom of an alkali metal group which are also called as group 1 atoms loses one electrons then such metals easily form a cation with one positive charge on it, in a same way the atoms of group 2 losses 2 electrons and forms a cations.
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