What does mean when it asks for the smaller ion

Explanation:

mg2+ would be the smaller ion this is because each ion has the same number of electrons however mg2+ has a greater number of protons and therefore is more charge dense and the outer electrons feel a greater pull from the nucleus.

I hope it's helpful!!

ca has a crystalline lattice with positive ions (since it is a metal, metals are positively charged) and electrons flow freely in its structure since the ca atoms have a large nuclear radius so the electrons are not tightly held, therefore the electrons can flow freely since they are not being highly attracted by the nucleus. moreover, one of the properties of metals are that they are conductors of electricity, thus proving ca is the element.