How do you predict the charge on a ion?

Answer 1

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The periodic table is organised so that all the elements in the same group (column going down the periodic table) have the same number of electrons in the outer electron shell.

The group 1A elements all have 1 electron in their outer shell. Since atoms want to have a full outer electron shell then all atoms in group one will loose 1 electron to from a 1+ ion.

Group 2A atoms all have 2 electron in outer shell, therefore loose 2 electrons and so all form ions with 2+ charge.

On the other side of the table, group 7A elements all have 7 electrons in their outer shell. They will want to gain one electron to form a 1- ion. And so forth.

Group 8A elements have full outer electron shells and are inert.

It gets more complicated in the middle of the periodic table. But elements in 1A, 2A, 6A and 7A will always act as described above to form ions.

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Answer 2

Look at the periodic table.
It is the minimum amount of atoms that the ion must gain/loose in order to obtain a noble gas configuration.