How do you use the periodic table to recall the ionic charge of an alkali metal, an alkaline earth metal, or aluminum

The last digit of the group number (the unit digit) is the number of valence electrons of the atoms.

So, group 1 elements (alkali metals) have 1 valence electron; gropup 2 elements (alkaline earth metal) have 2 valence electrons; and group 13 elements (to which aluminum belongs) have 3 valence electrons.

The metals form ions lossing their valence electrons, so the ionic charge will be determined in this form:

kind of element    group in                    number of               ionic charge
                             the periodic table     valence electrons

alkali metal           1                                1                                  +1

alkaline earth
metal                    2                                2                                 +2

aluminum             13                              3                                 +3

The ionic charge of these metals equals the last digit of the group number.