The atomic number of calcium is #20#, and the atomic number of argon (a noble gas) is #18#, so calcium is on the second column of the periodic table.
Since we are talking about the #2+# cation, it already lost two electrons. We can tell because each electron brings a #1-# charge, and so losing a #1-# charge is like gaining a #1+# charge.
Also, since neutral #"Ca"# is on the second column/group, it originally had #2# electrons. #2-2 = 0#, so #\mathbf("Ca"^(2+))# has no valence electrons .
Therefore, drawing the Lewis structure is actually not too hard; just write #"Ca"#, and mention somehow that it has a #2+# charge.
One way you could do it is to enclose #"Ca"# in square brackets and put the #2+# up top like so:
#["Ca"]^(2+)#
