Why would you never expect a Ca+ or a Na2+ ion to exist?

All elements "want" to be like a noble gas (very stable because all of its orbitals are filled). Since Na is in group 2A it "wants" to lose two electrons in order to be stable (has low electron ionization energy for the first two electrons) because it has two electrons in its sublevel.

Na2+ is very similar. The first electron ionization energy for Na is very low because it "wants" to be more like Ne (its in group 1A). However, once it is very stable, it is more difficult to remove the second electron, making it difficult for Na2+ to exist.

Hope this helps a bit! Oh, and never use the word "want" on a chem assignment. My teacher just explains it that way, but she says that if you ever personify the electrons like that on the AP test you'll get in trouble because the AP graders (and your teacher) will think that you're crazy...

Too unstable in nature it always wants to bond to something