[DarkLightA]

I know it forms like this:


Given that the Cl has a charge of 5+, why doesn't instead ClO4 form, with a double bonded O and three single bonded ones?

[rabolisk]

ClO₄^- does form, and it is called perchlorate. It has 3 double bonds and 1 single bond. ClO₄ as a neutral molecule, however, could not form. If you drew the Lewis structure as you suggested, you would have an incomplete octet at one of the single bonded oxygens. If it did form, it would be a radical.