A2- ion has 36 electrons and 45 neutrons. what is its correct symbol

Total electron element having  =  36

In order to find number of Protons we will subtract 2 from 36 because 2 electrons are extra with respect to protons, So number of protons are,

                                   # of protons  = 36 - 2  = 34

Means, this element has atomic number 34 and electronic configuration of this element is,

                             1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁴

Now, Valence Shell  =  4   = Element is present in Period 4

         Valence Electrons = 6 =  Element is present in Group 6

Result;
            This element is Selenium, with atomic number 34 and atomic mass 79 (# of protons + # of neutrons). And the correct symbol is,

                                              ⁷⁹₃₄Se⁻²

They are all full and don't need any more electrons in there outer shell.

The mass has escaped into a gas which you can not see so the law stays true.