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tht means the max no. of electrons in the first shell is 2. the max no. of electrons in the second shell is 8. and the max number of electrons in the third shell is 8....it goes on as the no. of electron in the atom increases. so the electron configuration for magnesium is 2.8.2. this shows tht the first and second shells are filled to the max. the third shell isn't. it has a 2+ charge because it has lost 2 electrons( which carries a negative charge), hence the atom is more positive than normal.
Your starting point for finding the electron configuration of the magnesiu mcation, #Mg^(2+)#, will be the electron configuration of the neutral magnesium atom, #Mg#. You know that magnesium is located in period 3, group 2 of the periodic table, and that it has an atomic number equal to #12#. This means that a neutral magnesium atom will have a total of #12# protons in its nucleus and #12# electrons surrounding its nucleus. Its electron configuration would look like this. #Mg:  1s^2 2s^2 2p^2 3s^2#. Now, when the magnesium cation is formed, two electrons, hence the #2+# charge of the cation, are lost by the mgnesium atom. These electrons, which are called valence electrons, come from magnesium's outermost energy level.