hey, radium has 88 electrons and that next step you have to determine which orbitals has lower energy when it is unfilled

1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,6s,4f,5d,6p,7s.... <-- first step

and next you start filling in the electrons

1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{6}5s^{2}4d^{10}5p^{6}6s^{2}4f^{14}5d^{10}6p^{6}7s^{2}

Check your number of electrons and lastly rearrange the orbitals in order. all the 1's comes first follow by 2's....

this is to ensure you will not make mistake if you are removing electrons from orbital. if you are very sure which orbials has lower energy when unfilled, you could practically skip first step.

alternatively you could write [Rn]7s^{2} much easier...

good luck to your test!!