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
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]7s2 much easier...
good luck to your test!!