Question

How many of the following electron configurations are allowed?
1s22s22p7 1s22s22p62d1 [Ne]4s24p4 [Ar]3d64s2 1s22s22p83s23p4

Answer 1

What about this do you not understand. By the way, leave space like this.
1s2 2s2 2p6 3s2 otherwise it makes no sense.