Which pair of atomic orbitals has the same energy in a krypton atom?

1s and 2s

2s and 2p

3px and 4px

4pz and 4py

4pz and 4py

Explanation:

The electron configuration of Kr is:

[Ar] 3d¹⁰ 4s² 4p⁶

4p has 3 subatomic orbitals: 4px, 4py and 4pz

As there are a total of 6 electrons each of the three 4p orbitals have two electrons with opposite spins. Hence 4py and 4pz have equal energy. Option D is correct.  

Option A, 1s and 2s is incorrect, as the energy of the 1st orbital is lower than that of the 2nd orbital.  

Option B, 2s and 2p is incorrect, as the energy of the s suborbital is lower than that of the p, even though they both belong to the same orbital.  

Option C, 3px and 4px is incorrect, as the energy of the 3rd orbital is lower than that of the 4th orbital.