According to Chemwiki, the next atomic orbital in the series 1s 2s 2p 3s 3p would be 4s and not 3d. As per Aufbaus principle, since the 4s orbital is at a lower energy level than the 3d orbital, electrons would first enter the 4s orbital. Hence the atomic series would be 1s 2s 2p 3s 3p 4s.
Aufbaus principle tells us the sequence in which various orbitals are filled with electrons. According to Aufbaus principle, orbitals are filled with electrons in order of their increasing energies. Based on their increasing energy levels, the atomic orbitals are arranged as follows:
1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d < 7p < 6fLearn more about Chemistry