Orbital Energies and Atomic Structure The energy of atomic orbitals increases as the principal quantum number, n, increases.
We can rationalize this observation by saying that the electron—electron repulsions experienced by pairing the electrons in the 5s orbital are larger than the gap in energy between the 5s and 4d orbitals. The 3d orbital is higher in energy than the 4s orbital. It is the loss, gain, or sharing of valence electrons that defines how elements react.
Figure 2. The first two electrons in lithium fill the 1s orbital and have the same sets of four quantum numbers as the two electrons in helium. Since the arrangement of the periodic table is based on the electron configurations, Figure 4 provides an alternative method for determining the electron configuration.
Figure 6 shows the lowest energy, or ground-state, electron configuration for these elements as well as that for atoms of each of the known elements. The valence shells of the inner transition elements consist of the n — 2 f, the n — 1 d, and the ns subshells.
This stability is such that an electron shifts from the 4s into the 3d orbital to gain the extra stability of a half-filled 3d subshell in Cr or a filled 3d subshell in Cu.