Text_Template

Module 1 : Atomic Structure

Lecture 6 : Multi-Electron Atoms

This pattern of filling continues until we reach the configuration 1s² 2s² 2p⁶ 3s² 3p⁶ . We may expect to fill 3d next. But at this stage 4s is lower in energy than 3d, and the 4s subshell gets filled first, giving the configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² , which is for Ca. After Ca, the 3d which was so far unfilled gets filled. This 3d series is the series of the first set of transition elements. The orbital energies are a function of atomic number and the variation of orbital energies with atomic number is shown in figure 6.2.

Figure 6.2 Orbital energy levels as a function of atomic numbers.

As the nuclear charge increases, the electrons, on the average are drawn closer to the nucleus and the orbital energies decrease with increasing nuclear charge. The order of filling the orbitals is 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p….. when the energy levels of two orbitals are very close (of the order of a few kJ/ mol, the filling order can change and whenever possible a larger number of unpaired electrons are favoured.