AUFBAUS LOGIC

1. AN INVESTIGATION हर हुस्न लाजवाब नहीं होता,और हर क्यों का,क्यों जवाब नहीं होता। Madelung

2. The aufbau principle states: In the ground state of an atom, atomic orbitals are filled by electrons in the order of their increasing energies. In other words, electrons will occupy the lowest-energy orbital first before filling higher-energy orbitals.

3. The etymology of aufbau Reader may think aufbau is the name of a scientist, but it is not. The word aufbau(auf-: up and bau: building) is a german word meaning building up or construction.

4. Madelung rule. (n + l rule) • from where did this rule originate. Is there any scientific reason behind the principle? Well, there is no such equation that dictates this rule. But there is a scientific logic behind it, and it is n + l rule.

5. The energy of an orbital mainly depends on two quantum numbers: the principal quantum number (n) and azimuthal quantum number (l). The principal quantum number corresponds to the shell while the azimuthal quantum number, subshell. As the value of both quantum numbers increases, the energy of an orbital also increases.

6. The Madelung rule, named after German physicist Erwin Madelung, states: • Electrons in an atom fill the orbital with the lowest value of n + l. • When two or more orbitals have the same value of n + l, the electron will occupy the orbital with the lowest value of n.

7. The energy of the orbital increases with n + l. Whenever there is a tie in the value, the energy increases with n. • In the diagram, each orbital along the diagonal has the same value of n + l (see below).

9. Many elements do not always follow the ordering of orbitals as predicted by the aufbau principle. • This is observed in transition metals, lanthanides and actinides.

10. Consider an example of scandium (Z = 21). Its electronic configuration is [Ar] 4s2 3d1 as per the principle, but this contradicts the spectroscopic observation. When the scandium atom is ionized, we presume the electron will be released from the highest energy orbital—3d orbitals as stated by the principle. However, the electron is released from 4s to form Sc+ having the electronic configuration [Ar] 3d1 4s1. This suggests that 3d have more energy than 4s. This behaviour is also seen in the remaining transition metals.

11. A similar trend exists in lanthanides and actinides. For example, the aufbau filling order of neodymium (Z = 60) is [Xe] 6s2 4f4, but the order of leaving is [Xe] 4f4 6s2. For heavier nuclei (Z ≥ 120), the aufbau principle becomes invalid. As the nuclear charge increases, the electrons, particularly nearer to the nucleus, experiences a heavy electrostatic force. The electrons of such nuclei have velocities approaching the speed of light. Thus, we need to account for the relativity theory to the quantum mechanic model.

12. We have already mentioned the exceptions to the rule. But why do we have exceptions? Is there any better rule than this? The answer is no. There is no simple mathematical formula that describes the electronic system. In an atom, we have two electrostatic interactions: the attraction between the positive nucleus and negative electrons, and the repulsion among the negative electrons. Every atom or ion tries to minimize the repulsion and try to reach the lowest energy configuration. And understanding such a system is very complicated, not easy.

13. In each element, the d orbital takes an extra electron from the s orbital, except in palladium where both electrons are consumed by the d orbital.

15. In all the above exceptions, the d orbital takes an electron from the f orbital; thorium and lawrencium are special cases. In thorium, 6d consumes both electrons from 5f while in lawrencium 6d is replaced by 7p. हर हुस्न लाजवाब नहीं होता और हर क्यों का जवाब नहीं होता।