Question

Explain why quantum numbers stop following the expected trend for elements at potassium, while they do follow the expected trend for ions.

Answer 1

Potassium is an alkali earth metal with electronic configuration as 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ and atomic number 19.

Quantum numbers stop following the trend for electronic configuration while filling electrons in orbitals. The first two electrons are filled in 1s orbital, leaving 17 electrons. The next two electrons are filled in the 2s orbital. The 2p orbital is the next available energy level and can hold six electrons. The next two electrons are filled in the 3s orbital. The 3p is the next energy level and can hold six electrons, leaving 1 electron. This 1 electron, if the trend was followed, was supposed to be filled in 3d. However, this is not the case. It is filled in the 4s orbital.

This is because, according to the Aufbau principle, electrons fill orbitals of lower energy first before moving to higher energy orbitals. Quantum numbers n and l are deciding factors of this energy, given by the (n+l) rule. ‘n’ here is the principle quantum number and ‘l’ is the azimuthal quantum number. Applying the above rule, for 4s and 3d orbitals

4s orbital has n=4 and l=0, thus n+l = 4+0 = 4

3d orbital has n=3 and l=2, thus n+l=3+2=5.

Thus, 4s has lower energy when compared to the 3d orbital. Hence,4s will be filled first before moving on to the 3d orbital.

Order of filling orbitals.

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