Question

What is the first f element in the 6^th period to which Hund's rule must be applied when writing the orbital diagram?

Answer 1

Lanthanum/Electron configuration

[Xe] 5d¹ 6s²

Cerium/Electron configuration

[Xe] 4f¹ 5d¹ 6s²

In 6th period first f element is ce.

Hund's rule of maximum multiplicity is a rule based on observation of atomic spectra, which is used to predict the ground state of an atom or molecule with one or more open electronic shells. The rule states that for a given electron configuration, the lowest energy term is the one with the greatest value of spin multiplicity. This implies that if two or more orbitals of equal energy are available, electrons will occupy them singly before filling them in pairs. The rule, discovered by Friedrich Hund in 1925, is of important use in atomic chemistry, spectroscopy, and quantum chemistry, and is often abbreviated to Hund's rule.

The multiplicity of a state is defined as 2S + 1, where S is the total electronic spin.^[2] A high multiplicity state is therefore the same as a high-spin state. The lowest-energy state with maximum multiplicity usually has unpaired electrons all with parallel spin. Since the spin of each electron is 1/2, the total spin is one-half the number of unpaired electrons, and the multiplicity is the number of unpaired electrons + 1. For example, the nitrogen atom ground state has three unpaired electrons of parallel spin, so that the total spin is 3/2 and the multiplicity is 4.

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