Hund’s first rule

Hund s first rule The ground term will be one of maximum spin-multiplicity (maximum S)... 

The ground state of carbon presents a new problem Which orbital does the sixth electron go into This question can be answered by doing either an experiment or a calculation. The answers are summarized in Hund s first rule ... 

With increasing atomic volume, one approaches the free atom limit where Hund s first rule postulates maximum spin, so that the individual spins of the electrons in a shell are aligned parallel. More generally, Pauli s exclusion principle implies that electrons with parallel spins have different spatial wavefunctions, reduces the Coulomb repulsion and is seen as exchange interaction. When the atoms are squeezed into a solid, some of the electrons are forced into common spatial wavefunctions, with antiparallel spins and reduction of the overall magnetic moment. At surfaces and interfaces, the reduced coordination reverses this effect, and a part of the atomic moment is recovered. 

Hund s first rule. S has the maximum value consistent with the Pauli exclusion principle. 

The ground state of an atom may be chosen by application of Hund s rules. Hund s first rule is that of maximum multiplicity. It states that the ground state will be that having the largest value of 5, in the case of carbon the 3F. Such a system having a maximum number of parallel spins will be stabilized by the exchanf e energy resulting from their more favorable spatial distribution compared with that of paired electrons (see Pauli principle. Chapter 2). 

This approach is relevant when the overlap between orbitals belonging to neighbouring sites is large (for instance in C - C covalent bonds). In case of orbital degeneracy, two low-lying states g,a) or u,a) are available for the electron pair which may then form a singlet or a triplet. Of course, the Hund s first rule tells us that the triplet is more stable, as often encountered. 

The ferromagnetic contribution 7tt dominated by yi is actually a misadventure of the Hund s first rule (Eermi hole), while the antiferromagnetic one 7f4, is mainly associated with the prohibition for the electrons to occupy the same orbital with the same spin. Both are demonstrations of Pauli s exclusion principle. The first term of Jf vanishes with the overlap, while the second... 

Hess law, 30, 85, J46 heteropolar bond, 23 heteropolymetallate, 223 high-spin complex, 213 HOMO, 75, 90 homopolar bond, 23 H ckel theory, 82 Hund s first rule, KL, 11, 70 hybridization, 73, 77 hydrazine, 162 hydride, 26,129,150 hydroformylation, 268 hydrogen bond, 79, 91, 151 hydrogen electrode, 140, 143 hydrogen peroxide, 171, 266 hydrogenation, 268 hydrothermal reaction, 41, 114, 257 hydroxide, 130,132,138,119,110 hypervalence, see octet expansion... 

Hund s first rule (often referred to simply as Hund s rule) in a set of degenerate orbitals, electrons may not be spin-paired in an orbital until each orbital in the set contains one electron electrons singly occupying orbitals in a degenerate set have parallel spins, i.e. they have the same values of m. ... 

B2. The six electrons should give the electron configuration (ox)2(a in the basis of Fig. 3-14b. The two electrons in the % orbital should have heir spins unpaired in accord with Hund s first rule, which applies to the filing of MO s as well as to atomic orbitals. On the other hand, according o Fig. 3-14a, the configuration would be (ffi)2(cr2)2(ff3)2, with no unpaired dectrons. Experiment shows that B2 has two unpaired electrons in a % irhital thus, Fig. 3-14b is still correct at B2. The bond order is 1 (one net... 

It is a general rule that if a group of n or less electrons occupies a set of n degenerate orbitals, they will spread themselves among the orbitals and give n unpaired spins. This is Hund s first rule, or the rule of maximum multiplicity. It means that pairing of electrons is an unfavorable process energy must be expended in order to make it occur. If two electrons are not only to... 

Hund s first rule the lowest energy corresponds to that configuration, which corresponds to the maximum of the spin angularmomentum S, where S = 5(5+ l)/i, with 5 either an integer or a half-integer. A large value of 5 requires a same-spin situation, which is only... 

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