31 votes

What is the physical basis for Hund's first rule?

Disclaimer My following answer is the "traditional" explanation of Hund's first rule, which is based on a smaller value of $V_\mathrm{ee}$ (electron-electron repulsions) in the triplet state arising ...
orthocresol's user avatar
22 votes
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What is the physical basis for Hund's first rule?

The lowest energy state has parallel spins to maximize the exchange energy. As you say, there's a Coulomb repulsion between two electrons to put them in the same orbital. There's also a quantum ...
Geoff Hutchison's user avatar
22 votes
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Degeneracy of second excited state of H-?

I think it is important to understand that for hydrogen atom (or any other one-electron system) all orbitals from the same shell have same energy. For instance, $E_\mathrm{2s} = E_\mathrm{2p}$, $E_\...
Wildcat's user avatar
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22 votes
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Why are protons more common than hydride ion?

This is because we live in a world dominated by oxygen and water. In other words, it is an oxidized world. Most metals occur naturally in the form of oxides, silicates, halides, or other derivatives. ...
Ivan Neretin's user avatar
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21 votes
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Difference between Coupled Cluster and Full CI

Full Coupled Cluster (FCC) vs. Full Configuration Interaction (FCI) The main theoretical difference is the way excitations are used. "Excitation" refers to putting one or more electrons in higher ...
TAR86's user avatar
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17 votes
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What do these labels for molecular electronic states mean?

As stated in this answer, these are irrep (irreducible representation) labels for molecular symmetry point groups. In the context of chemistry, point groups are usually introduced when learning about ...
pentavalentcarbon's user avatar
17 votes
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Help understanding how "steric effects" are distinct from "electronic effects"?

The normal distinction between "steric" and "electronic" is based on whether the effect is transmitted through space or through bonds All the normal physical interactions we ...
matt_black's user avatar
15 votes
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While filling electrons, we follow Aufbau principle, but not while removing them. Why is this so?

Usually when adding electrons based on the Aufbau principle, you go from one element to the next highest one, e.g. from $\ce{Ti}: \ce{[Ar] 4s^2 3d^2}$ to $\ce{V: [Ar] 4s^2 3d^3}$. Thus you add not ...
Feodoran's user avatar
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14 votes

Can an atom bond with more than 8 other atoms?

In october 2015, 5 months after all the current answers were given, a new cluster compound witn C.N. 16, $\ce{CoB16-}$, has been reported [1]: Here we report the observation of a large metal-doped ...
andselisk's user avatar
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13 votes

Why do atoms need 8 electrons to stabilize?

The valence orbitals of atoms are composed of suborbitals (s and p) there is 1 s suborbital which is spherical and can hold 2 electrons (one with up spin and one with down spin). There are 3 p ...
A.K.'s user avatar
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12 votes

Why does Boron only need 6 valence electrons unlike the standard 8?

The principle of full octet is that most reactivity patterns are described well if there is a driving force / energetic gain for full octets in main-group atoms. However, the only facet of an atom ...
Lighthart's user avatar
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12 votes

What is the physical basis for Hund's first rule?

Some understanding can be gained by looking at the symmetry of the orbital parts of the wavefunctions involved. The total wavefunction for electrons must be anti-symmetric with respect to exchanging ...
porphyrin's user avatar
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12 votes
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If the ferric ion is more stable than the ferrous ion, then why is it readily reduced to the latter?

As already pointed out, the notion of stability needs to be defined much more accurately. The fact that the standard reduction potential is positive has no direct bearing on the gas-phase stabilities ...
orthocresol's user avatar
12 votes
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"NBO diagrams" versus MO diagrams

A molecular orbital diagram is a schematic representation of how we interpret bonding in certain species. It is as much an accurate representation for a specific bonding situation as a Lewis structure ...
Martin - マーチン's user avatar
12 votes
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Is cobalt(II) in the hexaamminecobalt(II) complex high spin or low spin?

Theoretically, you cannot predict a priori whether a compound is high- or low-spin. For some reason, a lot of people seem to think that it depends only on the ligand and that it is possible to ...
orthocresol's user avatar
11 votes
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Effects of atmospheric gases on colour of aurora

There is good information at Glowing Gases - Aurorae There are many factors that need to be considered. Once an atom or molecule is excited, it can lose the energy by collision or by emission of ...
DavePhD's user avatar
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11 votes
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Why do rare earth metal oxides vary in color so much?

TL;DR f-f transitions in lanthanide ions give rise to the colours of the ions. The electronic states can be derived from quantum mechanics, and their energies vary from ion to ion, which give rise to ...
orthocresol's user avatar
11 votes

How P(C2H5)3 acts as a ligand?

As I already mentioned in my comment, phosphines such as $\ce{PEt3}$ can bind to transition metals with partially filled d orbitals as a σ-donor as well as a π-acceptor. The σ interaction is simply a ...
orthocresol's user avatar
11 votes
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How are the π bonds arranged in Osmium Tetroxide?

Your conception of $\pi$-bonds is a bit too restrictive. A $\pi$-bond is a bond in which there is one node along the internuclear axis. So, yes, two side-on $p$-orbitals do form a $\pi$ bond because ...
levineds's user avatar
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11 votes

Europium stability in +2 and +3 state

This is going to be a rehash of Oscar Lanzi's answer, but this point needs to be driven home, so I make no apologies. "Special" electron configurations - fully-filled or half-filled subshells - are ...
orthocresol's user avatar
11 votes
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Size consistency in computational chemisty methods

Size consistency is essentially the idea that if you were to determine the energy of two noninteracting systems A and B, you should obtain the same result if you sum the energy of A and B separately ...
Tyberius's user avatar
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11 votes
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Can a metal be forced to form an anion theoretically?

Absolutely! You will find these mostly in electride systems and off these, mostly in alkali metals. Here is an example research paper: "Superakali-Alkalide Interactions and Ion Pairing in Low-...
Evamentality's user avatar
10 votes
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What are complete active space methods and how are such spaces defined for molecules?

Multiconfigurational self-consistent field (MCSCF) calculations can be further categorized. What you are referring to is a complete active space SCF (CASSCF) calculation. In CASSCF, the 1-particle ...
pentavalentcarbon's user avatar
10 votes
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Is the shorthand electron configuration of Br- just [Kr]?

You could render bromide ion that way. But the outermost electrons in the "krypton core" are potentially reactive, valence electrons; therefore $\text{[Kr]}$ is not an accurate description of how the ...
Oscar Lanzi's user avatar
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10 votes

Fundamental doubts about energy levels vs. shells vs. subshells vs. orbitals

Shell, level and orbital can often be used interchangeably although they do have different meanings. The level represents an ordinal or numerical representation of the state and corresponding relative ...
Buck Thorn's user avatar
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9 votes

Why is the 5d orbital instead of the 4f orbital filled in lanthanum?

According to Ground-state configurations of ionic species I through XVI for Z = 57-74 and the interpretation of 4d-4f emission resonances in laser-produced plasmas Phys. Rev. A 25, 275 (where "I" ...
DavePhD's user avatar
  • 40.6k
9 votes

What do these labels for molecular electronic states mean?

To fully understand the meaning of these symbols, you need to understand molecular symmetry groups. If you know the point group symmetry of your molecule, you can construct a character table comprised ...
Paul's user avatar
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9 votes
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Why exactly does atomic radius increase down a group?

The description of $Z_\mathrm{eff}$ you gave is a bit too simplistic. As you correctly said, that would predict the same value of $Z_\mathrm{eff}$ for all elements in a group, which is not true. In ...
orthocresol's user avatar
9 votes

Why might a planar, conjugated molecule be higher in energy than a nonplanar system?

I reproduced your calculations with PBE/def2-SVP (using Orca). The energy "jump" is even smaller than with MP2, however the causes are probably the same. So here is my answer: The planar structure is ...
Ramon Crehuet's user avatar
9 votes

Would singlet oxygen in the engine cause a more efficient combustion?

This hypermileage scheme is one of the more interesting ones I've come across, so I think it's worth some discussion. I don't know much at all about the engineering aspects of internal combustion ...
Nicolau Saker Neto's user avatar

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