Hund's rule of Maximum Multiplicity
Encyclopedia
Hund's Rule of Maximum Multiplicity is an observational rule which states that a greater total spin state usually makes the resulting atom more stable. Accordingly, it can be taken 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
. As a result this rule is often abbreviated to Hund's Rule, ignoring Hund's other two rules.
As a result of Hund's rule, constraints are placed on the way atomic orbitals are filled using the Aufbau principle
. Before any two electron
s occupy an orbital
in a subshell, other orbitals in the same subshell
must first each contain one electron. Also, the electrons filling a subshell will have parallel spin before the shell starts filling up with the opposite spin electrons (after the first orbital gains a second electron). As a result, when filling up atomic orbitals, the maximum number of unpaired electrons (and hence maximum total spin state) is assured.
For example a p4 subshell arranges its electrons as [↑↓][↑][↑] rather than [↑↓][↑][↓] or [↑↓][↑↓][ ].
(DMX), the first organic
molecule
known to violate Hund's rule.
Friedrich Hund
Friedrich Hermann Hund was a German physicist from Karlsruhe known for his work on atoms and molecules.Hund worked at the Universities of Rostock, Leipzig, Jena, Frankfurt am Main, and Göttingen....
in 1925, is of important use in atomic chemistry, spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
, and quantum chemistry
Quantum chemistry
Quantum chemistry is a branch of chemistry whose primary focus is the application of quantum mechanics in physical models and experiments of chemical systems...
. As a result this rule is often abbreviated to Hund's Rule, ignoring Hund's other two rules.
Details
The increased stability of the atom, most commonly manifested in a lower energy state, arises because the high-spin state forces the unpaired electrons to reside in different spatial orbitals. A false, but commonly given, reason for the increased stability of high multiplicity states is that the different occupied spatial orbitals create a larger average distance between electrons, reducing electron-electron repulsion energy. In reality, it has been shown that the actual reason behind the increased stability is a decrease in the screening of electron-nuclear attractions. Total spin state is calculated as the total number of unpaired electrons + 1, or twice the total spin + 1 written as 2S+1.As a result of Hund's rule, constraints are placed on the way atomic orbitals are filled using the Aufbau principle
Aufbau principle
The Aufbau principle is used to determine the electron configuration of an atom, molecule or ion. The principle postulates a hypothetical process in which an atom is "built up" by progressively adding electrons...
. Before any two electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s occupy an orbital
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
in a subshell, other orbitals in the same subshell
Subshell
Subshell can refer to:* A subshell of an electron shell* A child process launched by a shell...
must first each contain one electron. Also, the electrons filling a subshell will have parallel spin before the shell starts filling up with the opposite spin electrons (after the first orbital gains a second electron). As a result, when filling up atomic orbitals, the maximum number of unpaired electrons (and hence maximum total spin state) is assured.
For example a p4 subshell arranges its electrons as [↑↓][↑][↑] rather than [↑↓][↑][↓] or [↑↓][↑↓][ ].
Exception
In 2004, researchers reported the synthesis of 5-dehydro-m-xylylene5-Dehydro-m-xylylene
5-Dehydro-m-xylylene is an aromatic organic triradical and the first known organic molecule to violate Hund's Rule.Its electronic ground state is an "open-shell doublet" rather than a quartet; that is, it contains three low-spin coupled unpaired electrons in three singly occupied molecular orbitals...
(DMX), the first organic
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
known to violate Hund's rule.