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Hunds Rule - Definition, Examples, Uses, Spin Multiplicity, FAQs

Hunds Rule - Definition, Examples, Uses, Spin Multiplicity, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:47 PM IST

The concept of chemical bonding explains how the atoms are combined to form the molecules. Atoms need to achieve a stable electronic configuration by filling their outermost shells. To achieve this stable electronic configuration the arrangement of electrons is guided by several rules and Hund's rule is one of them. Hund's rule states that the electrons are filled in the orbitals of the same energy levels with the parallel spin before pairing up. By doing this the electron-electron repulsion is minimized and the atom is stabilized.

This Story also Contains
  1. State Hund's Rule of Maximum Multiplicity
  2. What is Hund's rule?
  3. Explain Hund's rule with an example.
  4. Uses of Hund’s Rule:
  5. Some Solved Examples

State Hund's Rule of Maximum Multiplicity

The Aufbau principle tells us that the lowest energy orbitals will get filled up by electrons first. Thereafter, the electrons move on to energetically higher orbitals. The problem with this rule is that it does not tell about the order in which they will be filled in three 2p orbitals and five 3d orbitals consequently. Hund's Rule of Maximum Multiplicity depicts the perfect order of electrons filled up in energetically higher orbitals with higher principal numbers such as 2,3,4.. so on.. It states that for a particular electronic configuration, the term with maximum multiplicity is of the lowest energy. By this rule pairing of electrons in p, d and f orbitals cannot occur until each orbital of a given subshell contains one electron each or becomes singly occupied.

Background wave

What is Hund's rule?

Hund's rule states that:

1. For a particular electronic configuration, the electron having maximum spin multiplicity has the lowest energy. The multiplicity can be depicted as ( 2S+1), where S represents the total spin angular momentum of the electrons.

2. For a particular multiplicity, the term with the maximum value of total orbital angular momentum quantum numbers (L) occupies the lowest energy.

3. In an atom having the outermost subshell half-filled or less, for a particular term, the level with the lowest value of the total angular momentum quantum number (J) lies in the lowest energy. If the outermost shell is more than half-filled, the level with the highest value of J is the lowest in energy.

Where, total angular momentum quantum number, J = L + S

Explain Hund's rule with an example.

Hund's rule of maximum multiplicity was discovered by Friedrich Hund in the year 1925. Hund's principle states that, for a particular electronic configuration, the greatest value of spin multiplicity has the lowest energy term. It says if two or more than two orbitals having the same amount of energy are unoccupied then the electrons will start occupying them individually before they get paired up during filling up. This statement depends on the observation of atomic spectra, which is helpful in predicting the ground state of a molecule or an atom with one or more open electronic shells.

The electrons enter into an empty orbital before they get paired up. While considering the 1st statement, there comes a problem. The electrons repel each other as they are negatively charged. Hence, The electrons will not share orbitals to reduce repulsion. But when we consider the 2nd statement, the spins of unpaired electrons in singly occupied orbitals are the same. The spin of initial electrons at the sub-level decides what the spin of the upcoming electrons will be.

Example of Hund's rule (example of Hund's rule of maximum multiplicity)

For example, a nitrogen atom’s electronic configuration would be 1s22s22p3. The same orbital will be occupied by the two 2s electrons although different orbitals will be occupied by the three 2p electrons in accordance to Hund’s rule.

filling of electron according to hunds rule

approch

Let's consider carbon as an example.

The electronic configuration for carbon atoms: 1s22s22p2:

Here, The two 2s electrons will occupy the same orbital, whereas the two 2p electrons will be in different orbitals (and aligned in the same direction) in accordance with Hund's rule. Consider also the electronic configuration of oxygen. Oxygen has 8 electrons.

The electronic configuration can be written as 1s22s22p4.

The first two electrons will be paired up in the 1s orbital; the next two electrons will be paired up in the 2s orbital. The remaining 4 electrons, must be placed in the 2p orbitals. According to Hund’s rule, all orbitals will be singly occupied before being doubly occupied. Therefore, two p orbitals get one electron and one will have two electrons. Hund's rule also stipulates that all of the unpaired electrons must have the same spin. In keeping with convention, the unpaired electrons are drawn as "spin-up".

Spin Multiplicity Meaning

In spectroscopy and in quantum chemistry, the multiplicity of an energy level can be calculated by using 2S+1, where S indicates the total spin angular momentum. States of electrons with multiplicity 1, 2, 3, 4, and 5 are respectively called singlets, doublets, triplets, quartets, and quintets.

Spin Multiplicity Rule

According to the spin multiplicity rule, for a given electron configuration, the lowest energy term is the one with the greatest value of spin multiplicity. This indicates that if two or more orbitals of equal energy are available, electrons will occupy them singly before filling them in pairs.

Uses of Hund’s Rule:

Hund's rule of maximum multiplicity is a rule that is based on the observation of atomic spectra. This observation is used to predict the electronic configuration of the ground state of an atom or molecule with one or more open electronic shells. For example, from boron through neon, the electron filling order of the 2p orbitals follows Hund's Rule of maximum multiplicity. It has wide applications in atomic chemistry to predict the electronic configuration and spin of an electron, quantum chemistry, spectroscopy, etc.

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Some Solved Examples

Example.1 The 71st electron of an element X with an atomic number of 71 enters into the orbital:

1)6p

2) (correct)5d

3)4f

4)6s

Solution

The 71st electron will having an atomic number 71 will enter 5d.

Hence, the answer is the option (2).

Example.2 Which law indicates the pairing of electrons in the same orbital?

1)Newton’s first law

2) (correct)Hund’s rule

3)Aufbau principle

4)Pauli exclusion principle

Solution

Hund’s rule states that “pairing of electrons in the orbitals belonging to the same subshell (p, d or f) does not take place until each orbital belonging to that subshell has got one electron each. It is singly occupied”.

Hence, the answer is the option (2).

Example.3 Nitrogen has the electronic configuration 1s2,2s2,2px1,2py1,2pz1 and not 1s2,2s2,2px2,2py1,2pz0 which is determined by

1)Aufbau's Principle

2)Pauli's exclusion principle

3) (correct)Hund's rule

4)Uncertainty Principle

Solution

The above is determined by Hund's rule of maximum multiplicity which says that pairing will not start until and unless all the degenerate orbitals are singly occupied first.

Hence, the answer is the option (3).

Example.4 Which of these orbitals has the highest penetration effect?

1) (correct)1s

2)2p

3)3d

4)4s

Solution

Penetration effect of electron -

Due to the different shapes and orientations of different orbital, the penetration effect decreases from 1s to f.

n(s)>n(p)>n(d)>n(f)

Hence, the answer is the option (1).


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Frequently Asked Questions (FAQs)

1. Why is Hund's rule called the rule of maximum multiplicity?

hund's rule is called the rule of maximum multiplicity because out of the various possible electronic configurations, only that configuration is correct for which the total spin value is maximum.

2. What type of configurations violate Hund’s principle?

Every orbital of the same energy must have at least one electron which has identical spin before you deposit two in the same orbital.

3. What is stated at the 1st part of the hund’s rule?

The 1st part of the hund’s rule states that, for a particular electronic configuration, the electron having maximum spin multiplicity has the lowest energy. The multiplicity can be depicted as ( 2S+1), where S represents total spin angular momentum of the electrons.

The orbitals of the subshell will be singly occupied with electrons initially with parallel spin before pairing up occurs.

4. What is the nickname of the 1st part of the hund’s rule?

The nickname of the 1st part of the hund’s rule is spin-spin interaction.

5. Why is hund’s rule not applicable for all elements?

If Hund's rule is not applied the number of singly occupied orbitals or unpaired electrons will decrease gradually. Total number of unpaired electrons is 5, if Hund's rule is followed. Hence, there will be only one electron unpaired if this rule is violated. Hund’s rule is not applicable for NO molecule.

6. Why is Hund's rule called maximum multiplicity?

Hund's rule is called maximum multiplicity because out of the various possible electronic configurations, only that configuration is correct for which the total spin value is maximum.

7. What does spin multiplicity indicate?

Spin multiplicity indicates the total number of maximum orientations of spin angular momentum corresponding to the spin quantum number.

8. What happens if Hund's rule is not obeyed?

If Hund's rule is not obeyed, the electron will get paired more easily without all the d orbital getting parallelly occupied.

9. How does Hund's rule relate to electron configuration?
Hund's rule guides the arrangement of electrons in an atom's orbitals. It ensures that electrons fill orbitals of equal energy singly and with parallel spins before pairing up, which affects the overall electron configuration.
10. What is spin multiplicity and how does it relate to Hund's rule?
Spin multiplicity is the number of possible orientations of the total spin in a molecule or atom. Hund's rule maximizes spin multiplicity by promoting unpaired electrons, which leads to greater stability in many atoms.
11. Can you explain the concept of "parallel spins" in Hund's rule?
Parallel spins refer to electrons in different orbitals of the same energy level having the same spin direction (either all up or all down). This arrangement is favored by Hund's rule as it minimizes electron-electron repulsion.
12. Why do electrons follow Hund's rule?
Electrons follow Hund's rule to minimize repulsion between them. By occupying separate orbitals, they can stay farther apart, reducing electrostatic repulsion and lowering the overall energy of the atom.
13. How does Hund's rule affect the stability of atoms?
Hund's rule generally leads to more stable atomic configurations. By maximizing the number of unpaired electrons, it reduces electron-electron repulsion and often results in lower overall energy states for atoms.
14. What is the relationship between Hund's rule and electron spin?
Hund's rule promotes the alignment of electron spins in the same direction (parallel) when occupying orbitals of equal energy. This maximizes the total spin of the atom, which is often associated with greater stability.
15. How does Hund's rule relate to the concept of high-spin and low-spin complexes?
Hund's rule favors high-spin configurations in free atoms. In complexes, the competition between Hund's rule and the crystal field splitting determines whether a high-spin (following Hund's rule) or low-spin (pairing electrons) configuration is adopted.
16. What role does Hund's rule play in understanding the reactivity of elements?
Hund's rule affects the number and arrangement of unpaired electrons in an atom's outermost shell. This directly influences an element's valence and reactivity, as unpaired electrons are often available for bonding.
17. How does Hund's rule relate to the concept of spin-orbit coupling?
While Hund's rule deals with electron spin alignment, spin-orbit coupling involves the interaction between an electron's spin and its orbital angular momentum. In heavier atoms, spin-orbit coupling can sometimes overcome the effects predicted by Hund's rule.
18. What is the connection between Hund's rule and atomic term symbols?
Atomic term symbols describe the angular momentum state of an atom. Hund's rule helps determine the ground state term symbol by maximizing the total spin (S) and then the total orbital angular momentum (L), which are key components of term symbols.
19. What is Hund's rule?
Hund's rule is a principle in quantum chemistry that states electrons in an atom will occupy orbitals of equal energy individually before pairing up. This maximizes the total spin and minimizes electron repulsion.
20. What is the difference between Hund's rule and the Pauli exclusion principle?
While both deal with electron arrangement, the Pauli exclusion principle states that no two electrons in an atom can have the same four quantum numbers. Hund's rule, on the other hand, dictates how electrons of the same energy level are distributed across orbitals.
21. What is the significance of Hund's rule in understanding atomic structure?
Hund's rule is crucial for understanding atomic structure as it explains the distribution of electrons in orbitals, which affects an atom's chemical and physical properties, including its reactivity, magnetic behavior, and spectroscopic characteristics.
22. What is the relationship between Hund's rule and the Aufbau principle?
While the Aufbau principle determines the order in which orbitals are filled, Hund's rule dictates how electrons are distributed within orbitals of the same energy level. Together, they guide the complete electron configuration of atoms.
23. How does Hund's rule apply to the electron configuration of nitrogen?
For nitrogen, Hund's rule results in three unpaired electrons in the 2p orbitals. The electron configuration is 1s² 2s² 2p³, with each 2p orbital containing one electron with parallel spins, maximizing the total spin.
24. How does Hund's rule apply to transition metals?
Hund's rule is particularly important for transition metals as it explains their tendency to have unpaired electrons in d-orbitals. This results in their unique magnetic and spectroscopic properties.
25. What are some exceptions to Hund's rule?
While Hund's rule is generally reliable, there are exceptions, particularly in excited states or in some complex molecules. In these cases, other factors like orbital hybridization or molecular geometry may override Hund's rule.
26. How does Hund's rule influence the magnetic properties of atoms?
Hund's rule promotes unpaired electrons, which contribute to paramagnetism in atoms. Atoms with more unpaired electrons (following Hund's rule) tend to be more strongly paramagnetic.
27. What role does Hund's rule play in chemical bonding?
Hund's rule influences chemical bonding by affecting the number and arrangement of unpaired electrons in atoms. This impacts an atom's ability to form covalent bonds and determines its valence and reactivity.
28. What is the connection between Hund's rule and spectroscopy?
Hund's rule affects the electronic structure of atoms, which in turn influences their spectroscopic properties. The arrangement of electrons according to Hund's rule determines the possible electronic transitions, affecting absorption and emission spectra.
29. How does Hund's rule relate to the concept of exchange energy?
Exchange energy is the quantum mechanical effect that favors electrons with parallel spins. Hund's rule maximizes this exchange energy by promoting unpaired electrons with parallel spins, contributing to the stability of the atom.
30. How does Hund's rule affect the formation of molecular orbitals?
While Hund's rule primarily applies to atomic orbitals, it indirectly affects molecular orbital formation. The electron configurations determined by Hund's rule in individual atoms influence how these atoms combine to form molecular orbitals.
31. Can Hund's rule predict the color of transition metal complexes?
While Hund's rule doesn't directly predict color, it influences the number of unpaired electrons in transition metal ions. This affects the energy of d-d transitions, which are responsible for many of the colors observed in transition metal complexes.
32. What is the importance of Hund's rule in understanding ferromagnetism?
Hund's rule promotes unpaired electrons with parallel spins, which is crucial for ferromagnetism. Materials with many unpaired electrons aligned by Hund's rule are more likely to exhibit ferromagnetic properties.
33. How does Hund's rule apply to excited states of atoms?
While Hund's rule is most commonly applied to ground states, it can also influence excited states. However, in excited states, other factors may become more significant, sometimes leading to configurations that don't strictly follow Hund's rule.
34. What is the relationship between Hund's rule and electron affinity?
Hund's rule indirectly affects electron affinity. Atoms with half-filled or fully filled subshells (often a result of following Hund's rule) tend to have lower electron affinities, as adding an electron would disrupt this stable configuration.
35. How does Hund's rule influence the periodic trends in atomic properties?
Hund's rule contributes to periodic trends by affecting electron configurations. This influences trends in properties like atomic size, ionization energy, and magnetic behavior across the periodic table.
36. How does Hund's rule affect the stability of half-filled and fully-filled subshells?
Hund's rule contributes to the extra stability observed in atoms with half-filled or fully-filled subshells. This is due to the maximized exchange energy in half-filled shells and the symmetry in fully-filled shells, both of which lower the overall energy.
37. What is the significance of Hund's rule in computational chemistry?
In computational chemistry, Hund's rule is important for accurately predicting electronic structures and properties of atoms and molecules. Many computational methods incorporate Hund's rule to ensure correct electron configurations in calculations.
38. How does Hund's rule influence the magnetic moment of atoms?
Hund's rule maximizes the number of unpaired electrons in an atom, which directly affects its magnetic moment. The more unpaired electrons (with parallel spins) an atom has, the higher its magnetic moment will be.
39. What is the relationship between Hund's rule and electron correlation?
Hund's rule is related to electron correlation, which describes the interaction between electrons. By promoting configurations with unpaired electrons, Hund's rule minimizes electron-electron repulsion, which is a form of electron correlation.
40. How does Hund's rule contribute to the understanding of atomic spectra?
Hund's rule helps predict the ground state configuration of atoms, which is crucial for understanding atomic spectra. The transitions between states that follow Hund's rule and excited states give rise to the characteristic spectral lines of elements.
41. How does Hund's rule relate to the concept of electron shielding?
While Hund's rule doesn't directly address electron shielding, it affects the distribution of electrons in orbitals. This distribution influences the effective nuclear charge experienced by outer electrons, which is a key aspect of electron shielding.
42. What is the importance of Hund's rule in understanding chemical bonding in coordination compounds?
Hund's rule influences the number of unpaired electrons in transition metal ions, which affects their ability to form coordination compounds. The electron configuration determined by Hund's rule impacts the geometry, magnetic properties, and color of these compounds.
43. How does Hund's rule relate to the concept of spin-only magnetic moment?
The spin-only magnetic moment is calculated based on the number of unpaired electrons, which is often determined by Hund's rule. This concept is particularly useful for predicting the magnetic properties of many transition metal complexes.
44. What is the connection between Hund's rule and the stability of radical species?
Hund's rule can help explain the relative stability of some radical species. Radicals with unpaired electrons distributed according to Hund's rule may have lower energy and greater stability compared to those that violate the rule.
45. How does Hund's rule apply to the concept of hybridization?
While Hund's rule primarily deals with electron distribution in atomic orbitals, it can influence hybridization. The initial electron configuration determined by Hund's rule may be altered during hybridization to achieve lower energy molecular configurations.
46. What role does Hund's rule play in understanding the photoelectric effect?
Hund's rule helps determine the ground state electron configuration of atoms, which is crucial for understanding the photoelectric effect. The energy required to remove an electron (work function) is related to how the electrons are arranged according to Hund's rule.
47. How does Hund's rule contribute to the understanding of atomic size?
Hund's rule affects electron distribution, which indirectly influences atomic size. Atoms with more unpaired electrons (following Hund's rule) may have slightly larger radii due to increased electron-electron repulsion.
48. What is the significance of Hund's rule in quantum chemistry calculations?
In quantum chemistry calculations, Hund's rule is often used as a starting point for determining electron configurations. It helps in constructing initial wavefunctions and is crucial for accurate predictions of atomic and molecular properties.
49. How does Hund's rule affect the electron configuration of carbon?
In carbon, Hund's rule leads to two unpaired electrons in the 2p orbitals. This configuration (1s² 2s² 2p² with two unpaired 2p electrons) is more stable than alternatives with paired electrons in the 2p orbital.
50. How can you use an orbital diagram to illustrate Hund's rule?
An orbital diagram shows electron distribution using boxes (orbitals) and arrows (electrons). To illustrate Hund's rule, draw orbitals of equal energy side by side and fill them with single electrons (up arrows) before adding any down arrows for electron pairs.
51. Can you explain how Hund's rule applies to the lanthanide elements?
In lanthanides, Hund's rule applies to the filling of 4f orbitals. It predicts that these orbitals will be filled with one electron each before any pairing occurs, leading to many lanthanides having multiple unpaired electrons and strong paramagnetic properties.
52. Can you explain how Hund's rule applies to the electron configuration of transition metal ions?
For transition metal ions, Hund's rule dictates that electrons fill the d-orbitals singly with parallel spins before pairing. This often results in partially filled d-subshells with unpaired electrons, explaining many of the unique properties of transition metal compounds.
53. What role does Hund's rule play in determining the electron configuration of anions?
When forming anions, atoms gain electrons. Hund's rule still applies to these added electrons, influencing how they are distributed in the available orbitals and affecting the overall electronic structure and properties of the anion.
54. Can you explain how Hund's rule applies to the electron configuration of oxygen?
In oxygen, Hund's rule leads to two unpaired electrons in the 2p orbitals. The electron configuration is 1s² 2s² 2p⁴, with four electrons in the 2p orbitals arranged as two pairs and two unpaired electrons with parallel spins.
55. How does Hund's rule relate to the concept of electron promotion in excited states?
While Hund's rule primarily applies to ground states, understanding it is crucial for comprehending electron promotion. Excited states often involve configurations that deviate from Hund's rule, highlighting the energy difference between ground and excited states.
56. Can you explain how Hund's rule applies to the electron configuration of chromium?
Chromium is a notable example where Hund's rule leads to an unusual configuration. Instead of 3d⁴ 4s², one 4s electron is promoted to the 3d orbital, resulting in a 3d⁵ 4s¹ configuration. This maximizes the number of unpaired electrons, following Hund's rule.
57. What is the relationship between Hund's rule and crystal field theory?
Hund's rule and crystal field theory both influence electron distribution in transition metal complexes. While Hund's rule favors unpaired electrons, strong crystal fields can overcome this preference, leading to paired electrons and low-spin complexes.
58. How does Hund's rule contribute to our understanding of atomic and molecular orbitals?
Hund's rule is fundamental to understanding how electrons occupy atomic orbitals, which in turn forms the basis for molecular orbital theory. It helps explain the electronic structure of atoms and how these atoms combine to form molecules with specific properties.

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