Editing Molecular orbital (section)

===Bonding in molecular orbitals===

====Orbital degeneracy====
{{main|Degenerate orbital}}
Molecular orbitals are said to be degenerate if they have the same energy. For example, in the homonuclear diatomic molecules of the first ten elements, the molecular orbitals derived from the p<sub>x</sub> and the p<sub>y</sub> atomic orbitals result in two degenerate bonding orbitals (of low energy) and two degenerate antibonding orbitals (of high energy).<ref name="Gary L. Miessler 2004"/>

====Ionic bonds====
{{main|Ionic bond}}
In an ionic bond, oppositely charged [[ion]]s are bonded by [[electrostatic attraction]].<ref>{{Cite book|chapter-url=https://doi.org/10.1351/goldbook.IT07058|doi = 10.1351/goldbook.IT07058|chapter = Ionic bond|title = IUPAC Compendium of Chemical Terminology|year = 2009|isbn = 978-0-9678550-9-7}}</ref> It is possible to describe ionic bonds with molecular orbital theory by treating them as extremely [[polar bond]]s. Their bonding orbitals are very close in energy to the atomic orbitals of the [[anion]]. They are also very similar in character to the anion's atomic orbitals, which means the electrons are completely shifted to the anion. In computer diagrams, the orbitals are centered on the anion's core.<ref>{{Cite web |date=2020-08-06 |title=5.3.3: Ionic Compounds and Molecular Orbitals |url=https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Inorganic_Chemistry_(LibreTexts)/05%3A_Molecular_Orbitals/5.03%3A_Heteronuclear_Diatomic_Molecules/5.3.03%3A_Ionic_Compounds_and_Molecular_Orbitals |access-date=2024-06-06 |website=Chemistry LibreTexts |language=en}}</ref>

====Bond order====
{{main|Bond order}}
The bond order, or number of bonds, of a molecule can be determined by combining the number of electrons in bonding and antibonding molecular orbitals. A pair of electrons in a bonding orbital creates a bond, whereas a pair of electrons in an antibonding orbital negates a bond. For example, N<sub>2</sub>, with eight electrons in bonding orbitals and two electrons in antibonding orbitals, has a bond order of three, which constitutes a triple bond.

[[Bond strength]] is proportional to bond order—a greater amount of bonding produces a more stable bond—and [[bond length]] is inversely proportional to it—a stronger bond is shorter.

There are rare exceptions to the requirement of molecule having a positive bond order. Although Be<sub>2</sub> has a bond order of 0 according to MO analysis, there is experimental evidence of a highly unstable Be<sub>2</sub> molecule having a bond length of 245&nbsp;pm and bond energy of 10&nbsp;kJ/mol.<ref name = H&C /><ref>{{cite journal | last1 = Bondybey | first1 = V.E. | year = 1984 | title = Electronic structure and bonding of Be2 | journal = Chemical Physics Letters | volume = 109 | issue = 5| pages = 436–441 | doi = 10.1016/0009-2614(84)80339-5 | bibcode = 1984CPL...109..436B }}</ref>

====HOMO and LUMO====
{{main|HOMO and LUMO}}
The highest occupied molecular orbital and lowest unoccupied molecular orbital are often referred to as the HOMO and LUMO, respectively. The difference of the energies of the HOMO and LUMO is called the HOMO-LUMO gap. This notion is often the matter of confusion in literature and should be considered with caution. Its value is usually located between the fundamental gap (difference between ionization potential and electron affinity) and the optical gap. In addition, HOMO-LUMO gap can be related to a bulk material [[band gap]] or transport gap, which is usually much smaller than fundamental gap.{{cn|date=June 2022}}