Editing Electron (section)

=== Atoms and molecules ===
{{Main|Atom}}
[[File:Hydrogen Density Plots.png|right|thumb|upright=1.25|alt=A table of five rows and five columns, with each cell portraying a color-coded probability density|Probability densities for the first few hydrogen atom orbitals, seen in cross-section. The energy level of a bound electron determines the orbital it occupies, and the color reflects the probability of finding the electron at a given position.]]
An electron can be ''bound'' to the nucleus of an atom by the attractive Coulomb force. A system of one or more electrons bound to a nucleus is called an atom. If the number of electrons is different from the nucleus's electrical charge, such an atom is called an [[ion]]. The wave-like behavior of a bound electron is described by a function called an [[atomic orbital]]. Each orbital has its own set of quantum numbers such as energy, angular momentum and projection of angular momentum, and only a discrete set of these orbitals exist around the nucleus. According to the Pauli exclusion principle each orbital can be occupied by up to two electrons, which must differ in their [[spin quantum number]].

Electrons can transfer between different orbitals by the emission or absorption of photons with an energy that matches the difference in potential.<ref name=Tipler2003 />{{rp|159–160}} Other methods of orbital transfer include collisions with particles, such as electrons, and the [[Auger effect]].<ref>{{cite book
 | last = Burhop | first = E.H.S.
 | author-link = Eric Burhop
 | year = 1952
 | title = The Auger Effect and Other Radiationless Transitions
 | publisher = Cambridge University Press
 | pages = 2–3
 | isbn = 978-0-88275-966-1
}}</ref> To escape the atom, the energy of the electron must be increased above its [[Ionization energy|binding energy]] to the atom. This occurs, for example, with the [[photoelectric effect]], where an incident photon exceeding the atom's [[ionization energy]] is absorbed by the electron.<ref name=Tipler2003>{{cite book
 | last1=Tipler | first1=Paul
 | last2=Llewellyn | first2=Ralph
 | title = Modern Physics
 | publisher=Macmillan | year=2003 | edition=illustrated
 | isbn=978-0-7167-4345-3
}}</ref>{{rp|127–132}}

The orbital angular momentum of electrons is [[Angular momentum operator#Quantization|quantized]]. Because the electron is charged, it produces an orbital magnetic moment that is proportional to the angular momentum. The net magnetic moment of an atom is equal to the vector sum of orbital and spin magnetic moments of all electrons and the nucleus. The magnetic moment of the nucleus is negligible compared with that of the electrons. The magnetic moments of the electrons that occupy the same orbital (so called, paired electrons) cancel each other out.<ref>{{cite book
 | last = Jiles
 | first = D.
 | title = Introduction to Magnetism and Magnetic Materials
 | url = https://books.google.com/books?id=axyWXjsdorMC&pg=PA280
 | pages = 280–287
 | publisher = [[CRC Press]]
 | year = 1998
 | isbn = 978-0-412-79860-3
 | access-date = 2020-08-25
 | archive-date = 2021-01-26
 | archive-url = https://web.archive.org/web/20210126003325/https://books.google.com/books?id=axyWXjsdorMC&pg=PA280
 | url-status = live
 }}</ref>

The [[chemical bond]] between atoms occurs as a result of electromagnetic interactions, as described by the laws of quantum mechanics.<ref>{{cite book
 |last1        = Löwdin
 |first1       = P.O.
 |last2        = Erkki Brändas
 |first2       = E.
 |last3        = Kryachko
 |first3       = E.S.
 |title        = Fundamental World of Quantum Chemistry: A Tribute to the Memory of Per-Olov Löwdin
 |url          = https://books.google.com/books?id=8QiR8lCX_qcC&pg=PA393
 |pages        = 393–394
 |publisher    = Springer Science+Business Media
 |year         = 2003
 |isbn         = 978-1-4020-1290-7
 |access-date  = 2020-08-25
 |archive-date = 2022-02-04
 |archive-url  = https://web.archive.org/web/20220204071147/https://books.google.com/books?id=8QiR8lCX_qcC&pg=PA393
 |url-status   = live
}}</ref> The strongest bonds are formed by the [[Covalent bond|sharing]] or [[Electron transfer|transfer]] of electrons between atoms, allowing the formation of [[molecule]]s.<ref name=Pauling>{{cite book
 | last = Pauling | first = L.C.
 | title = The Nature of the Chemical Bond and the Structure of Molecules and Crystals: an introduction to modern structural chemistry
 | url = https://archive.org/details/natureofchemical0000paul_3ed/page/4
 | url-access = registration | pages =4–10
 | publisher = Cornell University Press | edition = 3rd | year = 1960
 | isbn = 978-0-8014-0333-0
}}</ref> Within a molecule, electrons move under the influence of several nuclei, and occupy [[molecular orbital]]s; much as they can occupy atomic orbitals in isolated atoms.<ref>{{cite book
 | last1 = McQuarrie
 | first1 = D.A.
 | last2 = Simon
 | first2 = J.D.
 | title = Physical Chemistry: A Molecular Approach
 | url = https://books.google.com/books?id=f-bje0-DEYUC&pg=PA325
 | publisher = University Science Books
 | year = 1997
 | pages = 325–361
 | isbn = 978-0-935702-99-6
 | access-date = 2020-08-25
 | archive-date = 2021-01-07
 | archive-url = https://web.archive.org/web/20210107160307/https://books.google.com/books?id=f-bje0-DEYUC&pg=PA325
 | url-status = live
 }}</ref> A fundamental factor in these molecular structures is the existence of [[electron pair]]s. These are electrons with opposed spins, allowing them to occupy the same molecular orbital without violating the Pauli exclusion principle (much like in atoms). Different molecular orbitals have different spatial distribution of the electron density. For instance, in bonded pairs (i.e. in the pairs that actually bind atoms together) electrons can be found with the maximal probability in a relatively small volume between the nuclei. By contrast, in non-bonded pairs electrons are distributed in a large volume around nuclei.<ref>{{cite journal
 |last=Daudel |first=R.
 |year=1974
 |title=The Electron Pair in Chemistry
 |journal=[[Canadian Journal of Chemistry]]
 |volume=52 |issue=8 |pages=1310–1320
 |doi=10.1139/v74-201
 |display-authors=etal
|doi-access=free
 }}</ref>