Editing Periodic table (section)

=== Electronegativity ===
[[Image:Electrostatic Potential.jpg|thumb|alt=A water molecule is put into a see-through egg shape, which is colour-coded by electrostatic potential. A concentration of red is near the top of the shape, where the oxygen atom is, and gradually shifts through yellow, green, and then to blue near the lower-right and lower-left corners of the shape where the hydrogen atoms are.|upright=1.5|right|Electrostatic potential map of a water molecule, where the oxygen atom has a more negative charge (red) than the positive (blue) hydrogen atoms]]
Another important property of elements is their [[electronegativity]]. Atoms can form [[covalent bond]]s to each other by sharing electrons in pairs, creating an overlap of valence orbitals. The degree to which each atom attracts the shared electron pair depends on the atom's electronegativity<ref name="Greenwood25" /> – the tendency of an atom towards gaining or losing electrons.<ref name="cartoon" /> The more electronegative atom will tend to attract the electron pair more, and the less electronegative (or more electropositive) one will attract it less. In extreme cases, the electron can be thought of as having been passed completely from the more electropositive atom to the more electronegative one, though this is a simplification. The bond then binds two ions, one positive (having given up the electron) and one negative (having accepted it), and is termed an [[ionic bond]].<ref name="cartoon" />

Electronegativity depends on how strongly the nucleus can attract an electron pair, and so it exhibits a similar variation to the other properties already discussed: electronegativity tends to fall going up to down, and rise going left to right. The alkali and alkaline earth metals are among the most electropositive elements, while the chalcogens, halogens, and noble gases are among the most electronegative ones.<ref name="Greenwood25" />

Electronegativity is generally measured on the Pauling scale, on which the most electronegative reactive atom ([[fluorine]]) is given electronegativity 4.0, and the least electronegative atom ([[caesium]]) is given electronegativity 0.79.<ref name="cartoon" /> In fact [[neon]] is the most electronegative element, but the Pauling scale cannot measure its electronegativity because it does not form covalent bonds with most elements.<ref>{{cite journal |doi=10.1021/ja00207a003 |title=Electronegativity is the average one-electron energy of the valence-shell electrons in ground-state free atoms|year=1989|author=Allen, Leland C.|journal=Journal of the American Chemical Society |volume=111|pages=9003–9014 |issue=25|bibcode=1989JAChS.111.9003A }}</ref>

An element's electronegativity varies with the identity and number of the atoms it is bonded to, as well as how many electrons it has already lost: an atom becomes more electronegative when it has lost more electrons.<ref name="Greenwood25">Greenwood and Earnshaw, pp. 25–6</ref> This sometimes makes a large difference: lead in the +2 oxidation state has electronegativity 1.87 on the Pauling scale, while lead in the +4 oxidation state has electronegativity 2.33.<ref>{{cite book |last1=Dieter |first1=R. K. |last2=Watson |first2=R. T. |chapter=Transmetalation reactions producing organocopper compounds |pages=443–526 |editor-last1=Rappoport |editor-first1=Z. |editor-last2=Marek |editor-first2=I. |title=The Chemistry of Organocopper Compounds |volume=1 |year=2009 |publisher=John Wiley & Sons |isbn=978-0-470-77296-6 |chapter-url=https://books.google.com/books?id=263AXB0Q6tAC |access-date=6 April 2022 |archive-date=17 October 2022 |archive-url=https://web.archive.org/web/20221017193845/https://books.google.com/books?id=263AXB0Q6tAC |url-status=live }}<!--specifically page 509--></ref>