Editing Periodic table (section)

=== Electron affinity ===
[[File:Electron affinity of the elements.svg|thumb|384px|right|Trend in electron affinities]]
The opposite property to ionisation energy is the [[electron affinity]], which is the energy released when adding an electron to the atom.<ref name="chemguideea" /> A passing electron will be more readily attracted to an atom if it feels the pull of the nucleus more strongly, and especially if there is an available partially filled outer orbital that can accommodate it. Therefore, electron affinity tends to increase down to up and left to right. The exception is the last column, the noble gases, which have a full shell and have no room for another electron. This gives the [[halogen]]s in the next-to-last column the highest electron affinities.<ref name="cartoon" />

Some atoms, like the noble gases, have no electron affinity: they cannot form stable gas-phase anions.<ref>{{cite journal |last1=Cárdenas |first1=Carlos |last2=Ayers |first2=Paul |first3=Frank |last3=De Proft |first4=David J. |last4=Tozer |first5=Paul |last5=Geerlings |date=2010 |title=Should negative electron affinities be used for evaluating the chemical hardness? |journal=Physical Chemistry Chemical Physics |volume=13 |issue=6 |pages=2285–2293 |doi=10.1039/C0CP01785J|pmid=21113528 }}</ref> (They can form metastable [[Resonance (particle physics)|resonances]] if the incoming electron arrives with enough kinetic energy, but these inevitably and rapidly [[Autoionization|autodetach]]: for example, the lifetime of the most long-lived He<sup>−</sup> level is about 359&nbsp;microseconds.)<ref>{{cite journal |last1=Schmidt |first1=H. T. |last2=Reinhed |first2=P. |first3=A. |last3=Orbán |first4=S. |last4=Rosén |first5=R. D. |last5=Thomas |first6=H. A. B. |last6=Johansson |first7=J. |last7=Werner |first8=D. |last8=Misra |first9=M. |last9=Björkhage |first10=L. |last10=Brännholm |first11=P. |last11=Löfgren |first12=L. |last12=Liljeby |first13=H. |last13=Cederquist |date=2012 |title=The lifetime of the helium anion |journal=Journal of Physics: Conference Series |volume=388 |issue= 1|pages=012006 |doi=10.1088/1742-6596/388/1/012006 |doi-access=free |bibcode=2012JPhCS.388a2006S }}</ref> The noble gases, having high ionisation energies and no electron affinity, have little inclination towards gaining or losing electrons and are generally unreactive.<ref name="cartoon" />

Some exceptions to the trends occur: oxygen and fluorine have lower electron affinities than their heavier homologues sulfur and chlorine, because they are small atoms and hence the newly added electron would experience significant repulsion from the already present ones. For the nonmetallic elements, electron affinity likewise somewhat correlates with reactivity, but not perfectly since other factors are involved. For example, fluorine has a lower electron affinity than chlorine (because of extreme interelectronic repulsion for the very small fluorine atom), but is more reactive.<ref name="chemguideea">{{cite web |url=https://www.chemguide.co.uk/atoms/properties/eas.html |title=Electron Affinity |last=Clark |first=Jim |date=2012 |website=Chemguide |access-date=30 March 2021 |archive-date=23 April 2021 |archive-url=https://web.archive.org/web/20210423195854/https://www.chemguide.co.uk/atoms/properties/eas.html |url-status=live }}</ref>