Editing Periodic table (section)

=== Ionisation energy ===
[[File:First Ionization Energy blocks.svg|thumb|right|512px|Graph of first ionisation energies of the elements in electronvolts (predictions used for elements 109–118)]]
The first [[ionisation energy]] of an atom is the energy required to remove an electron from it. This varies with the atomic radius: ionisation energy increases left to right and down to up, because electrons that are closer to the nucleus are held more tightly and are more difficult to remove. Ionisation energy thus is minimized at the first element of each period – hydrogen and the [[alkali metal]]s – and then generally rises until it reaches the [[noble gas]] at the right edge of the period.<ref name="cartoon" /> There are some exceptions to this trend, such as oxygen, where the electron being removed is paired and thus interelectronic repulsion makes it easier to remove than expected.<ref name="Greenwood294">Greenwood and Earnshaw, pp. 24–5</ref>

In the transition series, the outer electrons are preferentially lost even though the inner orbitals are filling. For example, in the 3d series, the 4s electrons are lost first even though the 3d orbitals are being filled. The shielding effect of adding an extra 3d electron approximately compensates the rise in nuclear charge, and therefore the ionisation energies stay mostly constant, though there is a small increase especially at the end of each transition series.<ref name="chemguideIE">{{cite web |url=https://www.chemguide.co.uk/atoms/properties/ies.html |title=Ionisation Energy |last=Clark |first=Jim |date=2016 |website=Chemguide |access-date=30 March 2021 |archive-date=22 April 2021 |archive-url=https://web.archive.org/web/20210422032340/https://www.chemguide.co.uk/atoms/properties/ies.html |url-status=live }}</ref>

As metal atoms tend to lose electrons in chemical reactions, ionisation energy is generally correlated with chemical reactivity, although there are other factors involved as well.<ref name="chemguideIE" />