Editing Periodic table (section)

=== Atomic radius ===
Historically, the physical size of atoms was unknown until the early 20th century. The first calculated estimate of the atomic radius of hydrogen was published by physicist [[Arthur Erich Haas|Arthur Haas]] in 1910 to within an order of magnitude (a factor of 10) of the accepted value, the [[Bohr radius]] (~0.529 Å). In his model, Haas used a single-electron configuration based on the classical atomic model proposed by [[J. J. Thomson]] in 1904, often called the [[plum-pudding model]].<ref>Haas, Arthur Erich (1884–1941) Uber die elektrodynamische Bedeutung des Planckschen Strahlungsgesetzes und uber eine neue Bestimmung des elektrischen Elementarquantums und der dimension des wasserstoffatoms. Sitzungsberichte der kaiserlichen Akademie der Wissenschaften in Wien. 2a, 119 pp 119–144 (1910). Haas AE. Die Entwicklungsgeschichte des Satzes von der Erhaltung der Kraft. Habilitation Thesis, Vienna, 1909. Hermann, A. Arthur Erich Haas, Der erste Quantenansatz für das Atom. Stuttgart, 1965 [contains a reprint]</ref>

[[Atomic radius|Atomic radii]] (the size of atoms) are dependent on the sizes of their outermost orbitals.<ref name=SB23>Siekierski and Burgess, pp. 23–26</ref> They generally decrease going left to right along the main-group elements, because the nuclear charge increases but the outer electrons are still in the same shell. However, going down a column, the radii generally increase, because the outermost electrons are in higher shells that are thus further away from the nucleus.<ref name="cartoon" /><ref name="chemguidear">{{cite web |url=https://www.chemguide.co.uk/atoms/properties/atradius.html |title=Atomic and Ionic Radius |last=Clark |first=Jim |date=2012 |website=Chemguide |access-date=30 March 2021 |archive-date=14 November 2020 |archive-url=https://web.archive.org/web/20201114002613/https://www.chemguide.co.uk/atoms/properties/atradius.html |url-status=live }}</ref> The first row of each block is abnormally small, due to an effect called [[kainosymmetry]] or primogenic repulsion:<ref>{{cite journal |last1=Cao |first1=Chang-Su |last2=Hu |first2=Han-Shi |last3=Li |first3=Jun |last4=Schwarz |first4=W. H. Eugen |date=2019 |title=Physical origin of chemical periodicities in the system of elements |journal=Pure and Applied Chemistry |volume=91 |issue=12 |pages=1969–1999 |doi=10.1515/pac-2019-0901 |s2cid=208868546 |doi-access=free }}</ref> the 1s, 2p, 3d, and 4f subshells have no inner analogues. For example, the 2p orbitals do not experience strong repulsion from the 1s and 2s orbitals, which have quite different angular charge distributions, and hence are not very large; but the 3p orbitals experience strong repulsion from the 2p orbitals, which have similar angular charge distributions. Thus higher s-, p-, d-, and f-subshells experience strong repulsion from their inner analogues, which have approximately the same angular distribution of charge, and must expand to avoid this. This makes significant differences arise between the small 2p elements, which prefer [[multiple bond]]ing, and the larger 3p and higher p-elements, which do not.<ref name=SB23/> Similar anomalies arise for the 1s, 2p, 3d, 4f, and the hypothetical {{Not a typo|5g}} elements:<ref name="Kaupp">{{cite journal |last=Kaupp |first=Martin |date=1 December 2006 |title=The role of radial nodes of atomic orbitals for chemical bonding and the periodic table |journal=Journal of Computational Chemistry |volume=28 |issue=1 |pages=320–25 |doi=10.1002/jcc.20522 |pmid=17143872 |s2cid=12677737 |doi-access=free }}</ref> the degree of this first-row anomaly is highest for the s-block, is moderate for the p-block, and is less pronounced for the d- and f-blocks.<ref name="PTSS2" />

In the transition elements, an inner shell is filling, but the size of the atom is still determined by the outer electrons. The increasing nuclear charge across the series and the increased number of inner electrons for shielding somewhat compensate each other, so the decrease in radius is smaller.<ref name="chemguidear" /> The 4p and 5d atoms, coming immediately after new types of transition series are first introduced, are smaller than would have been expected,<ref name="Greenwood29">Greenwood and Earnshaw, p. 29</ref> because the added core 3d and 4f subshells provide only incomplete shielding of the nuclear charge for the outer electrons. Hence for example gallium atoms are slightly smaller than aluminium atoms.<ref name=SB23/> Together with kainosymmetry, this results in an even-odd difference between the periods (except in the s-block){{efn|Properties of the p-block elements nevertheless do affect the succeeding s-block elements. The 3s shell in sodium is above a kainosymmetric 2p core, but the 4s shell in potassium is above the much larger 3p core. Hence while one would have already expected potassium atoms to be larger than sodium atoms, the size difference is greater than usual.<ref name=SB23/>}} that is sometimes known as secondary periodicity: elements in even periods have smaller atomic radii and prefer to lose fewer electrons, while elements in odd periods (except the first) differ in the opposite direction. Thus for example many properties in the p-block show a zigzag rather than a smooth trend along the group. For example, phosphorus and antimony in odd periods of group 15 readily reach the +5 oxidation state, whereas nitrogen, arsenic, and bismuth in even periods prefer to stay at +3.<ref name="PTSS2" /><ref>{{cite journal |last1=Imyanitov |first1=Naum S. |date=2018 |title=Is the periodic table appears doubled? Two variants of division of elements into two subsets. Internal and secondary periodicity |url= |journal=Foundations of Chemistry |volume=21 |issue= |pages=255–284 |doi=10.1007/s10698-018-9321-z |s2cid=254514910 |access-date=}}</ref> A similar situation holds for the d-block, with lutetium through tungsten atoms being slightly smaller than yttrium through molybdenum atoms respectively.<ref>{{cite journal |last1=Chistyakov |first1=V. M. |date=1968 |title=Biron's Secondary Periodicity of the Side d-subgroups of Mendeleev's Short Table |url=https://archive.org/details/sim_russian-journal-of-general-chemistry_1968-02_38_2/page/212/mode/2up |journal=Journal of General Chemistry of the USSR |volume=38 |issue=2 |pages=213–214 |doi= |access-date=6 January 2024}}</ref><ref name="Calc1">{{cite journal|author1=P. Pyykkö|author2=M. Atsumi|year=2009|title=Molecular Single-Bond Covalent Radii for Elements 1-118|journal=Chemistry: A European Journal|volume=15|issue=1|pages=186–197|doi=10.1002/chem.200800987|pmid=19058281}}</ref>

[[File:Pouring liquid mercury bionerd.jpg|thumb|right|Liquid mercury. Its liquid state at standard conditions is the result of relativistic effects.<ref name=PekkaPyykko/>]]
Thallium and lead atoms are about the same size as indium and tin atoms respectively, but from bismuth to radon the 6p atoms are larger than the analogous 5p atoms. This happens because when atomic nuclei become highly charged, [[special relativity]] becomes needed to gauge the effect of the nucleus on the electron cloud. These [[relativistic quantum chemistry|relativistic effects]] result in heavy elements increasingly having differing properties compared to their lighter homologues in the periodic table. [[Spin–orbit interaction]] splits the p&nbsp;subshell: one p&nbsp;orbital is relativistically stabilized and shrunken (it fills in thallium and lead), but the other two (filling in bismuth through radon) are relativistically destabilized and expanded.<ref name=SB23/> Relativistic effects also explain why [[gold]] is golden and [[mercury (element)|mercury]] is a liquid at room temperature.<ref name="PekkaPyykko">{{cite journal |doi=10.1021/ar50140a002 |title=Relativity and the periodic system of elements |year=1979 |last1=Pyykkö |first1=Pekka |last2=Desclaux |first2=Jean Paul |journal=Accounts of Chemical Research |volume=12 |issue=8 |page=276}}</ref><ref name="Norrby">{{cite journal |doi=10.1021/ed068p110 |title=Why is mercury liquid? Or, why do relativistic effects not get into chemistry textbooks? |year=1991 |last1=Norrby |first1=Lars J. |journal=Journal of Chemical Education |volume=68 |issue=2 |page=110 |bibcode = 1991JChEd..68..110N}}</ref> They are expected to become very strong in the late seventh period, potentially leading to a collapse of periodicity.<ref name=actrev/> Electron configurations are only clearly known until element 108 ([[hassium]]), and experimental chemistry beyond 108 has only been done for elements 112 ([[copernicium]]) through 115 ([[moscovium]]), so the chemical characterization of the heaviest elements remains a topic of current research.<ref name="Schändel 2003 277">{{cite book|title=The Chemistry of Superheavy Elements|last=Schädel|first=M.|year=2003|publisher=Kluwer Academic Publishers|location=Dordrecht|isbn=978-1-4020-1250-1|page=277}}</ref><ref name=moscovium>{{cite journal |last1=Yakushev |first1=A. |last2=Khuyagbaatar |first2=J. |first3=Ch. E. |last3=Düllmann |first4=M. |last4=Block |first5=R. A. |last5=Cantemir |first6=D. M. |last6=Cox |first7=D. |last7=Dietzel |first8=F. |last8=Giacoppo |first9=Y. |last9=Hrabar |first10=M. |last10=Iliaš |first11=E. |last11=Jäger |first12=J. |last12=Krier |first13=D. |last13=Krupp |first14=N. |last14=Kurz |first15=L. |last15=Lens |first16=S. |last16=Löchner |first17=Ch. |last17=Mokry |first18=P. |last18=Mošať |first19=V. |last19=Pershina |first20=S. |last20=Raeder |first21=D. |last21=Rudolph |first22=J. |last22=Runke |first23=L. G. |last23=Sarmiento |first24=B. |last24=Schausten |first25=U. |last25=Scherer |first26=P. |last26=Thörle-Pospiesch |first27=N. |last27=Trautmann |first28=M. |last28=Wegrzecki |first29=P. |last29=Wieczorek |date=23 September 2024 |title=Manifestation of relativistic effects in the chemical properties of nihonium and moscovium revealed by gas chromatography studies |journal=Frontiers in Chemistry |volume=12 |issue= |pages= |doi=10.3389/fchem.2024.1474820 |doi-access=free |pmid=39391836 |pmc=11464923 |bibcode=2024FrCh...1274820Y }}</ref>

The trend that atomic radii decrease from left to right is also present in [[ionic radius|ionic radii]], though it is more difficult to examine because the most common ions of consecutive elements normally differ in charge. Ions with the same electron configuration decrease in size as their atomic number rises, due to increased attraction from the more positively charged nucleus: thus for example ionic radii decrease in the series Se<sup>2−</sup>, Br<sup>−</sup>, Rb<sup>+</sup>, Sr<sup>2+</sup>, Y<sup>3+</sup>, Zr<sup>4+</sup>, Nb<sup>5+</sup>, Mo<sup>6+</sup>, Tc<sup>7+</sup>. Ions of the same element get smaller as more electrons are removed, because the attraction from the nucleus begins to outweigh the repulsion between electrons that causes electron clouds to expand: thus for example ionic radii decrease in the series V<sup>2+</sup>, V<sup>3+</sup>, V<sup>4+</sup>, V<sup>5+</sup>.<ref>Wulfsberg, pp. 33–34</ref>