# Periodic table The **periodic table** is the arrangement of the [[Chemical_element|chemical elements]] in order of increasing [[Atomic_number|atomic number]] Z, laid out in rows called periods and columns called groups so that elements with similar chemical behaviour fall beneath one another.[^openstax-table] It has seven periods and eighteen groups, and since the naming of nihonium, moscovium, tennessine and oganesson on November 30, 2016, its seventh row has been complete at 118 elements.[^iupac2016] The columns are the visible consequence of the [[Electron_configuration|electron configuration]]: elements in one group have the same arrangement of outer electrons, and the lengths of the periods, 2, 8, 8, 18, 18, 32 and 32, are the numbers of electrons it takes to fill each new set of subshells in the order the [[Aufbau_principle|Aufbau principle]] prescribes.[^bohr-nobel-lecture] In the microsim below the table becomes a landscape: the 118 tiles rise to a height set by one property that the reader selects, atomic radius, first [[Ionization_energy|ionization energy]], [[Electronegativity|electronegativity]] or electron affinity, and a Z marker that the reader slides along a period and down a group reads out the tile's values together with the trend rule, radius falling across a period and rising down a group, ionization energy the reverse.[^openstax-trends] The one equation on the display is the effective nuclear charge, Z_eff = Z − S, the number that shapes the whole landscape.[^openstax-trends] On the Chemistry flagship this article serves Part II — Modern principles › Matter, at the section *The periodic table and its trends* (row K9); it follows the Aufbau principle, whose Z control it inherits, and it precedes the compound, where the trends decide which elements give up electrons and which take them. ## Structure The table's rows are periods, numbered 1 to 7, and its columns are groups, numbered 1 to 18 from left to right.[^openstax-table] Each period begins where a new s subshell opens and ends at a [[Noble_gas|noble gas]] where the p subshell of the same shell closes, and the width of the period is the capacity of the subshells that fill within it, 2 for period 1, 8 for periods 2 and 3, 18 for periods 4 and 5 and 32 for periods 6 and 7.[^openstax-config] The 14-element [[Lanthanide|lanthanide]] and [[Actinide|actinide]] rows are usually printed below the main body to keep the table narrow. ### Group names and numbers Several groups carry names older than their numbers: group 1 is the [[Alkali_metal|alkali metals]], group 2 the [[Alkaline_earth_metal|alkaline earth metals]], group 15 the pnictogens, group 16 the chalcogens, group 17 the [[Halogen|halogens]] and group 18 the noble gases.[^openstax-table] Groups 3 to 12 hold the [[Transition_metal|transition metals]], and the lanthanides and actinides are the inner transition metals.[^openstax-table] ### Presentation forms The common 18-column form is a compromise between width and readability; the 32-column form keeps the f-block in place, with period 6 printed as one unbroken row of 32 tiles. Charles Janet's left-step table moves the s-block to the right-hand edge, so that each period ends at an s² element and the periods have lengths 2, 2, 8, 8, 18, 18, 32, 32, the groupings of the Madelung rule.[^scerri2007] The microsim's landscape is the 18-column form with the f-block set below, because the Z marker has to move along periods that read left to right. ### Electron configurations Group 1 atoms end in s¹, group 2 in s², groups 13 to 18 in s²p¹ to s²p⁶, and the transition metals fill d subshells one electron at a time between groups 3 and 12.[^openstax-config] The [[Block_(periodic_table)|blocks]] of the table, s, p, d and f, are named for the subshell being filled, and their widths are the subshell capacities 2, 6, 10 and 14.[^b054-capacity] The exceptions to the filling order, chromium and copper in period 4 and more in periods 5 and 6, are treated in the Aufbau article. ### Electron configuration table The full table of ground-state configurations is a page of its own, [[Periodic_table_(electron_configurations)]]. What the periodic table needs from it is the block summary: | Block | Subshell filling | Groups | Capacity | Periods | |---|---|---|---|---| | s | ns | 1–2 (and He) | 2 | 1–7 | | p | np | 13–18 | 6 | 2–7 | | d | (n − 1)d | 3–12 | 10 | 4–7 | | f | (n − 2)f | lanthanides, actinides | 14 | 6–7 | The capacities are the 2(2ℓ + 1) of the Aufbau rule; "(n − 1)" and "(n − 2)" mark subshells of an inner shell.[^b054-capacity] ## Variations Two placements are still argued, and both arise where a configuration rule and a chemical similarity point in different directions. ### Period 1 [[Hydrogen|Hydrogen]] has one s electron like the alkali metals and is one electron short of a closed shell like the halogens; it is usually printed above [[Lithium|lithium]], sometimes above fluorine, and sometimes floated above the table on its own.[^scerri2007] [[Helium|Helium]] has the s² configuration of group 2 but the inertness of group 18, and every ordinary table puts it above neon; only the left-step table, which follows configuration strictly, places it above [[Beryllium|beryllium]].[^scerri2007] ### Group 3 Whether group 3 continues from [[Scandium|scandium]] and [[Yttrium|yttrium]] to [[Lanthanum|lanthanum]] and [[Actinium|actinium]] or to [[Lutetium|lutetium]] and [[Lawrencium|lawrencium]] depends on where the f-block is cut. A 2021 provisional report of an IUPAC project chaired by Eric Scerri favours Sc, Y, Lu and Lr, on the grounds that this keeps the f-block at fourteen elements, the capacity of an f subshell, and avoids splitting the d-block into two uneven pieces; the report is explicit that the choice involves a degree of convention.[^scerri2021] ## Periodic trends The trends are the sim's subject. The landscape is built from a table of measured values, one per element, and the reader picks which property sets the height; nothing in it is calculated from theory, so the display is a picture of data. What explains the picture is the effective nuclear charge. An outer electron feels the full nuclear charge Z reduced by the screening S of the electrons inside it, Z_eff = Z − S; across a period Z rises by one at each step while the new electron enters the same shell and screens poorly, so Z_eff climbs, and down a group the outer electron sits in a new shell, farther out and better screened.[^openstax-trends] The hydrogen-like scalings r ∝ n²/Z_eff and IE ∝ Z_eff²/n² turn this into the two trend lines the sim draws beside the landscape; they are ILLUSTRATIVE, a display fit that reproduces the direction of each trend, not a measured law, and the reader is told so on the HUD.[^b054-radius] ### Atomic radius Covalent radius falls across a period as Z_eff pulls the same shell inward and rises down a group as each new shell is added.[^openstax-trends] The halogens make the group trend a clean staircase: [[Fluorine|fluorine]] 64 pm, [[Chlorine|chlorine]] 99 pm, [[Bromine|bromine]] 114 pm, [[Iodine|iodine]] 133 pm and astatine 148 pm.[^openstax-trends] On the landscape the alkali metals of the left edge are the peaks and the noble gases of the right edge are the floor of each period, and the whole surface tilts downward toward the top right corner. ### Ionisation energy The first ionization energy runs the other way, low at the left and bottom, high at the right and top, because a strongly held outer electron is expensive to remove.[^openstax-trends] The extremes among the natural elements are helium at 24.587 eV, about 2,372 kJ/mol, and [[Caesium|caesium]] at 3.894 eV, about 376 kJ/mol.[^nist-srd111] Two details of the surface matter. Across a period the rise is not smooth: [[Boron|boron]] lies below beryllium because its outermost electron is a 2p electron, less tightly held than beryllium's 2s.[^openstax-trends] And successive ionization energies show the shell structure directly: potassium's first ionization costs 418.8 kJ/mol and its second, which reaches into the closed argon core, 3,051.8 kJ/mol; for calcium the numbers are 589.8, 1,145.4 and then 4,912.4 kJ/mol, the jump arriving after the two 4s electrons are gone.[^openstax-ie-table] For [[Magnesium|magnesium]] the same jump, 738 and 1,451 kJ/mol and then 7,733 kJ/mol, is the reason the [[Born–Haber_cycle|Born–Haber cycle]] on the flagship's next row stops at MgCl₂.[^b054-mg] ### Electron affinity Electron affinity is the energy change when a gaseous atom accepts an electron; it is most negative, that is most exothermic, for the halogens, and chlorine's −348 kJ/mol is the most negative of all, more negative than fluorine's −322 kJ/mol because fluorine's small 2p shell is crowded.[^openstax-trends] The bonding textbook uses −349 kJ/mol for chlorine in its lattice cycles, a rounding difference between the two books.[^b054-ea] The sign convention varies between texts, and the sim states which it uses. ### Valence and oxidation states For the main-group elements the group number fixes the number of valence electrons, 1 and 2 for groups 1 and 2 and 3 to 8 for groups 13 to 18, which is why the valence-electron count of a molecule such as SiH₄ is 8, silicon's 4 plus four from hydrogen.[^b054-valence] The common [[Oxidation_state|oxidation states]] follow: +1 and +2 on the left, −1 and −2 on the right, and a spread of values in the transition metals whose d electrons can be lost one at a time. ### Electronegativity Electronegativity, an atom's pull on the electrons in a bond, rises across a period and falls down a group, so that fluorine is the most electronegative element and the heavy alkali metals the least.[^openstax-trends] The difference between two bonded atoms sorts the bond: on the Pauling scale the bonding textbook gives ΔEN = 0 for H–H, 0.9 for H–Cl and N–H, 1.9 for H–F, which it calls polar covalent, and 2.1 for Na–Cl, which is ionic.[^b054-en] On the landscape it is the one property that has no value for most of the noble gases. ### Metallicity Metals are the shiny, malleable conductors of heat and electricity; nonmetals are dull and poor conductors; metalloids conduct moderately.[^openstax-table] Metallic character falls across a period and rises down a group, the mirror of ionization energy, so the stair-step line that separates metals from nonmetals runs diagonally from boron to astatine. ### Further manifestations of periodicity Melting points, densities, ionic radii and the formulas of oxides and hydrides all repeat with the period, and the same Z_eff argument accounts for most of them. Relativistic effects, which grow roughly as Z², contract the 6s shell and are the textbook explanation of why [[Gold|gold]] is yellow and [[Mercury_(element)|mercury]] is liquid; by the seventh period they reorder the subshells themselves.[^pyykko2011] ## Classification of elements The elements are sorted in two ways at once. By block, an element is an s-, p-, d- or f-block element according to the subshell its last electron entered; groups 1, 2 and 13 to 18 are the main-group or representative elements, groups 3 to 12 the transition metals, and the two f rows the inner transition metals.[^openstax-table] By behaviour, an element is a metal, a [[Nonmetal|nonmetal]] or a [[Metalloid|metalloid]], the last a small diagonal band, boron, silicon, germanium, arsenic, antimony and tellurium, whose members conduct moderately and whose oxides can be either acidic or basic.[^openstax-table] The two schemes disagree at the edges: hydrogen is an s-block nonmetal, and the heavier p-block members tin, lead and bismuth are metals. The microsim colours its tiles by block and outlines the metal–nonmetal boundary, so that the reader can check, by switching properties, that the boundary runs where the ionization-energy and electronegativity landscapes begin to climb. ## History The table is the product of two centuries of sorting, and each stage added a physical reason for a pattern that had first been noticed as a regularity. ### Early history [[Antoine_Lavoisier|Antoine Lavoisier]]'s treatise of 1789 listed the substances then regarded as elementary.[^scerri2007] In 1829 Johann Döbereiner grouped elements in triads whose middle member had the mean atomic weight of the other two, and in 1865 John Newlands, ordering the elements by atomic weight, noticed that properties recurred at every eighth element, his "law of octaves".[^scerri2007] Neither scheme survived the elements it could not place. ### Mendeleev [[Dmitri_Mendeleev|Dmitri Mendeleev]] in 1869 and Lothar Meyer in 1870 independently arranged the elements by increasing atomic mass and recognised the periodic recurrence of their properties.[^openstax-table] Mendeleev's advantage was nerve: he left gaps for elements that had not been found and predicted the properties of the elements that would fill them, including one resembling aluminium and one resembling silicon,[^openstax-table] which were discovered as gallium in 1875 and germanium in 1886 with properties close to his predictions.[^scerri2007] ### Atomic number Mendeleev had ordered by mass and had to reverse a few pairs to keep the chemistry right. Henry Moseley's X-ray measurements of 1913 showed that the square root of the frequency of an element's Kα line rises linearly with an integer, the atomic number, and that this integer, not the mass, is the true ordering variable; his plot also exposed the gaps that later elements would fill.[^moseley1913][^openstax-table] The integer was identified with the nuclear charge of [[Ernest_Rutherford|Rutherford]]'s atom. ### Electron shells [[Niels_Bohr|Niels Bohr]]'s building-up scheme of 1921 to 1923 explained the period lengths as the closing of electron groups, and his Nobel lecture of December 11, 1922, set out the correspondence between the groups and the table.[^bohr-nobel-lecture] Wolfgang Pauli's exclusion principle of 1925 supplied the group sizes, 2n² per shell.[^pauli1925] In 1945 Glenn Seaborg proposed that the elements from actinium onward form an actinide series filling the 5f subshell, in parallel with the lanthanides; the proposal moved the heavy elements out of the d-block and gave the table its modern two-row footer.[^seaborg1951] ### Synthetic elements Every element beyond uranium, and technetium and promethium within the natural range, was first made rather than found.[^scerri2007] The seventh period was completed by nuclear synthesis, and the names of its last four members, nihonium, moscovium, tennessine and oganesson, were approved by IUPAC on November 30, 2016.[^iupac2016] ## Future extension beyond the seventh period An eighth period would open a 5g subshell and, by the Madelung rule, hold 50 elements. Pekka Pyykkö's 2011 Dirac–Fock calculations of atoms and ions up to Z = 172 suggest that relativistic effects scramble that order: the 8p and 9s subshells intrude among the 5g and 6f, so that a table extended past 118 would not simply repeat the pattern of period 7.[^pyykko2011] Whether such nuclei can be made and held long enough to have chemistry is a separate question that the calculations do not answer. On the microsim's landscape the eighth period does not exist: the data table stops at Z = 118 because the radius, ionization energy and electronegativity that set the tile heights have not been measured for any element beyond the seventh period, and the shape of the surface there is prediction, not data. The table's completion in 2016 is therefore a pause rather than an end, and the next tile, if it comes, will be named before it is measured. ## Alternative periodic tables Hundreds of alternative tables exist, and Scerri's history catalogues them: the left-step table, spiral and helical forms in which the periods wind around a cylinder, and three-dimensional models that place the blocks on separate planes.[^scerri2007] All contain the same information, because the atomic number and the electron configuration are fixed; they differ in which relationship they put next to which. The microsim's landscape is one more of them, a table whose third dimension is a measured property. ## See also - [[Periodic_trends]] - [[Ionization_energy]] - [[Electronegativity]] - [[Group_(periodic_table)]] - [[Period_(periodic_table)]] - [[Block_(periodic_table)]] - [[Aufbau_principle]] - [[Chemical_element]] ## Notes Explanatory notes are folded into the footnotes under References. ## References [^openstax-table]: Flowers, Paul; Neth, Edward; Robinson, William, et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 3, §3.6 "The Periodic Table" (seven periods and eighteen groups; group names; metals, nonmetals and metalloids; Mendeleev 1869 and Meyer 1870; Mendeleev's predictions; Moseley). https://openstax.org/books/chemistry-atoms-first-2e/pages/3-6-the-periodic-table (PDF pp. 115–184, page to pin) [^openstax-trends]: Flowers et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 3, §3.5 "Periodic Variations in Element Properties" (Z_eff = Z − shielding; covalent radii of the halogens 64, 99, 114, 133, 148 pm; ionization-energy and electronegativity trends; the Be–B drop; electron affinities of F −322 and Cl −348 kJ/mol). https://openstax.org/books/chemistry-atoms-first-2e/pages/3-5-periodic-variations-in-element-properties (PDF pp. 115–184, page to pin) [^openstax-ie-table]: Flowers et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 3, §3.5, Table 3.3, successive ionization energies in kJ/mol (K 418.8, 3051.8; Ca 589.8, 1145.4, 4912.4). Same URL as above. [^openstax-config]: Flowers et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 3, §3.4 "Electronic Structure of Atoms (Electron Configurations)" (filling order and the correspondence of groups to outer configurations). https://openstax.org/books/chemistry-atoms-first-2e/pages/3-4-electronic-structure-of-atoms-electron-configurations [^b054-capacity]: Blackstock, Lindsay; Brewer, Sharon; Cinel, Bruno (2022). *Chemical Bonding and Organic Chemistry*. Chapter 2, "Quantum Theory and Electronic Structure of Atoms", p. 171 (subshell capacities 2, 6, 10, 14). Open Textbook Library record: https://open.umn.edu/opentextbooks/textbooks/chemical-bonding-and-organic-chemistry [^b054-radius]: Blackstock, Brewer and Cinel (2022), *Chemical Bonding and Organic Chemistry*, Chapter 2, p. 184 (hydrogen-like orbit radius r_n = n²a₀/Z, the origin of the r ∝ n²/Z_eff display line) and Chapter 3, "Periodic Relationships Among the Elements", pp. 193–216 (page to pin). [^b054-mg]: Blackstock, Brewer and Cinel (2022), *Chemical Bonding and Organic Chemistry*, Chapter 4, "Chemical Bonding I: Basic Concepts", pp. 226–228 (Mg ionization energies 738, 1451 and 7733 kJ/mol in the MgClₙ cycles). [^b054-ea]: Blackstock, Brewer and Cinel (2022), *Chemical Bonding and Organic Chemistry*, Chapter 4, pp. 224–225 (EA of Cl taken as −349 kJ/mol in the NaCl cycle). [^b054-en]: Blackstock, Brewer and Cinel (2022), *Chemical Bonding and Organic Chemistry*, Chapter 4, p. 234 (ΔEN for H–H 0, H–Cl 0.9, Na–Cl 2.1, H–F 1.9 polar covalent, N–H 0.9). [^b054-valence]: Blackstock, Brewer and Cinel (2022), *Chemical Bonding and Organic Chemistry*, Chapter 4, p. 240 (valence electron counts: SiH₄ 8). [^nist-srd111]: National Institute of Standards and Technology. *Ground Levels and Ionization Energies for the Neutral Atoms*, NIST Standard Reference Database 111 (He 24.587 eV; Cs 3.894 eV; kJ/mol values derived at 96.485 kJ/mol per eV). https://www.nist.gov/pml/ground-levels-and-ionization-energies-neutral-atoms [^iupac2016]: International Union of Pure and Applied Chemistry (November 30, 2016). "IUPAC Announces the Names of the Elements 113, 115, 117, and 118." https://iupac.org/iupac-announces-the-names-of-the-elements-113-115-117-and-118/ [^scerri2021]: Scerri, Eric (2021). "Provisional Report on Discussions on Group 3 of the Periodic Table." *Chemistry International*, 43 (1). IUPAC / De Gruyter. https://doi.org/10.1515/ci-2021-0115 [^scerri2007]: Scerri, Eric R. (2007). *The Periodic Table: Its Story and Its Significance*. Oxford University Press (Döbereiner, Newlands, Mendeleev's predictions and their fulfilment, hydrogen and helium placement, Janet's left-step table, synthetic elements, alternative tables; page to pin). [^moseley1913]: Moseley, H. G. J. (1913). "The high-frequency spectra of the elements." *Philosophical Magazine*, Series 6, 26 (156): 1024–1034. https://doi.org/10.1080/14786441308635052 [^bohr-nobel-lecture]: Bohr, Niels (1922). "The Structure of the Atom." Nobel Lecture, December 11, 1922. NobelPrize.org. https://www.nobelprize.org/prizes/physics/1922/bohr/lecture/ [^pauli1925]: Pauli, W. (1925). "Über den Zusammenhang des Abschlusses der Elektronengruppen im Atom mit der Komplexstruktur der Spektren." *Zeitschrift für Physik*, 31: 765–783. https://doi.org/10.1007/BF02980631 [^seaborg1951]: Seaborg, Glenn T. (1951). "The Transuranium Elements: Present Status." Nobel Lecture, December 12, 1951. NobelPrize.org. https://www.nobelprize.org/prizes/chemistry/1951/seaborg/lecture/ [^pyykko2011]: Pyykkö, Pekka (2011). "A suggested periodic table up to Z ≤ 172, based on Dirac–Fock calculations on atoms and ions." *Physical Chemistry Chemical Physics*, 13 (1): 161–168. https://doi.org/10.1039/C0CP01575J ## Bibliography - Blackstock, Brewer and Cinel (2022). *Chemical Bonding and Organic Chemistry*. Chapter 3, "Periodic Relationships Among the Elements", pp. 193–216, and Chapter 4, pp. 217–240. Portal Book 054. - Flowers, Neth, Robinson et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 3, "Electronic Structure and Periodic Properties of Elements", pp. 115–184 (§3.4–§3.6). Portal Book 051. - Averill, Bruce; Eldredge, Patricia (2011). *General Chemistry: Principles, Patterns, and Applications*. Chapter 7, pp. 590–721 (periodic trends; page to pin). Portal Book 050. Open Textbook Library record: https://open.umn.edu/opentextbooks/textbooks/general-chemistry-principles-patterns-and-applications - Scerri (2007). *The Periodic Table: Its Story and Its Significance*. Oxford University Press. ## Further reading - Scerri (2021), the IUPAC provisional report on group 3, for the current state of the Lu/Lr question. - Pyykkö (2011), for what relativistic calculations say about an eighth period. ## External links - [The Periodic Table](https://openstax.org/books/chemistry-atoms-first-2e/pages/3-6-the-periodic-table), *Chemistry: Atoms First 2e*, OpenStax - [Ground Levels and Ionization Energies for the Neutral Atoms](https://www.nist.gov/pml/ground-levels-and-ionization-energies-neutral-atoms), NIST Standard Reference Database 111 - [IUPAC announcement of the names of elements 113, 115, 117 and 118](https://iupac.org/iupac-announces-the-names-of-the-elements-113-115-117-and-118/) <!-- MATTERSIM:BEGIN g24 — Matter & Energy Cluster microsim (framework build, specs/sims/Periodic_table.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework), pending deploy:** *Periodic table* will play here once `https://wikitube-3d-microsims.netlify.app/matter/Periodic_table.html` is live. <!-- pending: <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/matter/Periodic_table.html" data-title="Periodic table"></div> --> <!-- MATTERSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Periodic_table) : [Wikitube](https://en.wikitube.io/wiki/Periodic_table) · pinned revision [1372120384](https://en.wikipedia.org/w/index.php?oldid=1372120384) · 2026-09-11 ## Previous hub tags Hubs: `Life_Physics`. 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