Placements of Elements

Simply following electron configurations, hydrogen (electronic configuration 1s1) and helium (1s2) should be placed in groups 1 and 2, above lithium ([He]2s1) and beryllium ([He]2s2). However, such placing is rarely used outside of the context of electron configurations: When the noble gases (then called "inert gases") were first discovered around 1900, they were known as "group 0", reflecting no chemical reactivity of these elements known at that point, and helium was placed on the top that group, as it did share the extreme chemical inertness seen throughout the group. As the group changed its formal number, many authors continued to assign helium directly above neon, in group 18; one of the examples of such placing is the current IUPAC table.

Hydrogen's chemical properties are not very close to those of the alkali metals, which occupy group 1, and on that basis hydrogen is sometimes placed elsewhere: one of the most common alternatives is in group 17;one of the factors behind it is the strictly univalent predominantly non-metallic chemistry of hydrogen, and that of fluorine (the element placed on the top of group 17) is strictly univalent and non-metallic. Sometimes, to show how hydrogen has properties both corresponding to those of the alkali metals and the halogens, it may be shown in two columns simultaneously. Another suggestion is above carbon in group 14: placed that way, it fits well into the trend of increasing trends of ionization potential values and electron affinity values, and is not too far from the electronegativity trend, even though hydrogen cannot show the tetravalence characteristic of the heavier group 14 elements. Finally, hydrogen is sometimes placed separately from any group; this is based on how general properties of hydrogen differ from that of any group: unlike hydrogen, the other group 1 elements show extremely metallic behaviour; the group 17 elements commonly form salts (hence the term "halogen"); elements of any other group show some multivalent chemistry. The other period 1 element, helium, is sometimes placed separately from any group as well.The property that distinguishes helium from the rest of the noble gases (even though the extraordinary inertness of helium is extremely close to that of neon and argon) is that in its closed electron shell, helium has only two electrons in the outermost electron orbital, while the rest of the noble gases have eight.

Group 3 and its elements in periods 6 and 7
Chemical and physical arguments have been made in support of lutetium and lawrencium but the majority of authors seem unconvinced. Most working chemists are not aware there is any controversy. In December 2015 an IUPAC project was established to make a recommendation on the matter.
Lutetium and lawrencium
Lanthanides and actinides
Groups included in the transition metals
Still other chemists further exclude the group 3 elements from the definition of a transition metal. They do so on the basis that the group 3 elements do not form any ions having a partially occupied d shell and do not therefore exhibit any properties characteristic of transition metal chemistry.In this case, only groups 4–11 are regarded as transition metals. However, though the group 3 elements show few of the characteristic chemical properties of the transition metals, they do show some of their characteristic physical properties (on account of the presence in each atom of a single d electron).
Further periodic table extensions
Bohr model
Relativistic Dirac equation
Optimal form

Although scandium and yttrium are always the first two elements in group 3 the identity of the next two elements is not completely settled. They are commonly lanthanum and actinium, and less often lutetium and lawrencium. The two variants originate from historical difficulties in placing the lanthanides in the periodic table, and arguments as to where the f block elements start and end.[n 10][n 11] It has been claimed that such arguments are proof that, "it is a mistake to break the [periodic] system into sharply delimited blocks".A third variant shows the two positions below yttrium as being occupied by the lanthanides and the actinides.

Lanthanum and actinium

Right

La and Ac below Y

Lanthanum and actinium are commonly depicted as the remaining group 3 members.[n 12] It has been suggested that this layout originated in the 1940s, with the appearance of periodic tables relying on the electron configurations of the elements and the notion of the differentiating electron. The configurations of caesium, barium and lanthanum are [Xe]6s1, [Xe]6s2 and [Xe]5d16s2. Lanthanum thus has a 5d differentiating electron and this establishes "it in group 3 as the first member of the d-block for period 6".A consistent set of electron configurations is then seen in group 3: scandium [Ar]3d14s2, yttrium [Kr]4d15s2 and lanthanum [Xe]5d16s2. Still in period 6, ytterbium was assigned an electron configuration of [Xe]4f135d16s2 and lutetium [Xe]4f145d16s2, "resulting in a 4f differentiating electron for lutetium and firmly establishing it as the last member of the f-block for period 6".[135] Later spectroscopic work found that the electron configuration of ytterbium was in fact [Xe]4f146s2. This meant that ytterbium and lutetium—the latter with [Xe]4f145d16s2—both had 14 f-electrons, "resulting in a d- rather than an f- differentiating electron" for lutetium and making it an "equally valid candidate" with [Xe]5d16s2 lanthanum, for the group 3 periodic table position below yttrium.Lanthanum, however, has the advantage of incumbency since the 5d1 electron appears for the first time in its structure whereas it appears for the third time in lutetium, having also made a brief second appearance in gadolinium.

In terms of chemical behaviour, and trends going down group 3 for properties such as melting point, electronegativity and ionic radius,scandium, yttrium, lanthanum and actinium are similar to their group 1–2 counterparts. In this variant, the number of f electrons in the most common (trivalent) ions of the f-block elements consistently matches their position in the f-block.For example, the f-electron counts for the trivalent ions of the first three f-block elements are Ce 1, Pr 2 and Nd 3.

Right

Lu and Lr below Y

In other tables, lutetium and lawrencium are the remaining group 3 members.[n 13] Early techniques for chemically separating scandium, yttrium and lutetium relied on the fact that these elements occurred together in the so-called "yttrium group" whereas La and Ac occurred together in the "cerium group".Accordingly, lutetium rather than lanthanum was assigned to group 3 by some chemists in the 1920s and 30s.[n 14] Several physicists in the 1950s and 60s favoured lutetium, in light of a comparison of several of its physical properties with those of lanthanum.This arrangement, in which lanthanum is the first member of the f-block, is disputed by some authors since lanthanum lacks any f-electrons. However, it has been argued that this is not valid concern given other periodic table anomalies—thorium, for example, has no f-electrons yet is part of the f-block. As for lawrencium, its gas phase atomic electron configuration was confirmed in 2015 as [Rn]5f147s27p1. Such a configuration represents another periodic table anomaly, regardless of whether lawrencium is located in the f-block or the d-block, as the only potentially applicable p-block position has been reserved for nihonium with its predicted configuration of [Rn]5f146d107s27p1

Chemically, scandium, yttrium and lutetium (and presumably lawrencium) behave like trivalent versions of the group 1–2 metals.On the other hand, trends going down the group for properties such as melting point, electronegativity and ionic radius, are similar to those found among their group 4–8 counterparts. In this variant, the number of f electrons in the gaseous forms of the f-block atoms usually matches their position in the f-block. For example, the f-electron counts for the first five f-block elements are La 0, Ce 1, Pr 3, Nd 4 and Pm 5.

Right

Markers below Y

A few authors position all thirty lanthanides and actinides in the two positions below yttrium (usually via footnote markers). This variant emphasizes similarities in the chemistry of the 15 lanthanide elements (La–Lu), possibly at the expense of ambiguity as to which elements occupy the two group 3 positions below yttrium, and a 15-column wide f block (there can only be 14 elements in any row of the f block).[n 16]

The definition of a transition metal, as given by IUPAC, is an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell.[146] By this definition all of the elements in groups 3–11 are transition metals. The IUPAC definition therefore excludes group 12, comprising zinc, cadmium and mercury, from the transition metals category.

Some chemists treat the categories "d-block elements" and "transition metals" interchangeably, thereby including groups 3–12 among the transition metals. In this instance the group 12 elements are treated as a special case of transition metal in which the d electrons are not ordinarily involved in chemical bonding. The 2007 report of mercury(IV) fluoride (HgF4), a compound in which mercury would use its d electrons for bonding, has prompted some commentators to suggest that mercury can be regarded as a transition metal.Other commentators, such as Jensen, have argued that the formation of a compound like HgF4 can occur only under highly abnormal conditions; indeed, its existence is currently disputed. As such, mercury could not be regarded as a transition metal by any reasonable interpretation of the ordinary meaning of the term.

Elements with unknown chemical properties

Although all elements up to oganesson have been discovered, of the elements above hassium (element 108), only copernicium (element 112) and flerovium (element 114) have known chemical properties. The other elements may behave differently from what would be predicted by extrapolation, due to relativistic effects; for example, flerovium has been predicted to possibly exhibit some noble-gas-like properties, even though it is currently placed in the carbon group. More recent experiments have suggested, however, that flerovium behaves chemically like lead, as expected from its periodic table position.

Main article: Extended periodic table

Hydrogen (diatomic nonmetal)

Helium (noble gas)

Lithium (alkali metal)

Beryllium (alkaline earth metal)

Boron (metalloid)

Carbon (polyatomic nonmetal)

Nitrogen (diatomic nonmetal)

Oxygen (diatomic nonmetal)

Fluorine (diatomic nonmetal)

Neon (noble gas)

Sodium (alkali metal)

Magnesium (alkaline earth metal)

Aluminium (post-transition metal)

Silicon (metalloid)

Phosphorus (polyatomic nonmetal)

Sulfur (polyatomic nonmetal)

Chlorine (diatomic nonmetal)

Argon (noble gas)

Potassium (alkali metal)

Calcium (alkaline earth metal)

Scandium (transition metal)

Titanium (transition metal)

Vanadium (transition metal)

Chromium (transition metal)

Manganese (transition metal)

Iron (transition metal)

Cobalt (transition metal)

Nickel (transition metal)

Copper (transition metal)

Zinc (transition metal)

Gallium (post-transition metal)

Germanium (metalloid)

Arsenic (metalloid)

Selenium (polyatomic nonmetal)

Bromine (diatomic nonmetal)

Krypton (noble gas)

Rubidium (alkali metal)

Strontium (alkaline earth metal)

Yttrium (transition metal)

Zirconium (transition metal)

Niobium (transition metal)

Molybdenum (transition metal)

Technetium (transition metal)

Ruthenium (transition metal)

Rhodium (transition metal)

Palladium (transition metal)

Silver (transition metal)

Cadmium (transition metal)

Indium (post-transition metal)

Tin (post-transition metal)

Antimony (metalloid)

Tellurium (metalloid)

Iodine (diatomic nonmetal)

Xenon (noble gas)

Caesium (alkali metal)

Barium (alkaline earth metal)

Lanthanum (lanthanide)

Cerium (lanthanide)

Praseodymium (lanthanide)

Neodymium (lanthanide)

Promethium (lanthanide)

Samarium (lanthanide)

Europium (lanthanide)

Gadolinium (lanthanide)

Terbium (lanthanide)

Dysprosium (lanthanide)

Holmium (lanthanide)

Erbium (lanthanide)

Thulium (lanthanide)

Ytterbium (lanthanide)

Lutetium (lanthanide)

Hafnium (transition metal)

Tantalum (transition metal)

Tungsten (transition metal)

Rhenium (transition metal)

Osmium (transition metal)

Iridium (transition metal)

Platinum (transition metal)

Gold (transition metal)

Mercury (transition metal)

Thallium (post-transition metal)

Lead (post-transition metal)

Bismuth (post-transition metal)

Polonium (post-transition metal)

Astatine (metalloid)

Radon (noble gas)

Francium (alkali metal)

Radium (alkaline earth metal)

Actinium (actinide)

Thorium (actinide)

Protactinium (actinide)

Uranium (actinide)

Neptunium (actinide)

Plutonium (actinide)

Americium (actinide)

Curium (actinide)

Berkelium (actinide)

Californium (actinide)

Einsteinium (actinide)

Fermium (actinide)

Mendelevium (actinide)

Nobelium (actinide)

Lawrencium (actinide)

Rutherfordium (transition metal)

Dubnium (transition metal)

Seaborgium (transition metal)

Bohrium (transition metal)

Hassium (transition metal)

Meitnerium (transition metal (predicted))

Darmstadtium (transition metal (predicted))

Roentgenium (transition metal (predicted))

Copernicium (transition metal)

Nihonium (post-transition metal (predicted))

Flerovium (post-transition metal (predicted))

Moscovium (post-transition metal (predicted))

Livermorium (post-transition metal (predicted))

Tennessine (post-transition metal (predicted))

Oganesson (noble gas (predicted))

Ununennium (alkali metal (predicted))

Unbinilium (alkaline earth metal (predicted))

Unbiunium (superactinide (predicted))

Unquadquadium (superactinide (predicted))

Unquadpentium (superactinide (predicted))

Unquadhexium (superactinide (predicted))

Unquadseptium (superactinide (predicted))

Unquadoctium (superactinide (predicted))

Unquadennium (superactinide (predicted))

Unpentnilium (superactinide (predicted))

Unpentunium (superactinide (predicted))

Unpentbium (superactinide (predicted))

Unpenttrium (superactinide (predicted))

Unpentquadium (superactinide (predicted))

Unpentpentium (superactinide (predicted))

Unpenthexium (superactinide (predicted))

Unpentseptium (superactinide (predicted))

Unpentoctium (transition metal (predicted))

Unpentennium (transition metal (predicted))

Unhexnilium (transition metal (predicted))

Unhexunium (transition metal (predicted))

Unhexbium (transition metal (predicted))

Unhextrium (transition metal (predicted))

Unhexquadium (transition metal (predicted))

Unhexpentium (transition metal (predicted))

Unhexhexium (transition metal (predicted))

Unhexseptium (post-transition metal (predicted))

Unhexoctium (post-transition metal (predicted))

Unhexennium (post-transition metal (predicted))

Unseptnilium (post-transition metal (predicted))

Unseptunium (post-transition metal (predicted))

Unseptbium (noble gas (predicted))

Unbibium (superactinide (predicted))

Unbitrium (superactinide (predicted))

Unbiquadium (superactinide (predicted))

Unbipentium (superactinide (predicted))

Unbihexium (superactinide (predicted))

Unbiseptium (superactinide (predicted))

Unbioctium (superactinide (predicted))

Unbiennium (superactinide (predicted))

Untrinilium (superactinide (predicted))

Untriunium (superactinide (predicted))

Untribium (superactinide (predicted))

Untritrium (superactinide (predicted))

Untriquadium (superactinide (predicted))

Untripentium (superactinide (predicted))

Untrihexium (superactinide (predicted))

Untriseptium (superactinide (predicted))

Untrioctium (superactinide (predicted))

Untriennium (superactinide (predicted))

Unquadnilium (superactinide (predicted))

Unquadunium (superactinide (predicted))

Unquadbium (superactinide (predicted))

Unquadtrium (superactinide (predicted))

Periodic table with eight rows, extended to element

It is unclear whether new elements will continue the pattern of the current periodic table as period 8, or require further adaptations or adjustments. Seaborg expected the eighth period to follow the previously established pattern exactly, so that it would include a two-element s-block for elements 119 and 120, a new g-block for the next 18 elements, and 30 additional elements continuing the current f-, d-, and p-blocks, culminating in element 168, the next noble gas.More recently, physicists such as Pekka Pyykkö have theorized that these additional elements do not follow the Madelung rule, which predicts how electron shells are filled and thus affects the appearance of the present periodic table. There are currently several competing theoretical models for the placement of the elements of atomic number less than or equal to

Element with the highest possible atomic number

The number of possible elements is not known. A very early suggestion made by Elliot Adams in 1911, and based on the arrangement of elements in each horizontal periodic table row, was that elements of atomic weight greater than 256± (which would equate to between elements 99 and 100 in modern-day terms) did not exist. A higher—more recent—estimate is that the periodic table may end soon after the island of stability, which is expected to centre around element 126, as the extension of the periodic and nuclides tables is restricted by proton and neutron drip lines.Other predictions of an end to the periodic table include at element 128 by John Emsley,[3] at element 137 by Richard Feynman,and at element 155 by Albert Khazan.[3][n 17]

The Bohr model exhibits difficulty for atoms with atomic number greater than 137, as any element with an atomic number greater than 137 would require 1s electrons to be travelling faster than c, the speed of light. Hence the non-relativistic Bohr model is inaccurate when applied to such an element.

The relativistic Dirac equation has problems for elements with more than 137 protons. For such elements, the wave function of the Dirac ground state is oscillatory rather than bound, and there is no gap between the positive and negative energy spectra, as in the Klein paradox. More accurate calculations taking into account the effects of the finite size of the nucleus indicate that the binding energy first exceeds the limit for elements with more than 173 protons. For heavier elements, if the innermost orbital (1s) is not filled, the electric field of the nucleus will pull an electron out of the vacuum, resulting in the spontaneous emission of a positron; however, this does not happen if the innermost orbital is filled, so that element 173 is not necessarily the end of the periodic table.

The many different forms of periodic table have prompted the question of whether there is an optimal or definitive form of periodic table. The answer to this question is thought to depend on whether the chemical periodicity seen to occur among the elements has an underlying truth, effectively hard-wired into the universe, or if any such periodicity is instead the product of subjective human interpretation, contingent upon the circumstances, beliefs and predilections of human observers. An objective basis for chemical periodicity would settle the questions about the location of hydrogen and helium, and the composition of group 3. Such an underlying truth, if it exists, is thought to have not yet been discovered. In its absence, the many different forms of periodic table can be regarded as variations on the theme of chemical periodicity, each of which explores and emphasizes different aspects, properties, perspectives and relationships of and among the elements.[n 18]