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98 a highermore recentestimate is that the periodic

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[98] A higher—more recent—estimate is that the periodic table may end soon after the island of stability, [99] which is expected to center around element 126, as the extension of the periodic and nuclides tables is restricted by proton and neutron drip lines. [100] Other predictions of an end to the periodic table include at element 128 by John Emsley, [3] at element 137 by Richard Feynman [101] and at element 155 by Albert Khazan. [3][n 9] Bohr model 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 traveling faster than c , the speed of light. [102] Hence the non-relativistic Bohr model is inaccurate when applied to such an element. Relativistic Dirac equation 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. [103] 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; [104] however, this does not happen if the innermost orbital is filled, so that element 173 is not necessarily the end of the periodic table. [105] Placement of hydrogen and helium Hydrogen and helium are often placed in different places than their electron configurations would indicate; hydrogen is usually placed above lithium, in accordance with its electron configuration, but is sometimes placed above fluorine, [106] or even carbon, [106] as it also behaves somewhat similarly to them. Hydrogen is also sometimes placed in its own group, as it does not behave similarly enough to any element to be placed in a group with another. [107] Helium is almost always placed above neon, as they are very similar chemically, although it is occasionally placed above beryllium on account of having a comparable electron shell configuration (helium: 1s 2 ; beryllium: [He] 2s 2 ). [19] Groups included in the transition metals 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. [108] 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 recent discovery that mercury can use its d electrons in the formation of mercury(IV)
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