Bohrium facts for kids

Niels Bohr in 1939.

Bohrium is a chemical element. At the periodic table of the elements it is at position 107. The element is named in honor of Niels Bohr.

Mendeleev predicted that Bohrium would exist. He called the element eka-rhenium because of its location was near Rhenium in the Periodic Table. The chemistry of bohrium is like the chemistry of rhenium.

Isotopes

Bohrium has no stable or naturally occurring isotopes. Several radioactive isotopes have been synthesized in the laboratory, either by fusing two atoms or by observing the decay of heavier elements. Twelve different isotopes of bohrium have been reported with atomic masses 260–262, 264–267, 270–272, 274, and 278, one of which, bohrium-262, has a known metastable state. All of these but the unconfirmed ²⁷⁸Bh decay only through alpha decay, although some unknown bohrium isotopes are predicted to undergo spontaneous fission.

The lighter isotopes usually have shorter half-lives; half-lives of under 100 ms for ²⁶⁰Bh, ²⁶¹Bh, ²⁶²Bh, and ^262mBh were observed. ²⁶⁴Bh, ²⁶⁵Bh, ²⁶⁶Bh, and ²⁷¹Bh are more stable at around 1 s, and ²⁶⁷Bh and ²⁷²Bh have half-lives of about 10 s. The heaviest isotopes are the most stable, with ²⁷⁰Bh and ²⁷⁴Bh having measured half-lives of about 2.4 min and 40 s respectively, and the even heavier unconfirmed isotope ²⁷⁸Bh appearing to have an even longer half-life of about 11.5 minutes.

The most proton-rich isotopes with masses 260, 261, and 262 were directly produced by cold fusion, those with mass 262 and 264 were reported in the decay chains of meitnerium and roentgenium, while the neutron-rich isotopes with masses 265, 266, 267 were created in irradiations of actinide targets. The five most neutron-rich ones with masses 270, 271, 272, 274, and 278 (unconfirmed) appear in the decay chains of ²⁸²Nh, ²⁸⁷Mc, ²⁸⁸Mc, ²⁹⁴Ts, and ²⁹⁰Fl respectively. The half-lives of bohrium isotopes range from about ten milliseconds for ^262mBh to about one minute for ²⁷⁰Bh and ²⁷⁴Bh, extending to about 11.5 minutes for the unconfirmed ²⁷⁸Bh, which may have one of the longest half-lives among reported superheavy nuclides.

Predicted properties

Very few properties of bohrium or its compounds have been measured; this is due to its extremely limited and expensive production and the fact that bohrium (and its parents) decays very quickly. A few singular chemistry-related properties have been measured, but properties of bohrium metal remain unknown and only predictions are available.

Chemical

Bohrium is the fifth member of the 6d series of transition metals and the heaviest member of group 7 in the periodic table, below manganese, technetium and rhenium. All the members of the group readily portray their group oxidation state of +7 and the state becomes more stable as the group is descended. Thus bohrium is expected to form a stable +7 state. Technetium also shows a stable +4 state whilst rhenium exhibits stable +4 and +3 states. Bohrium may therefore show these lower states as well. The higher +7 oxidation state is more likely to exist in oxyanions, such as perbohrate, BhO−
4, analogous to the lighter permanganate, pertechnetate, and perrhenate. Nevertheless, bohrium(VII) is likely to be unstable in aqueous solution, and would probably be easily reduced to the more stable bohrium(IV).

The lighter group 7 elements are known to form volatile heptoxides M₂O₇ (M = Mn, Tc, Re), so bohrium should also form the volatile oxide Bh₂O₇. The oxide should dissolve in water to form perbohric acid, HBhO₄. Rhenium and technetium form a range of oxyhalides from the halogenation of the oxide. The chlorination of the oxide forms the oxychlorides MO₃Cl, so BhO₃Cl should be formed in this reaction. Fluorination results in MO₃F and MO₂F₃ for the heavier elements in addition to the rhenium compounds ReOF₅ and ReF₇. Therefore, oxyfluoride formation for bohrium may help to indicate eka-rhenium properties. Since the oxychlorides are asymmetrical, and they should have increasingly large dipole moments going down the group, they should become less volatile in the order TcO₃Cl > ReO₃Cl > BhO₃Cl: this was experimentally confirmed in 2000 by measuring the enthalpies of adsorption of these three compounds. The values are for TcO₃Cl and ReO₃Cl are −51 kJ/mol and −61 kJ/mol respectively; the experimental value for BhO₃Cl is −77.8 kJ/mol, very close to the theoretically expected value of −78.5 kJ/mol.

Physical and atomic

Bohrium is expected to be a solid under normal conditions and assume a hexagonal close-packed crystal structure (^c/_a = 1.62), similar to its lighter congener rhenium. Early predictions by Fricke estimated its density at 37.1 g/cm³, but newer calculations predict a somewhat lower value of 26–27 g/cm³.

The atomic radius of bohrium is expected to be around 128 pm. Due to the relativistic stabilization of the 7s orbital and destabilization of the 6d orbital, the Bh⁺ ion is predicted to have an electron configuration of [Rn] 5f¹⁴ 6d⁴ 7s², giving up a 6d electron instead of a 7s electron, which is the opposite of the behavior of its lighter homologues manganese and technetium. Rhenium, on the other hand, follows its heavier congener bohrium in giving up a 5d electron before a 6s electron, as relativistic effects have become significant by the sixth period, where they cause among other things the yellow color of gold and the low melting point of mercury. The Bh²⁺ ion is expected to have an electron configuration of [Rn] 5f¹⁴ 6d³ 7s²; in contrast, the Re²⁺ ion is expected to have a [Xe] 4f¹⁴ 5d⁵ configuration, this time analogous to manganese and technetium. The ionic radius of hexacoordinate heptavalent bohrium is expected to be 58 pm (heptavalent manganese, technetium, and rhenium having values of 46, 57, and 53 pm respectively). Pentavalent bohrium should have a larger ionic radius of 83 pm.

Experimental chemistry

In 1995, the first report on attempted isolation of the element was unsuccessful, prompting new theoretical studies to investigate how best to investigate bohrium (using its lighter homologs technetium and rhenium for comparison) and removing unwanted contaminating elements such as the trivalent actinides, the group 5 elements, and polonium.

In 2000, it was confirmed that although relativistic effects are important, bohrium behaves like a typical group 7 element. A team at the Paul Scherrer Institute (PSI) conducted a chemistry reaction using six atoms of ²⁶⁷Bh produced in the reaction between ²⁴⁹Bk and ²²Ne ions. The resulting atoms were thermalised and reacted with a HCl/O₂ mixture to form a volatile oxychloride. The reaction also produced isotopes of its lighter homologues, technetium (as ¹⁰⁸Tc) and rhenium (as ¹⁶⁹Re). The isothermal adsorption curves were measured and gave strong evidence for the formation of a volatile oxychloride with properties similar to that of rhenium oxychloride. This placed bohrium as a typical member of group 7. The adsorption enthalpies of the oxychlorides of technetium, rhenium, and bohrium were measured in this experiment, agreeing very well with the theoretical predictions and implying a sequence of decreasing oxychloride volatility down group 7 of TcO₃Cl > ReO₃Cl > BhO₃Cl.

2 Bh + 3 O₂ + 2 HCl → 2 BhO₃Cl + H₂

The longer-lived heavy isotopes of bohrium, produced as the daughters of heavier elements, offer advantages for future radiochemical experiments. Although the heavy isotope ²⁷⁴Bh requires a rare and highly radioactive berkelium target for its production, the isotopes ²⁷²Bh, ²⁷¹Bh, and ²⁷⁰Bh can be readily produced as daughters of more easily produced moscovium and nihonium isotopes.

Images for kids

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  Element 107 was originally proposed to be named after Niels Bohr, a Danish nuclear physicist, with the name nielsbohrium (Ns). This name was later changed by IUPAC to bohrium (Bh).

See also

In Spanish: Bohrio para niños