Ununtrium - Predicted Properties

Predicted Properties

Ununtrium is the first member of the 7p series of elements and the heaviest boron group element on the periodic table, below boron, aluminium, gallium, indium, and thallium. It is predicted to show many differences from its lighter homologues: a largely contributing effect is the spin–orbit (SO) interaction. It is especially strong for the superheavy elements, because their electrons move much faster than in lighter atoms, at velocities comparable to the speed of light, which is where the differences arise from. In relation to ununtrium atoms, it lowers the 7s and the 7p electron energy levels (stabilizing the corresponding electrons), but two of the 7p electron energy levels are stabilized more than the other four. The stabilization of the 7s electrons is called the inert pair effect, and the effect "tearing" the 7p subshell into the more stabilized and the less stabilized parts is called the subshell splitting. Computation chemists see the split as a change of the second (azimuthal) quantum number l from 1 to 1/2 and 3/2 for the more stabilized and less stabilized parts of the 7p subshell, respectively. For many theoretical purposes, the valence electron configuration may be represented to reflect the 7p subshell split as 7s27p_1/21. These effects stabilize lower oxidation states: the first ionization energy of ununtrium is expected to be 7.306 eV, the highest among the boron group elements. Hence, the most stable oxidation state of ununtrium is predicted to be the +1 state. Differences for other electron levels also exist. For example, the 6d electron levels (also split in halves, with four being 6d_3/2 and six being 6d_5/2) are both raised, so that they are close in energy to the 7s ones. Thus, the 6d electron levels, being destabilized, should be able to participate in chemical reactions in the earlier 7p elements (until around ununpentium), thus making them behave in some ways like transition metals and allow higher oxidation states. Ununtrium should hence also be able to show stable +2, +3 and +5 oxidation states. However, the +3 state should still be less stable than the +1 state, following periodic trends. Ununtrium should be the most electronegative among all the boron group elements: for example, in the compound UutUus, the negative charge is expected to be on the ununtrium atom rather than the ununseptium atom, the opposite of what would be expected from simple periodicity. The electron affinity of ununtrium is calculated to be around 0.68 eV; in comparison, that of thallium is 0.4 eV.

The simplest possible ununtrium compound is the monohydride, UutH. The bonding is provided by the 7p_1/2 electron of ununtrium and the 1s electron of hydrogen. However, the SO interaction causes the binding energy of ununtrium monohydride to be reduced by about 1 eV and the ununtrium–hydrogen bond length to decrease as the bonding 7p_1/2 orbital is relativistically contracted. The analogous monofluoride (UutF) should also exist. Ununtrium should also be able to form the trihydride (UutH₃), trifluoride (UutF₃), and trichloride (UutCl₃), with ununtrium in the +3 oxidation state. Because the 6d electrons are involved in bonding instead of the 7s ones, these molecules are predicted to be T-shaped and not trigonal planar. Although the polyfluoride anion UutF−
6 should be stable, the corresponding neutral fluoride UutF₅ should be unstable, spontaneously decomposing into the trifluoride and elemental fluorine. Ununtrium(I) is predicted to be more similar to silver(I) than thallium(I).

Ununtrium is expected to be much denser than thallium, having a predicted density of about 18 g/cm3, due to the relativistic stabilization and contraction of its 7s and 7p_1/2 orbitals. This is because it is estimated to have an atomic radius of about 170 pm, the same as that of thallium, even though periodic trends would predict it to have an atomic radius larger than that of thallium due to it being one period further down in the periodic table. The melting and boiling points of ununtrium are not definitely known, but have been calculated to be 430 °C and 1100 °C respectively, exceeding the values for gallium, indium, and thallium, following periodic trends.