Group 17 of the periodic table of the elements (see Table 1.1) consists of the so called halogen elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At) and tennessine (Ts). Halogen elements are characterised by the fact that their outer electron shell just misses one electron to form a stable noble gas configuration. As a result most halogen elements form stable anions in the −I oxidation stat, so called halide ions. However, in the halogen group the stability of the halide ion decreases from fluorine to astatine. In part for that reason, it is not known yet if tennessine, discovered only in 2010 (Organessian et al. 2010) behaves like a typical group 17 element which can also be the result of possible relativistic effects (see e.g. Pyykkö 2011a, 2011b). Most probably tennessine behaves much more like a metal, with the +1 oxidation state being expected the most stable oxidation state and the −1 oxidation state, which is the most stable oxidation state in the other halogens, being unstable (Norseev 2021).