<P> The 1s electron configuration of hydrogen, while superficially similar to that of the alkali metals (ns), is unique because there is no 1p subshell . Hence it can lose an electron to form the hydron H, or gain one to form the hydride ion H. In the former case it resembles superficially the alkali metals; in the latter case, the halogens, but the differences due to the lack of a 1p subshell are important enough that neither group fits the properties of hydrogen well . Group 14 is also a good fit in terms of thermodynamic properties such as ionisation energy and electron affinity, but makes chemical nonsense because hydrogen cannot be tetravalent . Thus none of the three placements are entirely satisfactory, although group 1 is the most common placement (if one is chosen) because the hydron is by far the most important of all monatomic hydrogen species, being the foundation of acid - base chemistry . As an example of hydrogen's unorthodox properties stemming from its unusual electron configuration and small size, the hydrogen ion is very small (radius around 150 fm compared to the 50--220 pm size of most other atoms and ions) and so is nonexistent in condensed systems other than in association with other atoms or molecules . Indeed, transferring of protons between chemicals is the basis of acid - base chemistry . Also unique is hydrogen's ability to form hydrogen bonds, which are an effect of charge - transfer, electrostatic, and electron correlative contributing phenomena . While analogous lithium bonds are also known, they are mostly electrostatic . Nevertheless, hydrogen can take on the same structural role as the alkali metals in some molecular crystals, and has a close relationship with the lightest alkali metals (especially lithium). </P> <P> The ammonium ion (NH) has very similar properties to the heavier alkali metals, acting as an alkali metal intermediate between potassium and rubidium, and is often considered a close relative . For example, most alkali metal salts are soluble in water, a property which ammonium salts share . Ammonium is expected to behave stably as a metal (NH ions in a sea of delocalised electrons) at very high pressures (though less than the typical pressure where transitions from insulating to metallic behaviour occur around, 100 GPa), and could possibly occur inside the ice giants Uranus and Neptune, which may have significant impacts on their interior magnetic fields . It has been estimated that the transition from a mixture of ammonia and dihydrogen molecules to metallic ammonium may occur at pressures just below 25 GPa . Under standard conditions, ammonium can form a metallic amalgam with mercury . </P> <P> Other "pseudo-alkali metals" include the alkylammonium cations, in which some of the hydrogen atoms in the ammonium cation are replaced by alkyl or aryl groups . In particular, the quaternary ammonium cations (NR) are very useful since they are permanently charged, and they are often used as an alternative to the expensive Cs to stabilise very large and very easily polarisable anions such as HI . Tetraalkylammonium hydroxides, like alkali metal hydroxides, are very strong bases that react with atmospheric carbon dioxide to form carbonates . Furthermore, the nitrogen atom may be replaced by a phosphorus, arsenic, or antimony atom (the heavier nonmetallic pnictogens), creating a phosphonium (PH) or arsonium (AsH) cation that can itself be substituted similarly; while stibonium (SbH) itself is not known, some of its organic derivatives are characterised . </P> <P> Cobaltocene, Co (C H), is a metallocene, the cobalt analogue of ferrocene . It is a dark purple solid . Cobaltocene has 19 valence electrons, one more than usually found in organotransition metal complexes, such as its very stable relative, ferrocene, in accordance with the 18 - electron rule . This additional electron occupies an orbital that is antibonding with respect to the Co--C bonds . Consequently, many chemical reactions of Co (C H) are characterized by its tendency to lose this "extra" electron, yielding a very stable 18 - electron cation known as cobaltocenium . Many cobaltocenium salts coprecipitate with caesium salts, and cobaltocenium hydroxide is a strong base that absorbs atmospheric carbon dioxide to form cobaltocenium carbonate . Like the alkali metals, cobaltocene is a strong reducing agent, and decamethylcobaltocene is stronger still due to the combined inductive effect of the ten methyl groups . Cobalt may be substituted by its heavier congener rhodium to give rhodocene, an even stronger reducing agent . Iridocene (involving iridium) would presumably be still more potent, but is not very well - studied due to its instability . </P>

In which group of the periodic table do the elements not form ions