<P> Many other substances are similar to the alkali metals in their tendency to form monopositive cations . Analogously to the pseudohalogens, they have sometimes been called "pseudo-alkali metals". These substances include some elements and many more polyatomic ions; the polyatomic ions are especially similar to the alkali metals in their large size and weak polarising power . </P> <P> The element hydrogen, with one electron per neutral atom, is usually placed at the top of Group 1 of the periodic table for convenience, but hydrogen is not normally considered to be an alkali metal; when it is considered to be an alkali metal, it is because of its atomic properties and not its chemical properties . Under typical conditions, pure hydrogen exists as a diatomic gas consisting of two atoms per molecule (H); however, the alkali metals only form diatomic molecules (such as dilithium, Li) at high temperatures, when they are in the gaseous state . </P> <P> Hydrogen, like the alkali metals, has one valence electron and reacts easily with the halogens, but the similarities end there because of the small size of a bare proton H compared to the alkali metal cations . Its placement above lithium is primarily due to its electron configuration . It is sometimes placed above carbon due to their similar electronegativities or fluorine due to their similar chemical properties . </P> <P> The first ionisation energy of hydrogen (1312.0 kJ / mol) is much higher than that of the alkali metals . As only one additional electron is required to fill in the outermost shell of the hydrogen atom, hydrogen often behaves like a halogen, forming the negative hydride ion, and is very occasionally considered to be a halogen on that basis . (The alkali metals can also form negative ions, known as alkalides, but these are little more than laboratory curiosities, being unstable .) An argument against this placement is that formation of hydride from hydrogen is endothermic, unlike the exothermic formation of halides from halogens . The radius of the H anion also does not fit the trend of increasing size going down the halogens: indeed, H is very diffuse because its single proton cannot easily control both electrons . It was expected for some time that liquid hydrogen would show metallic properties; while this has been shown to not be the case, under extremely high pressures, such as those found at the cores of Jupiter and Saturn, hydrogen does become metallic and behaves like an alkali metal; in this phase, it is known as metallic hydrogen . The electrical resistivity of liquid metallic hydrogen at 3000 K is approximately equal to that of liquid rubidium and caesium at 2000 K at the respective pressures when they undergo a nonmetal - to - metal transition . </P>

Why are alkali metals never found as free elements what is an example of an alkali metal