Hydrogen
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 (H2); however, the alkali metals only form diatomic molecules (such as dilithium, Li2) at high temperatures, when they are in the gaseous state.
Hydrogen, like the alkali metals, has one valence electron and reacts easily with the halogens, but the similarities end there. Its placement above lithium is primarily due to its electron configuration and not its chemical properties. It is sometimes placed above carbon due to their similar electronegativities or fluorine due to their similar chemical properties.
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 sometimes considered to be a halogen. (The alkali metals can also form negative ions, known as alkalides, but these are little more than laboratory curiosities, being unstable.) 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.
Read more about this topic: Alkali Metal, Characteristics, Other Similar Substances
Famous quotes containing the word hydrogen:
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