Main-group element Main-group element In chemistry and atomic physics, the main group is the group of elements whose lightest members are represented by helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, and fluorine as arranged in the periodic table of the elements. The main group includes the elements (except hydrogen, which is sometimes not included) in groups 1 and 2 (s-block), and groups 13 to 18 (p-block). The s-block elements are primarily characterised by one main oxidation state, and the p-block elements, when they have multiple oxidation states, often have common oxidation states separated by two units. Main-group elements (with some of the
Solid hydrogen Phase IV is (or is similar to) a structure of "Pc" symmetry, consisting of alternate layers of strongly bonded molecules and weakly bonded graphene-like sheets. Solid hydrogen Solid hydrogen is the solid state of the element hydrogen, achieved by decreasing the temperature below hydrogen's melting point of . It was collected for the first time by James Dewar in 1899 and published with the title "Sur la solidification de l'hydrogène" (English: On the solidification of hydrogen) in the "Annales de Chimie et de Physique", 7th series, vol. 18, Oct. 1899. Solid hydrogen has a density of 0.086 g/cm making it