Chemical element element (by mass) making up Earth, while oxygen is the most common element in the Earth's crust. Chemical elements constitute all of the ordinary matter of the universe. However astronomical observations suggest that ordinary observable matter makes up only about 15% of the matter in the universe: the remainder is dark matter; the composition of this is unknown, but it is not composed of chemical elements. The two lightest elements, hydrogen and helium, were mostly formed in the Big Bang and are the most common elements in the universe. The next three elements (lithium, beryllium and boron) were formed mostly
Period 6 element away rapidly, so much less is known about this element than its upper neighbors in the periodic table. Earlier studies have shown this element follows periodic trends, being the heaviest known halogen, with melting and boiling points being higher than those of lighter halogens. Until recently most of the chemical characteristics of astatine were inferred from comparison with other elements; however, important studies have already been done. The main difference between astatine and iodine is that the HAt molecule is chemically a hydride rather than a halide; however, in a fashion similar to the lighter halogens, it is known to