出典:Wiktionary
出典:『Wiktionary』 (2026/02/03 19:20 UTC 版)
chemical element (plural chemical elements)
出典:Wikipedia
出典:『Wikipedia』 (2011/06/10 01:42 UTC 版)
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Common examples of elements are sodium, calcium, iron, copper, carbon, and nitrogen. Although all known chemical matter is composed of these elements, chemical matter itself constitutes only about 15% of the total matter in the universe. The remainder is dark matter, which is not believed to consist of chemical elements as we know them since it does not contain protons, neutrons or electrons. As of May 2011, 118 elements have been identified, the latest being ununoctium in 2002. New elements of higher atomic number are discovered from time to time, as the synthetic products of man-made nuclear reactions, though ununennium (atomic number 119) has yet to be synthesized. Of the 118 known elements, only the first 94 are believed to occur naturally on Earth.