Chemical elements are types of atoms, with 118 observed and 98 occurring naturally on Earth. 20 are created artificially. Elements come from supernova, stellar, and Big Bang nucleosynthesis. The first stars formed 300 million years after the Big Bang, initiating stellar nucleosynthesis. Supernova nucleosynthesis creates elements heavier than iron.
A chemical element is a type of atom, such as hydrogen or oxygen. As of 2011, 118 elements had been observed, of which 98 occur naturally on Earth. 20 elements are created artificially in nuclear reactors or particle accelerator experiments. The first synthetic element to be created in substantial quantities was plutonium, element 94. Plutonium is also the heaviest atom found naturally on Earth. With a half-life of only 80 million years, plutonium is found in extremely small quantities in uranium ores.
Current chemical elements come from one of three sources: supernova nucleosynthesis, stellar nucleosynthesis, and Big Bang nucleosynthesis. Nucleosynthesis occurs when atomic nuclei are pressed together so tightly and at such high heat that they overcome the mutual repulsion of their electron shells and produce heavier nuclei. In this way, hydrogen nuclei can be fused into helium nuclei, which can in turn fuse into carbon nuclei, if sufficient temperature and pressure conditions are achieved.
In the beginning, the universe was so hot and dense that it consisted of nothing but free quarks – the constituents of protons and neutrons – electrons and radiation. After a millionth of a second, the quarks began to fuse into baryons – protons and neutrons. For the first twenty minutes after the Big Bang, the temperature of the universe exceeded that at the center of the brightest stars, with a density greater than air. During this time, protons and neutrons collided vigorously to form larger nuclei: deuterium and two isotopes of helium. 25 percent of all matter in the universe has been converted to helium, with about 75 percent being hydrogen, along with traces of heavier elements like lithium. This is similar to the current ratio of chemical elements.
The first stars formed about 300 million years after the Big Bang, initiating another form of nucleosynthesis called stellar nucleosynthesis. In stellar nucleosynthesis, the tightly packed matter at the center of a star undergoes nuclear fusion, releasing large amounts of energy and balancing the forces of gravity acting to collapse the star. This can be thought of as a continuously exploding H-bomb. Elements up to iron on the periodic table are formed in stellar nucleosynthesis.
To create an element heavier than iron requires another type of nucleosynthesis, supernova nucleosynthesis. Supernovae occur when stars catastrophically collapse after using up all of their nuclear fuel in their cores. The star’s atmospheric envelope collapses inward under gravity, bouncing off a core made of nearly incompressible “electronically degenerate” matter. During this sharp bounce, several percent of the star’s material is fused into heavier elements almost instantly. This releases enough energy for the supernova to outshine its host galaxy for days or weeks. Elements heavier than iron are synthesized during this incredibly energetic cosmic event.
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