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BookmarkIn chemistry and physics, an atom is the smallest particle of a chemical element that retains its chemical properties.
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An atom consists of a dense nucleus of positively-charged protons and electrically-neutral neutrons, surrounded by a much larger electron cloud consisting of negatively-charged electrons.
An atom is electrically neutral if it has the same number of protons as electrons.
The number of protons in an atom defines the chemical element to which it belongs, while the number of neutrons determines the isotope of the element. The first nuclei, including most of the helium and all of the deuterium in the universe, were theoretically created during big bang nucleosynthesis, about 3 minutes after the big bang.
The first atoms were theoretically created 380,000 years after the big bang, an epoch called recombination, when the universe cooled to allow electrons to become attached to nuclei.
Since then, atoms have been combined in stars through the process of nuclear fusion to generate atoms up to Iron. A human hair is about 1 million carbon atoms wide..
For more information about the topic Atom, read the full article at Wikipedia.org, or see the following related articles:
Proton — The proton is a subatomic particle with an electric charge of one positive fundamental unit and a mass of 938.3 MeV/c2, or about 1836 times the mass ... > read more
Subatomic particle — A subatomic particle is a particle smaller than an atom: it may be elementary or composite. Particle physics and nuclear physics concern themselves ... > read more
Nuclear reaction — In nuclear physics, a nuclear reaction is a process in which two nuclei or nuclear particles collide, to produce different products than the initial ... > read more
