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In chemistry and physics, an atom is the smallest particle of a chemical element that retains its chemical properties.

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.

Note: This article excerpts material from the Wikipedia article "Atom", which is released under the GNU Free Documentation License.

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