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Atom, classically, one of the minute, indivisible, homogeneous particles of which physical objects are composed; in 20th-century science, the name given to a relatively stable package of matter that is itself made up of at least 2 subatomic particles, and that defines an element. Every atom consists of a tiny nucleus (containing positively charged protons and electrically neutral neutrons) with which a number of negatively charged electrons are associated. The much smaller electrons occupy a hierarchy of orbitals that represent the atom's electronic energy levels and fill most of the space taken up by the atom. The number of protons in the nucleus of an atom (the atomic number, Z) defines the chemical element of which the atom is an example. In an isolated neutral atom the number of electrons equals the atomic number, but an electrically charged ion of the same atom has either a surfeit or a deficit of electrons. The number of neutrons in the nucleus (the neutron number, N) can vary among different atoms of the same element. Atoms with the same number of protons but different numbers of neutrons are called isotopes of the element in question. Most stable isotopes have slightly more neutrons than protons.Although the nucleus is very small, it contains nearly all the mass of the atom—protons and neutrons having very similar masses, the mass of the electron (about 0.05% of the proton mass) being almost negligible. The mass of an atom is roughly equal to the total number of its protons and neutrons. This number, Z + N, is known as the mass number of the atom, A, the mass of a proton being counted as 1. In equations representing nuclear reactions, the atomic number of an atom is often written as a subscript preceding the chemical symbol for the element, and the mass number as a superscript following it. Thus an atomic nucleus with a mass number 16 and containing 8 protons belongs to an atom of oxygen-16, written 8O16. The average of the mass numbers of the various naturally occurring isotopes of an element, weighted according to their relative abundance, gives the chemical atomic weight of the element. Subatomic particles fired into atomic nuclei can cause nuclear reactions that give rise either to new isotopes of the original element or to atoms of a different element. Such nuclear reactions emit alpha particles, or beta rays, sometimes accompanied by gamma rays.

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