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Protons and neutrons are bound together to form a nucleus by the nuclear force. The diameter of the nucleus is in the range of 1.70 fm (1.70 × 10 −15 m [7]) for hydrogen (the diameter of a single proton) to about 11.7 fm for uranium. [8]
One or more protons are present in the nucleus of every atom. They provide the attractive electrostatic central force which binds the atomic electrons. The number of protons in the nucleus is the defining property of an element, and is referred to as the atomic number (represented by the symbol Z).
A helium nucleus was presumed to have four protons plus two "nuclear electrons" (electrons bound inside the nucleus) to cancel two charges. At the other end of the periodic table, a nucleus of gold with a mass 197 times that of hydrogen was thought to contain 118 nuclear electrons in the nucleus to give it a residual charge of +79, consistent ...
A table or chart of nuclides is a two-dimensional graph of isotopes of the elements, in which one axis represents the number of neutrons (symbol N) and the other represents the number of protons (atomic number, symbol Z) in the atomic nucleus. Each point plotted on the graph thus represents a nuclide of a known or hypothetical chemical element.
The size of the 4 He nucleus has long been known to be in the order of magnitude of 1 fm. In an experiment involving the use of exotic helium atoms where an atomic electron was replaced by a muon, the nucleus size has been estimated to be 1.67824(83) fm. [4]
All the bound protons and neutrons in an atom make up a tiny atomic nucleus, and are collectively called nucleons. The radius of a nucleus is approximately equal to femtometres, where is the total number of nucleons. [42]
The mass number (symbol A, from the German word: Atomgewicht, "atomic weight"), [1] also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is approximately equal to the atomic (also known as isotopic) mass of the atom expressed in atomic mass units.
An atomic nucleus is shown here as a compact bundle of the two types of nucleons, protons (red) and neutrons (blue). In this picture, the protons and neutrons are shown as distinct, which is the conventional view in chemistry, for example.