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Its mass is slightly less than the mass of a neutron and approximately 1836 times the mass of an electron (the proton-to-electron mass ratio). Protons and neutrons, each with a mass of approximately one atomic mass unit, are jointly referred to as nucleons (particles present in atomic nuclei). One or more protons are present in the nucleus of ...
For other isotopes, the isotopic mass is usually within 0.1 u of the mass number. For example, 35 Cl (17 protons and 18 neutrons) has a mass number of 35 and an isotopic mass of 34.96885. [7] The difference of the actual isotopic mass minus the mass number of an atom is known as the mass excess, [8] which for 35 Cl is –0.03115.
Alternately, the atomic mass of a carbon-12 atom may be expressed in any other mass units: for example, the atomic mass of a carbon-12 atom is 1.992 646 882 70 (62) × 10 −26 kg. As is the case for the related atomic mass when expressed in daltons , the relative isotopic mass numbers of nuclides other than carbon-12 are not whole numbers, but ...
Electrons have been known since the late 19th century, mostly thanks to J.J. Thomson; see history of subatomic physics for details. Protons have a positive charge and a mass of 1.6726 × 10 −27 kg. The number of protons in an atom is called its atomic number.
The following table lists current measured masses and mass estimates for all the fermions, using the same scale of measure: millions of electron-volts relative to square of light speed (MeV/c 2). For example, the most accurately known quark mass is of the top quark (t) at 172.7 GeV/c 2, estimated using the on-shell scheme.
Most of the isotopes with atomic mass numbers below 14 decay to isotopes of carbon, while most of the isotopes with masses above 15 decay to isotopes of oxygen. The shortest-lived known isotope is nitrogen-10, with a half-life of 143(36) yoctoseconds , though the half-life of nitrogen-9 has not been measured exactly.
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To calculate the binding energy we use the formula Z (m p + m e) + N m n − m nuclide where Z denotes the number of protons in the nuclides and N their number of neutrons. We take m p = 938.272 0813 (58) MeV/c 2, m e = 0.510 998 9461 (30) MeV/c 2 and m n = 939.565 4133 (58) MeV/c 2. The letter A denotes the sum of Z and N (number