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; spins 1, 1, 3, 1). All four of these isotopes have the same number of protons and neutrons, and they all have an odd number for their nuclear spin. The only other observationally "stable" odd–odd nuclide is 180m 73 Ta (spin 9), the only primordial nuclear isomer, which has not yet been observed to decay despite experimental attempts. [5]
"Free" neutrons or protons are nucleons that exist independently, free of any nucleus. The free neutron has a mass of 939 565 413.3 eV/c 2, or 939.565 4133 MeV/c 2. This mass is equal to 1.674 927 471 × 10 −27 kg, or 1.008 664 915 88 Da. [4] The neutron has a mean-square radius of about 0.8 × 10 −15 m, or 0.8 fm, [20] and it is a spin-½ ...
For example, a neutral chlorine atom has 17 protons and 17 electrons, whereas a Cl − anion has 17 protons and 18 electrons for a total charge of −1. All atoms of a given element are not necessarily identical, however. The number of neutrons may vary to form different isotopes, and energy levels may differ, resulting in different nuclear ...
Protons and neutrons are best known in their role as nucleons, i.e., as the components of atomic nuclei, but they also exist as free particles. Free neutrons are unstable, with a half-life of around 13 minutes, but they have important applications (see neutron radiation and neutron scattering ).
The neutron number (symbol N) is the number of neutrons in a nuclide. Atomic number (proton number) plus neutron number equals mass number : Z + N = A . The difference between the neutron number and the atomic number is known as the neutron excess: D = N − Z = A − 2 Z .
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.
protons and neutrons have different masses, [7] [8] and different nuclides have different ratios of protons and neutrons. atomic masses are reduced, to different extents, by their binding energies. The ratio of atomic mass to mass number (number of nucleons) varies from 0.998 838 1346 (51) for 56 Fe to 1.007 825 031 898 (14) for 1 H.
where A = Atomic mass number (the number of protons Z, plus the number of neutrons N) and r 0 = 1.25 fm = 1.25 × 10 −15 m. In this equation, the "constant" r 0 varies by 0.2 fm, depending on the nucleus in question, but this is less than 20% change from a constant.