Isotopes, sub-atomic particles and relative atomic mass
A subatomic particle is a particle smaller than an atom: it may be elementary or composite. In 1905, Albert Einstein demonstrated the physical reality of the photons, hypothesized by Max Planck in 1900, in order to solve the problem of black body radiation in thermodynamics. In 1874, G. Johnstone Stoney postulated a minimum unit of electrical charge, for which he suggested the name electron in 1891. In 1897, J. J. Thomson confirmed Stoney's conjecture by discovering the first subatomic particle, the electron (now denoted e−). Subsequent speculation about the structure of atoms was severely constrained by Ernest Rutherford's 1907 gold foil experiment, showing that the atom is mainly empty space, with almost all its mass concentrated in a (relatively) tiny atomic nucleus. The development of the quantum theory led to the understanding of chemistry in terms of the arrangement of electrons in the mostly empty volume of atoms. Particle physics and nuclear physics concern themselves with the study of these particles, their interactions, and matter made up of them which do not aggregate into atoms. These particles include atomic constituents such as electrons, protons, and neutrons (protons and neutrons are actually composite particles, made up of quarks), as well as other particles such as photons and neutrinos which are produced copiously in the sun.
However, most of the particles that have been discovered and studied are not encountered under normal earth conditions; they are produced in cosmic rays and during scattering processes in particle accelerators.
Isotopes are variants of a particular chemical element. While all isotopes of a given element share the same number of protons, each isotope differs from the others in its number of neutrons. Radioactive isotopes
The existence of isotopes was first suggested in 1912 by the radiochemist Frederick...
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