<P> In the 1950s, the development of improved particle accelerators and particle detectors allowed scientists to study the impacts of atoms moving at high energies . Neutrons and protons were found to be hadrons, or composites of smaller particles called quarks . The standard model of particle physics was developed that so far has successfully explained the properties of the nucleus in terms of these sub-atomic particles and the forces that govern their interactions . </P> <P> Though the word atom originally denoted a particle that cannot be cut into smaller particles, in modern scientific usage the atom is composed of various subatomic particles . The constituent particles of an atom are the electron, the proton and the neutron; all three are fermions . However, the hydrogen - 1 atom has no neutrons and the hydron ion has no electrons . </P> <P> The electron is by far the least massive of these particles at 6969911000000000000 ♠ 9.11 × 10 kg, with a negative electrical charge and a size that is too small to be measured using available techniques . It was the lightest particle with a positive rest mass measured, until the discovery of neutrino mass . Under ordinary conditions, electrons are bound to the positively charged nucleus by the attraction created from opposite electric charges . If an atom has more or fewer electrons than its atomic number, then it becomes respectively negatively or positively charged as a whole; a charged atom is called an ion . Electrons have been known since the late 19th century, mostly thanks to J.J. Thomson; see history of subatomic physics for details . </P> <P> Protons have a positive charge and a mass 1,836 times that of the electron, at 6973167260000000000 ♠ 1.6726 × 10 kg . The number of protons in an atom is called its atomic number . Ernest Rutherford (1919) observed that nitrogen under alpha - particle bombardment ejects what appeared to be hydrogen nuclei . By 1920 he had accepted that the hydrogen nucleus is a distinct particle within the atom and named it proton . </P>

Atoms or molecules which carry a charge are called
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