page_title: History of subatomic physics
section_title: Inside the atom
hierarchical_section_title: History of subatomic physics / From atoms to nucleons / Inside the atom
caption_attribution_description: English: Since 1920s cloud chambers played an important role of particle detectors and eventually lead to the discovery of positron, muon and kaon.Čeština: Pohled na mlžnou komoru shora. Na pozorovací ploše právě probíhá rozpad thoriové řady, kdy se radon Rn-220 rozpadá alfa přeměnou na polonium Po-216, které se ihned poté rozpadá na olovo Pb-212Italiano: Sin dagli anni ‘20 del XX secolo, le camere a nebbia hanno avuto un ruolo importante nei rivelatori di particelle e hanno contribuito in modo significativo alla scoperta del positrone, del muone e del kaone.
context_page_description: The idea that matter consists of smaller particles and that there exists a limited number of sorts of primary, smallest particles in nature has existed in natural philosophy at least since the 6th century BC. Such ideas gained physical credibility beginning in the 19th century, but the concept of "elementary particle" underwent some changes in its meaning: notably, modern physics no longer deems elementary particles indestructible. Even elementary particles can decay or collide destructively; they can cease to exist and create particles in result.
  Increasingly small particles have been discovered and researched: they include molecules, which are constructed of atoms, that in turn consist of subatomic particles, namely atomic nuclei and electrons. Many more types of subatomic particles have been found. Most such particles were eventually found to be composed of even smaller particles such as quarks. Particle physics studies these smallest particles and their behaviour under high energies, whereas nuclear physics studies atomic nuclei and their constituents: protons and neutrons.
context_section_description: By 1914, experiments by Ernest Rutherford, Henry Moseley, James Franck and Gustav Hertz had largely established the structure of an atom as a dense nucleus of positive charge surrounded by lower-mass electrons.
  These discoveries shed a light to the nature of radioactive decay and other forms of transmutation of elements, as well as of elements themselves. It appeared that atomic number is nothing else than (positive) electric charge of the atomic nucleus of a particular atom. Chemical transformations, governed by electromagnetic interactions, do not change nuclei – that's why elements are chemically indestructible. But when the nucleus change its charge and/or mass (by emitting or capturing a particle), the atom can become the one of another element. Special relativity explained how the mass defect is related to the energy produced or consumed in reactions. The branch of physics that studies transformations and the structure of nuclei is now called nuclear physics, contrasted to atomic physics that studies the structure and properties of atoms ignoring most nuclear aspects. The development in the nascent quantum physics, such as Bohr model, led to the understanding of chemistry in terms of the arrangement of electrons in the mostly empty volume of atoms.
  In 1918, Rutherford confirmed that the hydrogen nucleus was a particle with a positive charge, which he named the proton. By then, Frederick Soddy's researches of radioactive elements, and experiments of J. J. Thomson and F.W. Aston conclusively demonstrated existence of isotopes, whose nuclei have different masses in spite of identical atomic numbers. It prompted Rutherford to conjecture that all nuclei other than hydrogen contain chargeless particles, which he named the neutron.
  Evidences that atomic nuclei consist of some smaller particles (now called nucleons) grew; it became obvious that, while protons repulse each other electrostatically, nucleons attract each other by some new force (nuclear force). It culminated in proofs of nuclear fission in 1939 by Lise Meitner (based on experiments by Otto Hahn), and nuclear fusion by Hans Bethe in that same year. Those discoveries gave rise to an active industry of generating one atom from another, even rendering possible (although it will probably never be profitable) the transmutation of lead into gold; and, those same discoveries also led to the development of nuclear weapons.
caption: Cloud chambers played an important role as particle detectors in the early days of subatomic physics. Some particles including positron we even discovered by using this device
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