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  2. Atom - Wikipedia

    en.wikipedia.org/wiki/Atom

    An atom consists of a nucleus of protons and generally neutrons, ... Chemistry is the science that studies these changes. ... By definition, ...

  3. Atomic physics - Wikipedia

    en.wikipedia.org/wiki/Atomic_physics

    Bohr Model of the Atom The Bohr model, proposed by Niels Bohr in 1913 , is a revolutionary theory describing the structure of the hydrogen atom. It introduced the idea of quantized orbits for electrons, combining classical and quantum physics.

  4. History of atomic theory - Wikipedia

    en.wikipedia.org/wiki/History_of_atomic_theory

    The current theoretical model of the atom involves a dense nucleus surrounded by a probabilistic "cloud" of electrons. Atomic theory is the scientific theory that matter is composed of particles called atoms. The definition of the word "atom" has changed over the years in response to scientific discoveries.

  5. Atomic, molecular, and optical physics - Wikipedia

    en.wikipedia.org/wiki/Atomic,_molecular,_and...

    In a neutral atom, the system will emit a photon of the difference in energy. However, if the lower state is in an inner shell, a phenomenon known as the Auger effect may take place where the energy is transferred to another bound electrons causing it to go into the continuum. This allows one to multiply ionize an atom with a single photon.

  6. Atomic nucleus - Wikipedia

    en.wikipedia.org/wiki/Atomic_nucleus

    The adoption of the term "nucleus" to atomic theory, however, was not immediate. In 1916, for example, Gilbert N. Lewis stated, in his famous article The Atom and the Molecule, that "the atom is composed of the kernel and an outer atom or shell." [12] Similarly, the term kern meaning kernel is used for nucleus in German and Dutch.

  7. Matter - Wikipedia

    en.wikipedia.org/wiki/Matter

    Three divisions of matter are recognized in science: masses, molecules and atoms. A Mass of matter is any portion of matter appreciable by the senses. A Molecule is the smallest particle of matter into which a body can be divided without losing its identity. An Atom is a still smaller particle produced by division of a molecule.

  8. Atomic mass - Wikipedia

    en.wikipedia.org/wiki/Atomic_mass

    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 ...

  9. Chemistry - Wikipedia

    en.wikipedia.org/wiki/Chemistry

    Chemistry is the scientific study of the properties and behavior of matter. [1] It is a physical science within the natural sciences that studies the chemical elements that make up matter and compounds made of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they undergo during reactions with other substances.