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An atom (from the Greek atomos for "uncut") is the smallest, irreducible constituent of a stable element. Usually, the term refers to chemical atoms, which are the constituents of molecules and are not divisible by chemical reactions but are now known to be composed of even smaller particles called quarks.

  • In present-day usage "atom" is the name for a particular subunit of matter—not at all the smallest—consisting of an elaborate organization of particles into which it can be divided.
  • In chemical terms the atom is the smallest unit of matter that has the characteristic properties of one of the 109 known elements, such as iron or hydrogen.
  • In these terms the atom is the unit of matter that contains a small, dense, positively charged nucleus surrounded by sufficient negative electrons to make the whole electrically neutral.
  • The lightest atom, hydrogen, contains only one of these subunits in its nucleus and has a nuclear diameter of about 3 × 10–13 cm—about1/100,000 of the atomic diameter.
  • Moseley gave an important confirmation of this picture in 1913 when he showed that each element bombarded by high-speed electrons in an X-ray tube gives off a unique spectrum of X-ray wavelengths.