Atom

• Atom


  The basic building block of all matter and chemistry. Atoms cannot be divided into smaller parts by ordinary chemical processes.
• Atom


  • Most of the atom is empty space
  • Consists of three basic types of subatomic particles: protons, neutrons, and electrons
  • Protons and neutrons form the atom's central nucleus
  • Electrons circle the nucleus in orbital paths called shells
• Neutral atom


  Has an equal number of protons (in the nucleus) and electrons (surrounding the nucleus), so the positive and negative charges are balanced
• Ion


  An ordinary atom that either gains or loses electrons, becoming a positive ion or a negative ion
• Subatomic particles

  • Protons
  • Neutrons
  • Electrons
  • Quarks
  • Leptons
• Chemical element


  Made up of only one type of atom, e.g. gold contains only gold atoms, neon contains only neon atoms
• Atomic number


  The total number of protons in the nucleus of an atom, which determines the element
• Valence shell


  The outermost electron shell of an atom, which determines the atom's chemical activity
• Atoms with the same number of electrons in the outer shell have similar chemical properties
• Nucleus


  • Size ranges from about 4 fm for a light nucleus such as carbon to 15 fm for a heavy nucleus such as lead
  • Virtually all the mass of the atom is concentrated in the nucleus
• Protons


  Massive, positively charged particles
• Neutrons


  Have no charge and are slightly more massive than the protons
• Nuclei can have anywhere from 1 to nearly 300 protons and neutrons
• The lightest nucleus, that of hydrogen, is 1,836 times more massive than an electron, while heavy nuclei are nearly 500,000 times more massive
• Atomic number (Z)


  The number of units of positive charge (protons) in the nucleus
• A neutral atom has an equal number of protons and electrons so that the positive and negative charges exactly balance
• It is the number of protons in the nucleus that determines the chemical properties of an atom
• Isotopes


  Nuclei with the same number of protons but different numbers of neutrons
• The number of neutrons in a nucleus affects the mass of the atom but not its chemical properties
• Atomic mass number


  The sum of the number of protons and neutrons in the nucleus
• Atomic mass unit


  1/12 of the mass of an atom of carbon-12, or 1.660538921 × 10^-24 gram
• Electron

  • Has a negative electric charge
  • Charge was first measured by Robert Millikan between 1909 and 1910
  • Charge is 1.602176565 × 10^-19 coulomb
• The charge on the proton is equal in magnitude to that on the electron but opposite in sign
• Spin


  An intrinsic property of the electron, where it can only exist at certain specific rates like 1/2, 1, 3/2, 2, etc. times a basic unit of rotation
• Fermions


  Particles with half-integer spin, like electrons, protons and neutrons
• Pauli exclusion principle


  No two fermions can occupy the same state
• Magnetic moment


  The strength of a magnetic field related to the torque experienced by a magnetic object, for electrons it is -9.28 × 10^-24 joule per tesla