Atoms

• The Atomic Theory


  The concept of atoms dates back to ancient Greece, proposed by Leucippus and Democritus
• Atomic Structure


  Protons, neutrons and electrons: Protons in the nucleus, neutrons also in the nucleus, electrons in energy levels or electron shells
• Atomic Mass


  The total mass of protons, neutrons, and electrons in an atom (measured in u or amu); affects an element's identity and chemical properties
• Electrons and Atomic Mass

  Electrons don't contribute significantly to atomic mass due to their small mass and large number in an atom
• Numbers on Chemical Symbols

  Mass number (A) = number of protons + neutrons; charge number (Z) = number of protons; determines element's identity and chemical properties
• Atomic Number on Chemical Symbols

  The number of protons in an atom's nucleus, determining element identity, chemical behavior, and properties
• Protons and electrons are the same in chemical elements.

  For example, if the atomic number is 6, there are 6 protons and 6 electrons.
• Electrons do not have enough mass to contribute to the overall mass of elements because they are in the electron shell and rather than the nucleus.
• Protons and neutrons make up the nucleus of an atom.