Atomic Structure

• Atom


  The smallest unit of an element that retains the properties of that element. It consists of a nucleus (containing protons and neutrons) and electrons orbiting the nucleus.
• Molecule


  A group of two or more atoms bonded together. For example, O₂ is a molecule of oxygen, and H₂O is a molecule of water.
• Sub-atomic Particles


  • Protons
  • Neutrons
  • Electrons
• Protons


  • Position: In the nucleus
  • Relative Mass: 1
  • Relative Charge: +1
• Neutrons


  • Position: In the nucleus
  • Relative Mass: 1
  • Relative Charge: 0 (neutral)
• Electrons


  • Position: Orbiting the nucleus in electron shells
  • Relative Mass: 1/1836 (almost negligible compared to protons and neutrons)
  • Relative Charge: -1
• Atomic Number (Z)


  The number of protons in the nucleus of an atom. It defines the element.
• Mass Number (A)


  The total number of protons and neutrons in the nucleus of an atom.
• Isotopes


  Atoms of the same element (same number of protons) that have different numbers of neutrons, and therefore different mass numbers.
• Relative Atomic Mass (Ar)


  The weighted average mass of the atoms in a naturally occurring sample of an element, relative to one-twelfth of the mass of a carbon-12 atom.
• Calculating Relative Atomic Mass (Ar)

  1. Multiply the mass of each isotope by its relative abundance (percentage or fraction)

  2. Add these values together

  3. Divide by the total relative abundance (usually 100 if percentages are used)
• The relative atomic mass of chlorine is 35.5