caption: Organization of the Periodic Table Groups or Families  Numbered 1 through 18  Vertical columns  Elements in each group have similar chemical and physical properties  Elements in each group have the same number of valence electrons Periods  Numbered 1 through 7  Horizontal rows  Increase by 1 proton and 1 electron as you go from left to right  Increase in mass number as you go from left to right  Increase in atomic number as you go from left to right
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