Periodic table

• Periods


  Horizontal rows (1-7). Properties change progressively across a period.
• Groups


  Vertical columns (1-18). Elements in the same group have similar properties and the same number of valence electrons.
• Group 1 (Alkali Metals)


  • Highly reactive, especially with water
  • 1 valence electron
  • Examples: Lithium (Li), Sodium (Na)
• Group 2 (Alkaline Earth Metals)


  • Reactive, but less so than Group 1
  • 2 valence electrons
  • Examples: Magnesium (Mg), Calcium (Ca)
• Group 17 (Halogens)


  • Very reactive non-metals
  • 7 valence electrons
  • Examples: Fluorine (F), Chlorine (Cl)
• Group 18 (Noble Gases)

  • Inert (non-reactive) gases
  • Full valence shell
  • Examples: Helium (He), Neon (Ne)
• Atomic Number


  Number of protons in an atom; determines the element
• Atomic Mass


  Average mass of an atom, roughly equal to the number of protons and neutrons
• Valence Electrons


  Electrons in the outermost shell; determine chemical properties and reactivity
• Elements are arranged by increasing atomic number
• Groups share similar properties
• Periods show trends in properties