Valence electrons can be determined quickly by the element’s position in the periodic table, with a few caveats for transition metals. Direct method (main- group elements)

• Count by group: For main-group elements (the s- and p-block), the number of valence electrons equals the group number, with a small exception for helium.
  • Group 1 → 1 valence electron
  • Group 2 → 2 valence electrons
  • Groups 13–18 (often labeled 3A–8A) correspond to 3–8 valence electrons respectively
  • Helium in Group 18 has 2 valence electrons, not 8
• Practical tip: Ignore the d-block (transition metals) when using this rule; they don’t follow the same simple pattern.

Alternative method (electron configuration)

• Write the electron configuration of the atom.
• The electrons in the outermost shell (the valence shell) are the valence electrons.
  • Example: Sodium—1s2 2s2 2p6 3s1 → outermost shell has 1 electron → 1 valence electron
  • Example: Oxygen—1s2 2s2 2p4 → outermost shell (second shell) has 6 electrons → 6 valence electrons

Notes and caveats

• Transition metals (groups 3–12) do not follow the same simple group-number rule due to d-electron involvement. In many introductory contexts, valence electrons are taken from the outermost s and, for some elements, the nearby d orbitals, but counting can be more nuanced.
• For noble gases, the valence electron count follows the same group rule, with helium as the notable exception.

If you’d like, share a specific element you’re studying, and I’ll show you both the group-rule approach and the electron configuration method for that element.