You can determine how many valence electrons an element has by either looking at its position in the periodic table or by examining its electron configuration.

Using the Periodic Table

• For main-group elements (Groups 1, 2, and 13–18), the number of valence electrons corresponds to the group number:
  • Groups 1 and 2 have 1 and 2 valence electrons, respectively.
  • For Groups 13–18, the number of valence electrons equals the units digit of the group number. For example, Group 15 elements have 5 valence electrons.
• Transition metals (Groups 3–12) are an exception because their valence electrons include electrons in both the outermost shell and the (n−1)d subshell, making the count more complex

Using Electron Configuration

• Write out the electron configuration of the element.
• Identify the electrons in the outermost shell (the highest principal quantum number, n).
• The number of electrons in this outermost shell is the number of valence electrons.
• For example, oxygen has the electron configuration 1s² 2s² 2p⁴, so it has 6 valence electrons in the 2s and 2p orbitals

Summary

• Valence electrons are the electrons in the atom's outermost shell that participate in chemical bonding

• The periodic table group number is a quick way to find the number of valence electrons for main-group elements.
• Electron configuration gives a detailed method to count valence electrons by examining the outermost shell

This knowledge helps predict an element’s chemical behavior and bonding tendencies.