What are valence electrons and how do you find them?

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t-nez eNotes educator| Certified Educator

Valence electrons are the electrons in an atom's highest energy level, farthest from the nucleus. These are the electrons that are lost, gained or shared when the atom forms chemical bonds to other atoms. The number of valence electrons an atom has can be determined from its position on the periodic table.

There are two ways to number the groups of the periodic table. One is number from 1 to 18. The other is to number the main group elements, which are the s and p blocks with the Roman numerals I-VIII and the letter A. The transition metals are then numbered with Roman numerals and the letter B. For the main group element IA-VIIIA, the main group number equals the number of valence electrons:

  • Group IA - alkali metals - 1 valence electron
  • Group IIA - alkaline earth metals - 2 valence electrons
  • Group IIIA - 3 valence electrons
  • Group IVA - 4 valence electrons
  • Group VA - 5 valence electrons
  • Group VIA- 6 valence electrons
  • Group VII - halogens - 7 valence electrons
  • Group VIII - noble gases - 8 valence electrons

The valence electrons of transition metals are more complicated. In general, they're the highest level s and d electrons, which is usually the number of spaces that the element is from the left. Some transition metals have electron configurations that are exceptions to the aufbau principle and their valence electrons might include only one or no s-electrons. 

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