Our final video in this sequence introduces the idea of an outer shell, or valence electrons, in an electron configuration. Because valence electrons are in orbitals in the outermost shell, they are the electrons that can be involved in chemical reactions.
Valence electrons are the electrons in the outermost shell, or energy level, of an atom. For example, oxygen has six valence electrons, two in the 2s subshell and four in the 2p subshell. We can write the configuration of oxygen's valence electrons as 2s²2p⁴.
Source: Khan Academy, https://youtu.be/cZTdBsAjPPk
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