Valence electrons: Clues about chemical reactions

Knowing the number of electrons that are an atom’s outermost energy level gives you a big clue about how that atom will react.  When chemists study chemical reactions, they study the transfer or sharing of electrons. The electrons more loosely held by the nucleus the electrons in the energy level farthest away from the nucleus — are the ones that are gained, lost, or shared.

Electrons are negatively charged, and the nucleus has a positive charge due to the protons. The protons attract and hold the electrons,  but the farther away the electrons are, the less the attractive force.

The electrons in the outermost energy level are commonly called valence electrons. Chemists really only consider the electrons in the s and p orbitals in the energy level that’s currently being filled as valence electrons. In the electron configuration for oxygen, 1s²2s²2p⁴, energy level 1 is filled, and there are two electrons in the 2s orbital and four electrons in the 2p orbital for a total of six valence electrons. Those valence electrons are the ones lost, gained, or shared.