Why Are Valence Electrons Important?
Valence electrons are the electrons in the outermost energy level of an atom — in the energy level that is farthest away from the nucleus. When chemists study chemical reactions, they study the transfer or sharing of electrons.
The electrons that are more loosely held by the nucleus (the valence electrons, those furthest away from the nucleus) are the ones that are gained, lost, or shared.
Electrons are negatively charged, while 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.
Chemists really only consider the electrons in the s and p orbitals in the energy level that is currently being filled as valence electrons.
In the electron configuration for oxygen (shown below), energy level 1 is filled, and there are 2 electrons in the 2s orbital and 4 electrons in the 2p orbital for a total of 6 valence electrons. Those valence electrons are the ones lost, gained, or shared.
Being able to determine the number of valence electrons in a particular atom gives you a big clue as to how that atom will react.