A valence shell is an atomic shell that’s farthest from an atom’s nucleus. In other words, it’s an atom’s outermost shell or energy level, housing electron(s) that actively participate in a chemical reaction such as a chemical bond. The electron(s) in this shell could also break away from its parent atom to become a free electron. A free electron is basically any electron that isn’t attached to any atom, ion, or molecule. By the way, valence shell electrons are referred to as valence electrons.
The valence shell’s occupancy levels, or its number of electrons, determine whether the atom is receptive to an inter-atom chemical reaction or not. As the shells encircling or orbiting an atom’s nucleus can only hold a specific number of electrons, electrons will move to the valence shell if they don’t find space within inner shells.
As aforementioned, valence electrons participate actively in a chemical reaction. However, the level of reactivity depends on the number of electrons in the valence shell. In case the valence shell can hold a maximum of 8 electrons and it has all the eight spots filled, then the electrons in the valence shell would behave similar to the electrons in the inner shells. In other words, they won’t be chemically reactive or prompt the parent atom to form a chemical bond with another atom.