In chemistry, valence electrons are the electrons in the outside or valence electron shell of an atom. They determine the valency of the atom which is important in how a chemical element reacts with other elements. Elements which have eight valence electrons (noble gases) are inert and they do not tend to create chemical reactions with other elements in the Periodic table group.
The number of valence electrons is also important for determining the group of an element in the Periodic table, because an element's number of valence electron is the same as the number of its group in the periodic table. To calculate the amount of valence electrons present in an element you have to find the last number of the electron configuration.