text: According to the modern concept about the structure of atom, the atom consists of shells with different energy levels, the farther from nucleus, the higher the energy. The electrons in the last shell are known as valence electrons. for eg. Sodium has the electronic configuration 1s2 2s1. So as valence shell is 2 therefore the no. of valence electron is 1. You should also study about the Quantum numbers for a better understanding.