Lektronic Theory of Valency Kossel and Levis in 1916 forwarded Theory of Valency, which was completed by Langworth in 1919. This theory is based on given assumptions. The capacity of an atom to take part in chemical reaction is determined by the number of valence electrons, that is, electrons present in outermost orbid. Here, the electron present inside the valence cell are called core electron and electron present in outermost cell are called valence electrons. Atoms are most stable in duplicate or octet state. They try to acquire in this state, which is cause of chemical combination. Atoms try to be in duplicate state like helium or in octet state like neon, argon, krypton, etc. During chemical combination, atom adjust their electronic configuration to nearest inert gas by loss, or gain, or mutual saving of electrons, and try to complete its duplicate or octet state. The number of electrons lost, or gained, or shared is called Valency of that element. The attractive force which hold atoms in compound is called chemical bond, in this way chemical bond is formed between atoms, and compound exist.