first 20 elements = Hydrogen(H), Helium(He), Lithium(Li), Beryllium(Be), Boron(B), Carbon(C), Nitrogen(N), Oxygen(O), Fluorine(F), Neon(Ne), Sodium(Na), Magnesium(Mg), Aluminum(Al), Silicon(Si), Phosphorus(P), Sulphur(s), Chlorine(Cl), Argon(Ar), Potassium(K), Calcium(Ca).