a = Direct Lattice Vectors; b = Reciprocal Lattice Vectors; 

X = Symbol of the element; A = Mass Number ;  

P = Number of Protons; N= Number of Nutrons

A = Mass number; Z= Atomic Number = Number of protons present in the nucleus 

C= Number of Carbon atoms present ; H = Number of Hydrogen atoms present; X= Number of Halogen atoms present (Cl, Br, I or F);
N= Number of Nitrogen atoms present; DBE = Double bond equivalent a.k.a Level of Unsaturation

K₁= The equilibrium constant at absolute temperature T₁ ; K₂=The equilibrium constant at absolute temperature T₂ ;
ΔH^o= The standard enthalpy change ; T₁ & T₂ = Temperature; R= Gas constant

P = Pressure ; V = Volume ;  a , b = Dieterici Constants ; n= Number of Moles ; R= Universal Gas Constant ; T= Temperature

I = Ionic Strength ; z =  Charge number of  an ionic specie ; f = Activity coefficient ;

A= A  constant that depends on temperature.
(A is assumed to be 0.509 mol^-1/2 kg-1/2 which corresponds to water at 25^oC)

ΔH= Enthalpy Change ; ΔU = (U_final - U_initial) Internal Energy Change;  ΔV = (V_final - V_initial)Volume Change;

I = Ionic Strength ; c_i =  concentration of the i^th ion ; z_i = Charge of the i^th ion

K_eq = Equilibrium Constant of Adsorbtion;  P = Partial Pressure or Concentration of the absorbate;  θ = The fraction of adsorbent surface covered by adsorbate