This question can be explained in a variety of ways, but the easiest can be that elements exist, and since each has a different atomic mass, all atoms are not identical in size and mass.
Atomic mass unit is the answer
The statement that describes the chemical reaction is D chlorine gas reacts with potassium bromide to form potassium chloride in solution and liquid bromide. The symbol "Cl" represents chlorine. The symbols in the brackets show the physical state of the substance, (g) is gaseous, (s) is solid, (aq) is aqueous and (l) is liquid.