"If all the elements are arranged in the order of their atomic weights, a periodic repetition of properties is obtained. This is expressed by the law of periodicity." 
Dmitri Mendeleev, Principles of Chemistry, Vol. 2, 1902, P. F. Collier, p17. 

"We have here a proof that there is in the atom a fundamental quantity, which increases by regular steps as one passes from one element to the next. This quantity can only be the charge on the central positive nucleus, of the existence of which we already have definite proof." 
Henry Moseley, Philosophical Magazine, Vol. 26, 1913, p1030. 

"The chemistry of an atom depends only on the number of electrons, which equals the number of protons and is called the atomic number. Chemistry is simply numbers, an idea Pythagoras would have liked. If you are an atom with one proton, you are hydrogen; two, helium;....." 
Carl Sagan, Cosmos, 1980, Random House, p223. Photo: NASA.