"Mendeleev noticed that there were regular repeating patterns and similarities between different elements when they were ordered in terms of their atomic weights. Atomic weight was originally calculated from the average mass of one mole of the element relative to hydrogen (though the modern comparison is with carbon-12...). However, even Mendeleev was aware that classification based on atomic weight was flawed; for example, the elements tellurium and iodine occurred in the wrong order on the basis of their atomic weight, with iodine having a lower atomic weight but rightly having the properties of the halogen group (chlorine, fluorine, etc)... Mendeleev therefore decided to reverse the periodic sequence in their case. It was only in the twentieth century, when the structure of the atom based on the protons and the neutrons was identified, that the use of atomic number rather than atomic weight provided the ideal basis for the periodic table classification system. The atomic number of an element refers to its number of protons... Because the chemical properties of an element are determined by the number of negatively charged electrons, which in the non-ionozed form of the element matches the number of protons, there are many elements that have a number of isotopes, that is, have different numbers of neutrons, affecting the atomic weight but not the atomic number. Hence, Mendeleev's tellurium and iodine anomoly turned out to be because the tellurium he had sourced contained a preponderance of a heavier tellurium isotope giving it a greater atomic weight than idodine."