What is an atomic number and atomic mass?
“The atomic number originally was used to signify the element’s location in the periodic table. Dmitri Mendeleev arranged the known elements in increasing order of atomic weight and grouped by their similar chemical properties.[1] However, placing the elements in strict order of atomic weight resulted in some mismatches. Iodine and tellurium, if listed by atomic weight, appeared to be in the wrong order; and would fit better by chemical properties if their places in the table were swapped.[2] Placing them in the order which fit their properties most closely, their number in the table was their atomic number. This number appeared to be related to the mass of the atom but, as the discrepancy showed, reflected some property other than mass. The anomalies in this sequence were finally explained after research by Henry Gwyn Jeffreys Moseley in 1913.[3] Moseley discovered a strict relationship between the x-ray diffraction spectra of elements, and their correct location in the periodic table