who was the first one to discover numbers for atoms?
Henry Moseley
Atomic number
he square root of the frequency of these photons increased from one target to the next in an arithmetic progression. This led to the conclusion (Moseley's law) that the atomic number does closely correspond (with an offset of one unit for K-lines, in Moseley's work) to the calculated electric charge of the nucleus, i.e. the element number "Z". 
the density of osmium which is the densest metal It is approximately twice as dense as lead and slightly denser than iridium. what is osmium? It is a chemical element with the symbol Os and atomic number 76. where did it get its name from? From Greek word  "osme" meaning "smell".
wiki:1630841
Atomic number [SEP] Moseley's 1913 experiment
The square root of the frequency of these photons increased from one target to the next in an arithmetic progression. This led to the conclusion (Moseley's law) that the atomic number does closely correspond (with an offset of one unit for K-lines, in Moseley's work) to the calculated electric charge of the nucleus, i.e. the element number "Z". Among other things, Moseley demonstrated that the lanthanide series (from lanthanum to lutetium inclusive) must have 15 members—no fewer and no more—which was far from obvious from known chemistry at that time.