Question

who was the first scientist who discovered that atomic number is more precise than atomic mass

Answer 1

Henry Mosley was the first scientist who discovered that atomic number is more precise than mass number.

Answer 2

Dmitri Mendeleev
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Dmitri Mendeleev was the first scientist who discovered that atomic number is more precise than atomic mass

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1) He was a Russian chemicalist 

2) He formulated the theory of Periodic table of chemical elements