Which fact was most likely discovered between the time of Mendeleev’s table and the time of Moseley’s table that helped Moseley develop his version?

The number of protons in an atom is different than the atom’s total mass.  

Explanation:

Mendeleev knew only that, with some anomalies, the properties of elements varied periodically with their atomic masses.

Moseley's X-ray experiments enabled him to remove these anomalies and to show that the properties of elements varied with their atomic numbers.  

Chadwick later discovered that the mass differences were caused by the presence of neutrons.

explanation:

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