Modern periodic table is based on the atomic number of the elements. the experiment which proved the significance of the atomic number was?
A) Mullikan’s oil drop experiment
B) Moseley’s work on X-ray spectra
C) Bragg’s work on X-ray diffraction
D) Discovery of X-rays by Roentgen

Hint: As we know that according to Mendeleev the properties of elements are a periodic function of their atomic weights. But scientists worked on this theory and proved that the physical and chemical properties of the elements are periodic functions of their atomic number.

Complete step by step answer:
We know that according to Mendeleev the properties of elements are a periodic function of their atomic weights. But it was violated at certain places while giving the appropriate position to the few elements on the basis of their properties.

-After development of atomic structure theories about subatomic particles, in $1913$, Henry Moseley an English physicist observed the regularities in the characteristics X-ray spectra of elements where he found a plot of square root of the frequency $\sqrt v $, where $v$ is the frequency of X-rays emitted against the atomic number (Z) gave a straight line and not the plot of $\sqrt v $ against atomic weights or atomic masses. So, he modified Mendeleev’s periodic table and defined the modern periodic law as:
“The Physical and Chemical properties of the elements are periodic functions of their atomic number”.
Mullikan’s oil drop experiment was basically performed to measure the elementary electric charge, Bragg’s work on X-ray diffraction provided the crystal parameters from its X-ray patterns and Roentgen discovered the X-rays accidently while testing if the cathode rays could pass through the glass or not.

Therefore, option (B) is correct.

Note: After Moseley’s experiment it has become easy to understand the significance of the quantum numbers and electronic configuration in periodicity of the elements. The periodic law is essentially the consequence of periodic variation in electronic configuration, which determines the physical and chemical properties of elements and their compounds.