Question 6: How did Moseley discover that the atomic number (Z) is the fundamental property of an element?

Henry Moseley experimentally found a relation between the X-rays and atomic number (Z) of the element used as cathode. He found from this experiment that the atomic number (Z) is the fundamental property of an element which determined most of the characteristics of the element.

Moseley’s experiment diagram is shown below. He used cathodes of different material and obtained spectrum of the emitted X-rays in each case. The study led him to the conclusion that the positive charge on the nucleus is the fundamental property of the element. He observed that the wavelength of the X-rays in each case was the characteristic of each element he used as anode. Based on his observations, he formulated a law called the Moseley’s law. It states that “the square root of the frequency, √v is directly proportional to the atomic number (Z) of the element.”

Mathematically,

√v ∝ Z. For more accurate results, Moseley modified the result for characteristic X-rays as  √v ∝  Z-b.

Putting ‘a’ as the constant of proportionality, the law can be written as,

Knowing the frequency of the X-rays, the atomic number, Z can be calculated.

The law showed that the properties of the element depend on the atomic number Z of the element and not its mass number. Secondly, some new elements are discovered on the basis of this law. Similarly the atomic number of rare earth’s element are determined with the help of this law.

Thus it proved that the atomic number Z is a fundamental property of the elements.