In 1911, a Dutch physicist, Antonius van den Broek, published his hypothesis that the atomic number of each element should represent the number of protons in the nucleus. At the time, atomic numbers were assigned by each element’s placement on the periodic table designed by Mendeleev. Therefore, atomic numbers were assigned by weight. Van den Broek’s theory was revolutionary, but he had no proof to support his hypothesis.
Two years later, Henry Moseley, a British physicist who had worked with Ernest Rutherford, began bombarding metals with electrons. He had learned that when metals are hit with electrons, they emit x-rays. Moseley wanted to measure the frequency of the x-rays. Frequency is the number of wave cycles a ray completes in one second.
Moseley found that each element created an x-ray with a different frequency, but his data didn’t make sense when he compared it to the atomic weight of the elements. He assumed that the frequencies of the x-rays would increase as the atomic weight increased, but they didn’t. Instead, the frequencies were changing based on the electric charge of the nucleus. The electric charge of the nucleus is caused by the number of protons in the nucleus.
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