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In a discussion on the topic of the atomic theory, prominent names such as Rutherford, Bohr, and Curie instantly come into mind, but on the other hand, despite being a lesser known person, English physicist Henry Moseley has contributed revolutionizing concepts regarding the chemical concept of the atomic number. In his short 29-year-old life, Moseley had completely redefined the formerly accepted idea, which had been stating that the atomic number of an element is somewhat a semi-arbitrary sequential number, just barely based on the sequence of atomic masses. This had meant that the atomic numbers were not strictly defined by their atomic mass, as many chemists at that time, such as the infamous Russian Dmitri Ivanovich Mendeleev, had changed them to fit in their desirable positions. For example, the metals cobalt and nickel had been assigned the atomic numbers 27 and 28, respectively, based on their known chemical and physical properties, even though they have nearly the same atomic masses. In fact, the atomic mass of cobalt is slightly larger than that of nickel, which would have placed them in backwards order if they had been placed in the Periodic Table blindly according to atomic mass. In order to fix this baseless mindset, starting from 1913, he began to utilize the method of X-ray spectroscopy in physics, using Bragg's

Essay - Henry Moseley

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English essay about Henry Moseley

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In a discussion on the topic of the atomic theory, prominent names such as Rutherford, Bohr, and Curie instantly come into mind, but on the other hand, despite being a lesser known person, English physicist Henry Moseley has contributed revolutionizing concepts regarding the chemical concept of the atomic number. In his short 29-year-old life, Moseley had completely redefined the formerly accepted idea, which had been stating that the atomic number of an element is somewhat a semi-arbitrary sequential number, just barely based on the sequence of atomic masses. This had meant that the atomic numbers were not strictly defined by their atomic mass, as many chemists at that time, such as the infamous Russian Dmitri Ivanovich Mendeleev, had changed them to fit in their desirable positions. For example, the metals cobalt and nickel had been assigned the atomic numbers 27 and 28, respectively, based on their known chemical and physical properties, even though they have nearly the same atomic masses. In fact, the atomic mass of cobalt is slightly larger than that of nickel, which would have placed them in backwards order if they had been placed in the Periodic Table blindly according to atomic mass. In order to fix this baseless mindset, starting from 1913, he began to utilize the method of X-ray spectroscopy in physics, using Bragg's diffraction law to determine the X-ray wavelengths. This was a ground-breaking scheme at that time, and soon Moseley discovered a systematic mathematical relationship between the wavelengths of the X-rays produced and the atomic numbers of the metals that were used as the targets in X-ray tubes. This has become known as Moseley's law. On the contrary with the previous impressions, Moseley's experiments in X-ray spectroscopy directly shows from their physical features that cobalt and nickel have the different atomic numbers, 27 and 28, and that they are placed in the Periodic Table correctly by Moseley's measurements of their atomic numbers. Therefore, upon the founding of the Moseleys law, his discovery demonstrated that the atomic numbers of elements are not just rather arbitrary numbers based on chemistry and the intuition of chemists, but rather, they have a solid experimental basis from the physics of their X-ray spectra. Sometime in the first half of 1914, Moseley resigned from his position at Manchester, with plans to return to Oxford and continue his physics research there. However, World War I broke out in August 1914, and Moseley turned down this job offer in order to enlist in the Royal Engineers of the British Army instead. Moseley served as a technical officer in communications during the Battle of Gallipoli, in Turkey, beginning in April 1915, where he was killed in action on 10 August 1915. Moseley's death in World War I was a great loss causing the British government to institute a policy of no longer allowing its prominent and promising scientists to enlist for combat duty in the armed forces of the Crown.