1 янв 1913 г. - Henry Moseley

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It wasn’t until 1913, six years after Mendeleev’s death that the final piece of the puzzle fell into place. The periodic table was arranged by atomic mass, and this nearly always gives the same order as the atomic number. However, there were some exceptions (like iodine and tellurium, see above), which didn’t work. Mendeleev had seen that they needed to be swapped around, but it was Moseley that finally determined why.


He fired the newly-developed X-ray gun at samples of the elements, and measured the wavelength of X-rays given. He used this to calculate the frequency and found that when the square root of this frequency was plotted against atomic number, the graph showed a perfect straight line. He’d found a way to actually measure atomic number. When the First World War broke out, Moseley turned down a position as a professor at Oxford and became an officer in the Royal Engineers. He was killed by a sniper in Turkey in August 15, and many people think that Britain lost a future Nobel prize winner.


Within 10 years of his work, the structure of the atom had been determined through the work of many prominent scientists of the day, and this explained further why Moseley’s X-rays corresponded so well with atomic number. The idea behind the explanation is that when an electron falls from a higher energy level to a lower one, the energy is released as electromagnetic waves, in this case X-rays. The amount of energy that is given out depends on how strongly the electrons are attracted to the nucleus. The more protons an atom has in its nucleus, the more strongly the electrons will be attracted and the more energy will be given out. As we know, atomic number is also known as proton number, and it is the amount of protons that determine the energy of the X-rays.


After years of searching, at last we had a periodic table that really worked, and the fact that we still use it today is testament to the huge achievement of these and many other great minds of the last two centuries of scientific discovery.

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