- 1866: English chemist William Odling proposes a method of arranging the elements based on their valence, the number of electrons in an atom's outermost shell that determines its chemical properties.
- 1869: Russian chemist Dmitri Mendeleev independently proposes a periodic table of elements and presents it to the Russian Chemical Society. Mendeleev's table is more comprehensive and accurate than previous attempts, and he predicts the properties of several missing elements.
- 1870: French chemist Julius Lothar Meyer independently publishes a periodic table of elements that is very similar to Mendeleev's table. Meyer's table also predicts the properties of missing elements and provides evidence for the periodicity of chemical properties.
- 1914: English physicist Henry Moseley discovers a relationship between the wavelength of X-rays emitted by an element and its atomic number. This discovery confirms the placement of elements in the periodic table based on their atomic number rather than their atomic weight, and leads to the modern definition of the periodic table.
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- 1866: English chemist William Odling proposes a method of arranging the elements based on their valence, the number of electrons in an atom's outermost shell that determines its chemical properties.
- 1869: Russian chemist Dmitri Mendeleev independently proposes a periodic table of elements and presents it to the Russian Chemical Society. Mendeleev's table is more comprehensive and accurate than previous attempts, and he predicts the properties of several missing elements.
- 1870: French chemist Julius Lothar Meyer independently publishes a periodic table of elements that is very similar to Mendeleev's table. Meyer's table also predicts the properties of missing elements and provides evidence for the periodicity of chemical properties.
- 1914: English physicist Henry Moseley discovers a relationship between the wavelength of X-rays emitted by an element and its atomic number. This discovery confirms the placement of elements in the periodic table based on their atomic number rather than their atomic weight, and leads to the modern definition of the periodic table.