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What's Inside an Atom? Protons, Electrons, and Neutrons!

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Henry Moseley (1887–1915) was a British physicist known for his work on atomic structure. He discovered that the atomic number (not atomic weight) is the key to organizing elements in the periodic table, based on the frequencies of X-ray emissions. His law, Moseley's Law, helped resolve inconsistencies in the periodic table and predicted the existence of elements like technetium. Moseley died young in World War I, but his contributions remain vital to the understanding of atomic theory.

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Dmitri Mendeleev (1834–1907) was a Russian chemist, best known for creating the periodic table of elements. In 1869, he organized the elements by their properties and predicted the existence of undiscovered elements. His work was crucial to the development of modern chemistry and the current organization of the elements in the periodic table.

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The first periodic table was created by Dmitri Mendeleev in 1869. He organized the known elements by their atomic masses and noticed that their properties repeated at regular intervals. This allowed him to group them into columns, or groups, with similar properties. Mendeleev left gaps for elements that had not yet been discovered, accurately predicting their properties. While his table was not perfect, it was crucial for the organization of the elements and the development of chemistry, laying the foundation for the modern table, which is organized by atomic number.