The periodic table organizes the chemical elements in an orderly fashion according to their atomic number and properties. Dmitri Mendeleev created one of the first periodic tables in 1869 by arranging the elements in order of increasing atomic mass, which allowed for the prediction of undiscovered elements. The modern periodic table arranges elements by atomic number and places them into rows called periods and columns called groups based on their chemical and physical properties. Elements within the same group have similar properties including their valence electrons and the types of ions they form.
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