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The periodic table organizes all known elements by atomic number (number of protons) and groups them by shared chemical properties. Elements in the same column (group) share the same number of valence electrons — which drives similar reactivity. Elements in the same row (period) have the same number of electron shells. Dmitri Mendeleev published the first version in 1869; he left gaps for elements not yet discovered and correctly predicted their properties. Today the table has 118 confirmed elements — 94 occur naturally, the rest are synthetic.