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Chapter Twelve: Atoms and the Periodic Table • • • • 12.1 12.2 12.3 12.4 The Structure of the Atom Electrons The Periodic Table of Elements Properties of the Elements What does a library, grocery store, department store, and phone book have in common? The first periodic table • Russian scientist Demitri Mendeleev discovered a set of patterns that applied to all the elements. • Mendeleev noticed a pattern of properties when he arranged the elements in order of increasing atomic mass. • Mendeleev used the melting point, density, color, atomic mass, and number of chemical bonds of elements to organize the periodic table. • Mendeleev arranged the elements in order of increasing mass so that elements with similar properties were in the same column. • Mendeleev used the properties of existing elements to predict properties of undiscovered elements. • The close match between Mendeleev’s predictions and the actual properties of new elements showed how useful his periodic table could be. • Periodic means “in a regular repeated pattern.” • In the periodic table, element properties repeat in each row or period. • British scientist Henry Moseley measured elements by the size of their positive charge (or the number of protons) • The periodic table changed from being organized by atomic mass to atomic number. 12.3 The Periodic Table • The periodic table organizes the elements according to how they combine with other elements (chemical properties). • The periodic table is organized in order of increasing atomic number. 12.3 The Periodic Table • The periodic table is further divided into periods and groups. • Each horizontal row is called a period. • Each vertical column is called a group.