Organizing the Elements Dmitri Mendeleev discovered a set of patterns that applied to all the elements. He found that some elements have similar chemical and physical properties. For example: fluorine and chlorine are gases form similar compounds and are very corrosive Silver and copper are shiny metals that tarnish if exposed to the air. Mendeleev looked each element’s melting point, density, color, atomic mass, and number of chemical bonds it could form. Mendeleev noticed that a pattern of properties appeared when he arranged the elements in order of increasing atomic mass. In 1869 he published the first periodic table. He arranged the 63 known elements according to their atomic mass and similar properties. In the modern periodic table, the properties of the elements repeat in each period or row of the table. Periodic means “in a regular, repeated pattern.” New elements were added as they were discovered. In the early 1900’s the table was rearranged in the order of atomic number (number of protons). Atomic number – number of protons Average Atomic Mass – result of an average of all the isotopes of an element Isotope are atoms with the same number of protons, but a different number of neutrons found in the nucleus of the atom. Carbon-12 Carbon-13 Organization of the Periodic Table The properties of an element can be predicted from its location in the periodic table. Periods are the horizontal rows. As you move across a period from left to right, properties of the elements change according to a pattern. There are 7 periods. Groups are the elements that are in a column. Each column is called a group or family. There are 18 groups. Most groups are named for the first element in the column. The elements in each group have similar characteristics.