Chapter 5: The Periodic Table Study Guide Section 1: Arranging the Elements Dmitri Mendeleev-1869 arranged the elements in order of increasing atomic mass. pattern appeared. Those that had similar properties occurred in a repeating pattern. -periodic: “happening at regular intervals.” table became known as the periodic table of the elements. used pattern he found to predict their properties. 1886- gaps were found; He was right. Henry Moseley- 1914 -British scientist determined the number of protons (atomic #) in the atom. All elements fit in Mendeleev’s table when they were arranged by atomic #. Periodic law: states that the repeating chem. + phys. Properties of elements change periodically with the elements’ atomic #. Elements are classified as: metals The number of electrons in the outer energy level of an atom in one characteristic that helps determines which category an element belongs in. Metals: most have few electrons in outer energy level. most are solid at RT. Properties: Shiny Ductile: drawn into thin wires. Good conductors of electric current. Malleable: flattened with a hammer. Good conductors of thermal energy. Nonmetals: -found to the right of the zigzag. -most have a complete set of electrons in outer level. more than ½ are gases at RT. Properties: Not malleable or ductile. Not shiny. Poor conductors of thermal energy and electric current. Metalloids (Semiconductors): -border the zigzag. ½ the complete set of electrons in outer level. Properties: (both metal + nonmetal) Shiny or brittle Hard Very brittle Good conductor of electric current at high temp. Each horizontal row of elements (left to right) on the periodic table are called periods. -phys. + chem. Properties in a row follow a repeating pattern as you move across the table. Each vertical column of elements (top to bottom) on the periodic table is called a group. -elements often have similar...