Chapter 12 - The Periodic Table Section 1 – Arranging the Elements (pp. 334-343

DEFINITIONS Periodic – having a regular, repeating pattern Periodic Law – states that the chemical and physical properties of elements are periodic functions of their atomic numbers

DISCOVERING A PATTERN Dmitri Mendeleev began looking for patterns among properties of elements He included density, appearance, atomic mass, melting point

DISCOVERING A PATTERN Mendeleev saw a repeating pattern to the elements. When the elements were arranged in order of increasing atomic mass, he observed similar chemical and physical properties in every eighth element. There were some elements missing in this arrangement.

CHANGING THE ARRANGEMENT Moseley determined the number of protons (the atomic number) of each element. He arranged the elements according to their atomic number. The Periodic Law says that the properties of each element is a periodic function of its atomic number.

FINDING YOUR WAY AROUND THE PERIODIC TABLE There are 3 classes of elements. They are: Metals Nonmetals Metalloids (or semiconductors)

METALS Most of the elements are metals. Metals are found to the left of the zigzag line on the Periodic Table. Most have few electrons in their outer energy level Most are solid at room temperature.

NONMETALS Found to the right of the zigzag line on the Periodic Table. Most have an almost complete set of electrons in their outer energy level. More than half are gases at room temperature.

METALLOIDS (SEMICONDUCTORS) Border the zigzag line on the Periodic Table Have about a half-complete set of electrons in their outer energy levels.

PERIODIC TABLE Each horizontal row of elements is called a period. Periods go from left to right. Each column is called a group or family. Groups (or families) go from top to bottom.