I.

Introduction

Periodic Table, table of the chemical elements arranged to illustrate patterns of recurring chemical and physical properties. Elements, such as oxygen, iron, and gold, are the most basic chemical substances and cannot be broken down by chemical reactions. All other substances are formed from combinations of elements. The periodic table provides a means of arranging all the known elements and even those yet to be discovered.

Although all elements differ from one another, some show similarities that allow scientists to categorize them. Around 1870, Russian chemist Dmitry Mendeleyev used these similarities to construct the original periodic table. Since that time, newly discovered elements have been added and the table’s layout has changed; however, the modern periodic table conveys essentially the same information as the one that Mendeleyev created. In the modern table, elements with similar properties fall into columns called groups or families. Group 1 of the periodic table, for example, contains a number of soft metals, all of which react vigorously with water to form hydrogen gas.