2.5 Periodic table of the elements

In 1869 the Russian chemist Dmitri Mendeleev (1834-1907) found that when he arranged the elements in order of atomic weight, he could place them in horizontal rows, one row under the other, so that the elements in any one vertical column have similar properties. A tabular arrangement of elements in rows and columns, highlighting the regular repetition of properties of the elements, is called a periodic table.

Periods and Groups

The basic structure of the periodic table is its division into rows and columns, or periods and groups. A period consists of the elements in any one horizontal row of the periodic table. A group consists of the elements in any one column of the periodic table.

The first period of elements is short, consisting of only hydrogen (H) and helium (He). The second period has 8 elements, beginning with lithium (Li) and ending with neon (Ne). There is then another period of 8 elements, and this is followed by a period having 18 elements, beginning with potassium (K) and ending with krypton (Kr). The fifth period also has 18 elements. The sixth period actually consists of 32 elements, but in order for the row to fit on a page, part of it appears at the bottom of the table. Otherwise the table would have to be expanded, with the additional elements placed after lanthanum (La, atomic number 57). The seventh period, though not complete, also has some of its elements placed as a row at the bottom of the table.

Metals, Nonmetals and Metalloids

The elements of the periodic table in are divided into metals on the left and nonmetals on the right. A metal is a substance or mixture that has a characteristic luster, or shine, and is generally a good conductor of heat and electricity. Except for mercury, the metallic elements are solids at room temperature (about 20°C). They are more or less malleable (can be hammered into sheets) and ductile (can be drawn into wire).

A nonmetal is an element that does not exhibit the characteristics of a metal. Most of the nonmetals are gases (for example, chlorine and oxygen) or solids (for example, phosphorus and sulfur). The solid nonmetals are usually hard, brittle substances. Bromine is the only liquid nonmetal.

Most of the elements bordering the staircase line in the periodic table are metalloids, or semimetals. A metalloid, or semimetal, is an element having both metallic and nonmetallic properties. These elements, such as silicon (Si) and germanium (Ge), are usually good semiconductors – elements that, when pure, are poor conductors of electricity at room temperature but become moderately good conductors at higher temperatures.

Chemical Substances: Formulas and Names

Atomic theory has developed steadily since Dalton's time and has become the cornerstone of chemistry. It results in an enormous simplification: all of the millions of compounds we know today are composed of the atoms of just a few elements. In the next section, we look more closely at how we describe the composition and structure of chemical substances in terms of atoms.