The first column of the periodic table, Group 1, contains elements that are soft, shiny solids. These alkali metals include lithium, sodium, potassium, rubidium, and cesium. At the other end of the table, fluorine, chlorine, bromine, iodine, and astatine appear in the next-to-last column. These are the halogens, or Group 17 elements. These four elements exist as diatomic molecules, so their formulas have the form X2. A sample of chlorine appears in Figure 2 (lesson twenty one). Each alkali metal combines with any of the halogens in a 1:1 ratio to form a white crystalline solid. The general formula of these compounds is AX, where A represents the alkali metal and X represents the halogen ( AX = NaCl, LiBr, CsBr, KI, etc.).
The elements in the second column of the table (Group 2) are the alkaline earth metals. These resemble the alkali metals in their appearance, but they have different chemical properties. For example, each of these metals combines with the halogens in a 1:2 ratio (AX2 = CaCl2, MgBr2, BaBr2, etc.). Each also reacts with atmospheric oxygen to form a solid with the formula AO (BaO, CaO, etc.).
The last column of the periodic table contains the noble gases, or Group 18 elements, all of which occur in nature as gases. With a few exceptions, these elements do not undergo chemical reactions.
The elements in Groups 3 through 12 are known as transition metals. The elements in rows 6 and 7 that are normally shown below the rest of the table are the inner transition metals, subdivided into lanthanides (row 6) and actinides (row 7). All other elements are main group elements.
The periodic table is a useful way to organize chemical properties. To help you see the patterns, the periodic table on the inside front cover of this book highlights the various groups of elements. As you learn more about chemical structure and behavior, you will discover the principles that account for similarities and differences in the chemical behavior of the elements.