Chemistry Essentials for Dummies
Chapter 3. The Periodic Table
Arranging Elements in the Periodic Table
Look at the periodic table in Figure 3-1. The elements are arranged in order of increasing atomic number. The atomic number (number of protons) is located right above the element symbol. Under the element symbol is the atomic mass, or atomic weight. Atomic mass is a weighted average of all naturally occurring isotopes (see Chapter 2 for details).
Notice that two rows of elements — Ce-Lu (commonly called the lanthanides) and Th-Lr (the actinides) — have been pulled out of the main body of the periodic table. If they were included in the main body of the periodic table, the table would be much wider.
Using the periodic table, you can classify the elements in many ways. Two quite useful ways are
✓ Metals, nonmetals, and metalloids
✓ Families and periods
Grouping metals, nonmetals, and metalloids
Elements can be metals, nonmetals, or metalloids. In this section, I explain their properties.
If you look carefully at Figure 3-1, you can see a stair-stepped line starting at boron (B), atomic number 5, and going all the way down to polonium (Po), atomic number 84. Except for germanium (Ge) and antimony (Sb), all the elements to the left of that line can be classified as metals. Figure 3-2 shows the metals.
These metals have properties that you normally associate with the metals you encounter in everyday life. They’re solid at room temperature (with the exception of mercury, Hg, a liquid), shiny, good conductors of electricity and heat, ductile (they can be drawn into thin wires), and malleable (they can be easily hammered into very thin sheets). All these metals tend to lose electrons easily (see Chapter 5 for more info). As you can see, the vast majority of the elements on the periodic table are classified as metals.
Figure 3-2: The metals.
Except for the elements that border the stair-stepped line (more on those in a second), the elements to the right of the line, along with hydrogen, are classified as nonmetals. These elements are in Figure 3-3.
Figure 3-3: The nonmetals.
Nonmetals have properties opposite those of the metals. The nonmetals are brittle, aren’t malleable or ductile, and are poor conductors of both heat and electricity. They tend to gain electrons in chemical reactions. Some nonmetals are liquids at room temperature.
The elements that border the stair-stepped line in the periodic table are classified as metalloids, and they’re in Figure 3-4.
The metalloids, or semimetals, have properties that are somewhat of a cross between metals and nonmetals. They tend to be economically important because of their unique conductivity properties (they only partially conduct electricity), which make them valuable in the semiconductor and computer chip industry. (The term Silicon Valley doesn’t refer to a valley covered in sand; silicon, one of the metalloids, is used in making computer chips.)
Figure 3-4: The metalloids.
Arranging elements by families and periods
REMEMBER. The periodic table is composed of horizontal rows and vertical columns. Here’s how they’re named and numbered:
✓ Periods: The seven horizontal rows are called periods. The periods are numbered 1 through 7 on the left-hand side of the table (see Figure 3-1). Within each period, the atomic numbers increase from left to right.
Members of a period don’t have very similar properties. Consider the first two members of period 3: sodium (Na) and magnesium (Mg). In reactions, they both tend to lose electrons (after all, they are metals), but sodium loses one electron, and magnesium loses two. Chlorine (Cl), down near the end of the period, tends to gain an electron (it’s a nonmetal).
✓ Families: The vertical columns are called groups, or families. The families may be labeled at the top of the columns in one of two ways. The older method uses roman numerals and letters. Many chemists (especially academic ones like me) prefer and still use this method, so that’s what I use in describing the features of the table. The newer method simply uses the numbers 1 through 18.
The members of a family do have similar properties. Consider the IA family, starting with lithium (Li) and going through francium (Fr) (don’t worry about hydrogen, because it’s unique, and it doesn’t really fit anywhere). All these elements tend to lose only one electron in reactions. And all the members of the VIIA family tend to gain one electron.