Chemical Foundations: Elements, Atoms, and Ions
Introduction to the Periodic Table
· To learn about various features of the periodic table.
· To learn some of the properties of metals, nonmetals, and metalloids.
In any room where chemistry is taught or practiced, you are almost certain to find a chart called the periodic table hanging on the wall. This chart shows all of the known elements and gives a good deal of information about each. As our study of chemistry progresses, the usefulness of the periodic table will become more obvious. This section will simply introduce it.
A simple version of the periodic table is shown in Fig. 4.9. Note that each box of this table contains a number written over one, two, or three letters. The letters are the symbols for the elements. The number shown above each symbol is the atomic number (the number of protons and also the number of electrons) for that element. For example, carbon has atomic number :
The periodic table.
Lead has atomic number :
Note that the elements are listed on the periodic table in order of increasing atomic number. They are also arranged in specific horizontal rows and vertical columns. The elements were first arranged in this way in 1869 by Dmitri Mendeleev, a Russian scientist. Mendeleev arranged the elements in this way because of similarities in the chemical properties of various “families” of elements. For example, fluorine and chlorine are reactive gases that form similar compounds. It was also known that sodium and potassium behave very similarly. Thus the name periodic table refers to the fact that as we increase the atomic numbers, every so often an element occurs with properties similar to those of an earlier (lower-atomic-number) element. For example, the elements
all show similar chemical behavior and so are listed vertically, as a “family” of elements.
A family of elements with similar chemical properties that lie in the same vertical column on the periodic table is called a group . Groups are often referred to by the number over the column (see Fig. 4.9). Note that the group numbers are accompanied by the letter A on the periodic table in Fig. 4.9. For simplicity we will delete the A’s when we refer to groups in the text. Many of the groups have special names. For example, the first column of elements (Group 1) has the name alkali metals . The Group 2 elements are called the alkaline earth metals , the Group 7 elements are the halogens , and the elements in Group 8 are called the noble gases . A large collection of elements that spans many vertical columns consists of the transition metals .
Most of the elements are metals . Metals have the following characteristic physical properties:
Physical Properties of Metals
1. Efficient conduction of heat and electricity
2. Malleability (they can be hammered into thin sheets)
3. Ductility (they can be pulled into wires)
4. A lustrous (shiny) appearance
For example, copper is a typical metal. It is lustrous (although it tarnishes readily); it is an excellent conductor of electricity (it is widely used in electrical wires); and it is readily formed into various shapes, such as pipes for water systems. Copper is one of the transition metals—the metals shown in the center of the periodic table. Iron, aluminum, and gold are other familiar elements that have metallic properties. All of the elements shown to the left of and below the heavy “stair-step” black line in Fig. 4.9 are classified as metals, except for hydrogen (Fig. 4.10).
The elements classified as metals and as nonmetals.
The relatively small number of elements that appear in the upper-right corner of the periodic table (to the right of the heavy line in Figs. 4.9 and 4.10) are called nonmetals . Nonmetals generally lack those properties that characterize metals and show much more variation in their properties than metals do. Whereas almost all metals are solids at normal temperatures, many nonmetals (such as nitrogen, oxygen, chlorine, and neon) are gaseous and one (bromine) is a liquid. Several nonmetals (such as carbon, phosphorus, and sulfur) are also solids.
The elements that lie close to the “stair-step” line as shown in blue in Fig. 4.10 often show a mixture of metallic and nonmetallic properties. These elements, which are called metalloids or semimetals , include silicon, germanium, arsenic, antimony, and tellurium.
As we continue our study of chemistry, we will see that the periodic table is a valuable tool for organizing accumulated knowledge and that it helps us predict the properties we expect a given element to exhibit. We will also develop a model for atomic structure that will explain why there are groups of elements with similar chemical properties.
Interactive Example 4.5. Interpreting the Periodic Table
For each of the following elements, use the periodic table in the front of the book to give the symbol and atomic number and to specify whether the element is a metal or a nonmetal. Also give the named family to which the element belongs (if any).
a. Iodine (symbol ) is element 53 (its atomic number is ). Iodine lies to the right of the stair-step line in Fig. 4.10 and thus is a nonmetal. Iodine is a member of Group 7, the family of halogens.
b. Magnesium (symbol ) is element 12 (atomic number ). Magnesium is a metal and is a member of the alkaline earth metal family (Group 2).
c. Gold (symbol ) is element 79 (atomic number ). Gold is a metal and is not a member of a named vertical family. It is classed as a transition metal.
d. Lithium (symbol ) is element 3 (atomic number ). Lithium is a metal in the alkali metal family (Group 1).
Self-Check: Exercise 4.5
· Give the symbol and atomic number for each of the following elements. Also indicate whether each element is a metal or a nonmetal and whether it is a member of a named family.
See Problems 4.53 and 4.54.
Chemistry in Focus Putting the Brakes on Arsenic
The toxicity of arsenic is well known. Indeed, arsenic has often been the poison of choice in classic plays and films—watch Arsenic and Old Lace sometime. Contrary to its treatment in the aforementioned movie, arsenic poisoning is a serious, contemporary problem. For example, the World Health Organization estimates that million people in Bangladesh are at risk from drinking water that contains large amounts of naturally occurring arsenic. Recently, the Environmental Protection Agency announced more stringent standards for arsenic in U.S. public drinking water supplies. Studies show that prolonged exposure to arsenic can lead to a higher risk of bladder, lung, and skin cancers as well as other ailments, although the levels of arsenic that induce these symptoms remain in dispute in the scientific community.
Cleaning up arsenic-contaminated soil and water poses a significant problem. One approach is to find plants that will remove arsenic from the soil. Such a plant, the Chinese brake fern, has been shown to have a voracious appetite for arsenic. Research led by Lena Q. Ma, a chemist at the University of Florida in Gainesville, has shown that the brake fern accumulates arsenic at a rate times that of the average plant. The arsenic, which becomes concentrated in fronds that grow up to feet long, can be easily harvested and hauled away. Researchers are now investigating the best way to dispose of the plants so that the arsenic can be isolated. The fern (Pteris vittata) looks promising for putting the brakes on arsenic pollution.
See Problem 4.54