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CliffsNotes Chemistry Quick Review

John Wiley & Sons

Chapter One

Chapter Check-In

Discovering the building blocks of all substances

Understanding the arrangement of the periodic table

Learning about atomic number and atomic weight

With all the different substances that exist, you may be surprised to learn that they are formed from a relatively small number of elements. For example, carbon is one of 112 known elements. Yet carbon can combine with other elements (like hydrogen and oxygen) to form thousands of substances (like sugar, alcohol, and plastics). Although some elements have been known from the earliest times, most were discovered during the last 300 years.

Each element is unique with its own characteristics. Each element is represented conveniently by a symbol. For example: H is for hydrogen, O for oxygen, and Cl for chlorine. In addition, each element has its own atomic number and atomic weight. The symbols of the elements, along with their respective atomic numbers and atomic weights, are displayed in a special arrangement called the periodic table. Although the elements are arranged in order of increasing atomic number, the elements with similar chemical characteristics appear in columns. This organization is useful for studying chemistry.

Discovery and Similarity

The modern science of chemistry began during the eighteenth century, when several brilliant natural philosophers classified the products of decomposition into a small number of fundamental substances. For example, in 1774, the Englishman Joseph Priestley discovered that when the red powder mercuric oxide was heated, it decomposed to liquid metal mercury and to a colorless gas capable of supporting combustion. (This gas was later named oxygen.) Most substances similarly can be decomposed into several simpler substances by heat or by an electrical current; however, the most fundamental substances cannot be broken down further, even with extraordinary temperature or electric voltage. These basic building blocks of all other substances are known as the chemical elements.

When the French chemist Antoine Lavoisier published his famous list of elements in 1789, there were only 33 elements, several of which were erroneous. By 1930, the diligent labors of thousands of chemists had increased the tally of naturally occurring chemical elements to 90. More recently, physicists in high-energy laboratories have been able to create about 20 highly radioactive, unstable elements that do not exist naturally on Earth, although they are probably produced in the hot cores of some stars.

The number of chemical elements has now reached 112, and the list is growing. Fortunately for students, only about 40 are relevant to basic chemistry. Please take a glance at the periodic table of chemical elements (found at the end of this chapter) and find calcium, element number 20. You need to be acquainted with the symbol and general properties of the 20 simplest elements up to calcium, plus another 20 of chemical significance that you will encounter in this book.

Notice that the key concepts of chemistry are set in boldface on their first appearance in the text to alert you to their importance. These terms are used repeatedly in this book, and you cannot master chemistry without understanding them. You can find the definitions of these key concepts in the glossary at the end of this book.

Groups of elements in nature have similar chemical properties. Helium, neon, argon, krypton, and xenon are all colorless gases, only two of which combine with other elements under very special conditions; their lack of reactivity leads to the name inert gases (or noble gases) for this group of similar elements. By contrast, fluorine and chlorine are corrosive greenish gases that form salts when they readily combine with metals, hence the name halogens (salt formers) for fluorine, chlorine, bromine, and iodine.

As a final example of a group of elements with similar properties, the metallic elements lithium, sodium, and potassium have such low densities that they float on water and are so highly reactive that they spontaneously burn by extracting oxygen from the water itself. These light metals form strong alkalis and appropriately are called the alkali metals. You should locate each of these columns of similar elements, as shown in Figure 1-1, on the periodic table.

Similar elements also occur in the same natural environment. For instance, the halogens are markedly concentrated in seawater. (The major salt in ocean brines is sodium chloride.) The other halogens are extracted from seawater that has been further concentrated—bromine from salt beds formed by evaporation and iodine from kelp, which grows in oceans.

The first indispensable key to making sense out of the extensive system of facts and principles called chemistry is the rule that the behavior of an element or compound can be predicted from similar substances.

Atomic Masses

By the early nineteenth century, chemists were striving to organize their rudimentary knowledge of the chemical elements. It was known that differing masses of elements reacted to form compounds. For example, they found that 3 grams of magnesium metal reacted with precisely 2 grams of oxygen to form magnesium oxide with no residual magnesium or oxygen. The same mass of oxygen, however, required 5 grams of calcium metal to react completely to form calcium oxide. Table 1-1 summarizes these relative combining masses.

Chemists gradually discovered that such relative masses in chemical reactions were fundamental characteristics of the elements. The English chemist John Dalton realized that all the known combining masses were nearly whole-number multiples of the combining mass of the lightest element—hydrogen. In 1803, he proposed an atomic theory in which all other elements would be built from multiple hydrogen atoms. Consequently, he based his scale of atomic masses on hydrogen being equal to 1.

Although Dalton's theory was found to be unrealistically simple, he did compel chemists to adopt a standard scale of atomic weights. Because the combining mass of oxygen is approximately 16 times that of hydrogen, the preceding chart can be revised, as shown in Table 1-2.

The modern masses for calcium, magnesium, and oxygen are still nearly in the 5:3:2 ratios of the original masses. Notice especially that the atomic mass of hydrogen is not precisely equal to 1, because the atomic mass scale is now based on the most common variety of carbon being exactly 12 atomic mass units. Dalton's bold conjecture that all the heavier elements have masses that are integral multiples of hydrogen is not strictly valid, but his theory was a good approximation that eventually led to the discovery of the particles composing the atoms.

The Periodic Table

In 1869, the Russian chemist Dmitri Mendeleyev published his great systematization called the periodic table. He arranged all known chemical elements in order of their atomic masses and found that similar physical and chemical properties recurred every 7 elements for the lighter elements and every 17 elements for the heavier ones. (The inert gases had not been discovered at that time; the correct values for similar properties are 8 and 18.) The periodic table is based on atomic masses and similar properties. In each row, the atomic masses...