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, on the periodic table.

figure Three groups of similar elements.

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.