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There are more than 100 species of atoms. These species are referred to as elements. Most of the known elements—for example, mercury, helium, gold, hydrogen, and oxygen—occur naturally on earth; others are not usually found in nature but are made by humans—for example, europium and americium. A reasonable explanation for the absence of some elements from nature is that if and when they were formed they proved too unstable to survive in detectable amounts into the present.

Figure 1.1 Electrostatic charge.

Figure 1.2 The NaCl molecule is the smallest unit of salt that retains the characteristics of salt.

All the elements have been assigned symbols or abbreviated chemical names: gold, Au, mercury, Hg; helium, He. Some symbols are obvious abbreviations of the English name; others are derived from the original Latin name of the element, for example, Au is from aurum, the Latin word for gold.

All of the known elements, both natural and those made by humans, are organized in the periodic table. In Figure 1.3, the elements that have a stable state are shown in white boxes; those that occur only in a radioactive form are shown in gray boxes. The number appearing above each element’s abbreviation is referred to as the atomic number, which will be discussed later in this chapter.

Figure 1.3 Periodic table.

The elements in the periodic table are arranged in columns (called groups) and rows (called periods). In general, elements within groups demonstrate similar properties. This is because elements in a group often have similar numbers of electrons in their outer shell; outer shell electron configurations are more important in determining how an atom interacts with other elemental atoms. The lanthanides and actinides are special groups of elements, conventionally shown in rows, separated and placed below the table. These two groups have the same number of outer‐shell electrons and share many common properties.