Atoms and Molecules of the Elements

The smallest particle of an element is the atom, an extremely small particle. A copper atom has a diameter of 2.8 x 1 O 10 meter, and it would take about 3.5 million of them to form a line just 1 mm long, about the thickness of a dime. Most of the elements exist as atoms, and when an element is represented by its symbol, you can think of it (at this point) as symbolizing one atom of that element. The symbol He indicates 1 atom of helium, for example. 

Seven elements, all of them nonmetals, do not exist as individual atoms in their pure form. Rather, for reasons you will learn later (see The Diatomic Elements table below), two (sometimes more) atoms join together to form a molecule, a particle composed of two or more atoms bonded together. These seven elements exist as diatomic molecules (di-meaning two, -atomic meaning atom), and are listed in the following table. 

Element Formula of the Diatomic Molecule Name of the Element as a Diatomic Molecule 

Some elements form still larger molecules, such as 03 (trioxygen, ozone), P4 (tetraphosphorus), and S8 (octasulfur). The prefixes tri-, tetra-, and octa- mean 3, 4, and 8, respectively. Remember that the molecules of these elements exist only when they are in their pure form or in mixtures, not when they are part of a compound. 

The composition of compounds was the focus of much research in the early days of chemistry and, in the early 1800s, resulted in the development of two of the most important laws of nature. 

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