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Formation of Molecules



In a covalent bond, a relatively small number of atoms are involved in the sharing of electrons. The combination of atoms that results forms a separate unit rather than the large crystal lattices characteristic of ionic compounds.

The combination of atoms formed by a covalent bond is called a molecule (MAHL-ih-kyool). A molecule is the smallest particle of a covalently bonded substance that has all the properties of that substance. This means that 1 molecule of water, for example, has all the characteristics of a glass of water, a bucket of water, or a pool of water. But if a molecule of water were broken down into atoms of its elements, the atoms would not have the same properties as the molecule.
Molecules are represented by chemical formulas. Like a chemical formula for an ionic crystal, the chemical formula for a covalent molecule contains the symbol of each element involved in the bond. Unlike a chemidal formula for an ionic crystal, however, the chemi al formula for a molecule shows the exact number f atoms of each element involved in the bond. The subscripts, or small numbers placed to the lower right of the symbols, show the number of atoms of each element. When there is only 1 atom of an element, the subscript 1 is not written. It is understood t be 1. Thus, a hydrogen chloride molecule has t e formula HCl. What would be the formula for a molecule that has 1 carbon (C) atom and 4 chlorine (Cl) atoms?

Covalently bonded solids tend to have low melting points. Some covalent substances, however, do not have low melting points. They have rather high melting points. This is because molecules of these substances are very large. The molecules are large because the atoms involved continue to bond to one another. These substances are called network solids, Carbon in the form of graphite is an example of a network solid. So too is silicon dioxide, the main ingredient in sand. Certain glues also form networks of atoms whose bonds are difficult to break. This accounts for the holding properties of such glues.

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