Learning Objectives Describe covalent compounds as existing as discrete molecules. Explain that the formula of a covalent compound specifies the number of atoms in the molecule. You have already seen examples of substances that contain covalent bonds. One substance mentioned previously was water (\(\ce{H2O}\)). You can tell from its formula that it is not an ionic compound; it is not composed of a metal and a nonmetal. Consequently, its properties are different from those of ionic compounds. Electron Sharing Previously, we discussed ionic bonding in which atoms gain or lose electrons and the resulting positively charged cations and negatively charged anions become attracted to one another. Covalent bonds , on the other hand, form from the sharing of electrons between atoms. This concept can be illustrated by using two hydrogen atoms. We can represent the two individual hydrogen atoms as follows: In contrast, when two hydrogen atoms get close enough together to share their electrons, they can be represented as follows: By sharing their electrons, both hydrogen atoms now have two electrons. This arrangement is more stable than when the two atoms each have a single electron. The sharing of electrons between atoms is called a covalent bond , and the two electrons that join atoms in a covalent bond are called a bonding pair of electrons. A discrete group of atoms connected by covalent bonds is called a molecule —the smallest part of a covalent compound that retains the chemical identity of that compound. Chemists frequently use Lewis diagrams to represent covalent bonding in molecular substances. For example, the Lewis diagrams of two separate hydrogen atoms are as follows: The Lewis diagram of two hydrogen atoms sharing electrons looks like this: This depiction of molecules is simplified further by using a dash to represent a covalent bond. The hydrogen molecule is then represented as follows: Remember that the dash, also referred to as a single bond, represents a pair of electrons. The bond in a hydrogen molecule, measured as the distance between the two nuclei, is about 7.4 × 10 −11 m, or 74 picometers (pm; 1 pm = 1 × 10 −12 m). This particular bond length represents a balance between several forces: the attractions between oppositely charged electrons and nuclei, the repulsion between two negatively charged electrons, and the repulsion between two positively charged nuclei. If the nuclei were closer together, they would repel each other more strongly; if the nuclei were farther apart, there would be less attraction between the positive and negative particles. The nature of covalent bonding and of molecules is the overriding subject of the first term of General Chemistry (CH221). For now, it is sufficient to know the following points: Covalent bonding occurs primarily between nonmetal atoms. A metalloid atom paired with a nonmetal atom can also form a covalent bond. Covalent bonds result from two atoms sharing one or more pairs of electrons. When a covalent bond forms, the resulting molecule is more stable than the atoms it came from. The two atoms participating in a covalent bond become geometrically fixed in space relative to one another. Two or more atoms covalently bonded together from a molecule. Lewis structures are used to represent molecules. How to draw and interpret Lewis structures will be covered in CH221.