Types of Chemical Compounds and Their Formulas

Generally speaking, two fundamental kinds of chemical bonds hold together the atoms in a compound. Covalent bonds, which involve a sharing of electrons between atoms, give rise to molecular compounds. Ionic bonds, which involve a transfer of electrons from one atom to another, give rise to ionic compounds. In this section we consider only the basic features of molecular and ionic compounds that we need as background for the early blog posts. Our in-depth discussion of chemical bonding are explained in other posts.

Molecular Compounds

A molecular compound is made up of discrete units called molecules, which typically consist of a small number of nonmetal atoms held together by covalent bonds. Molecular compounds are represented by chemical formulas, symbolic representations that, at minimum, indicate

1.     the elements present

2.     the relative number of atoms of each element

In the formula for water, the constituent elements are denoted by their symbols. The relative numbers of atoms are indicated by subscripts. Where no subscript is written, the number 1 is understood.

Another example of a chemical formula is CCl₄, which represents the compound carbon tetrachloride. The formulas H₂O and CCl₄ both represent distinct entities—molecules. Thus, we can refer to water and carbon tetrachloride as molecular compounds.

An empirical formula is the simplest formula for a compound; it shows the types of atoms present and their relative numbers. The subscripts in an empirical formula are reduced to their simplest whole-number ratio. For example, P₂O₅ is the empirical formula for a compound whose molecules have the formula P₄O₁₀. Generally, the empirical formula does not tell us a great deal about a compound. Acetic acid (C₂H₄O₂), formaldehyde (CH₂O, used to make certain plastics and resins), and glucose (C₆H₁₂O₆, blood sugar) all have the empirical formula CH₂O.

A molecular formula is based on an actual molecule of a compound. In some cases, the empirical and molecular formulas are identical, such as CH₂O for formaldehyde. In other cases, the molecular formula is a multiple of the empirical formula. A molecule of acetic acid, for example, consists of eight atoms—two C atoms, four H atoms, and two O atoms, so the molecular formula of acetic acid is C₂H₄O₂. This is twice the number of atoms in the formula unit (CH₂O). Empirical and molecular formulas tell us the combining ratio of the atoms in the compound, but they show nothing about how the atoms are attached to each other. Other types of formulas, however, do convey this information. Figure 3-1 shows several representations of acetic acid, the acid constituent that gives vinegar its sour taste.

A structural formula shows the order in which atoms are bonded together in a molecule and by what types of bonds. Thus, the structural formula of acetic acid tells us that three of the four H atoms are bonded to one of the C atoms, and the remaining H atom is bonded to an O atom. Both of the O atoms are bonded to one of the C atoms, and the two C atoms are bonded to each other. The covalent bonds in the structural formula are represented by lines or dashes (—). One of the bonds is represented by a double dash (=) and is called a double covalent bond. Differences between single and double bonds are discussed later in other blog posts. For now, just think of a double bond as being a stronger or tighter bond than a single bond.

A condensed structural formula, which is written on a single line, is an alternative, less cumbersome way of showing how the atoms of a molecule are connected. Thus, the acetic acid molecule is represented as either CH₃COOH or CH₃CO₂H. With this type of formula, the different ways in which the H atoms are attached are still apparent.

Condensed structural formulas can also be used to show how a group of atoms is attached to another atom. Consider methylpropane, C₄H₁₀, in Figure 3-2(b). The structural formula shows that there is a —CH₃ group of atoms attached to the central carbon atom. In the condensed structural formula, this is indicated by enclosing the CH₃ in parentheses to the right of the atom to which it is attached, thus CH₃CH(CH₃)CH₃. Alternatively, because the central C atom is bonded to each of the other three C atoms, we can write the condensed structural formula CH(CH₃)₃.

Organic compounds are made up principally of carbon and hydrogen, with oxygen and/or nitrogen as important constituents in many of them. Each carbon atom forms pure covalent bonds. Organic compounds can be very complex, and one way of simplifying their structural formulas is to write structures without showing the C and H atoms explicitly. We do this by using a line-angle formula (also referred to as a line structure), in which lines represent chemical bonds. A carbon atom exists wherever a line ends or meets another line, and the number of H atoms needed to complete each carbon atom’s four bonds are assumed to be present. The symbols of other atoms or groups of atoms and the bond lines joining them to C atoms are written explicitly. The formula of the complex male hormone molecule testosterone, seen in Figure 3-2(c), is a line-angle formula.

Molecules occupy space and have a three-dimensional shape, but empirical and molecular formulas do not convey any information about the spatial arrangements of atoms. Structural formulas can sometimes show this, but usually the only satisfactory way to represent the three-dimensional structure of molecules is with models. In a ball-and-stick model, atoms are represented by small balls, and the bonds between atoms by sticks (see Figure 3-1). Such models help us to visualize distances between the nuclei of atoms (bond lengths) and the geometrical shapes of molecules. Ball-and-stick models are easy to draw and interpret, but they can be somewhat misleading. Chemical bonds are forces that draw atoms in a molecule into direct contact. The atoms are not held apart as implied by a ball-and-stick model.

A space-filling model shows that the atoms in a molecule occupy space and that they are in actual contact with one another. Certain computer programs generate images of space-filling models such as those shown in Figures 3-1 and 3-2. A space-filling model is a more accurate representation of the size and shape of a molecule because it is constructed to scale (that is, a nanometer-size molecule is magnified to a millimeter or centimeter scale).

The acetic acid molecule is made up of three types of atoms (C, H, and O) and models of the molecule reflect this fact. Different colors are used to distinguish the various types of atoms in ball-and-stick and space-filling models (see Fig. 3-3). You will notice that the colored spheres are of different sizes, which correspond to the size differences between the various atoms in the periodic table.

The various depictions of molecules just discussed will be used throughout this book. In fact, visualization of the sizes and shapes of molecules and interpretation of the physical and chemical properties in terms of molecular sizes and shapes is one of the most important aspects of modern chemistry.

Ionic Compounds

Chemical combination of a metal and a nonmetal usually results in an ionic compound. An ionic compound is made up of positive and negative ions joined together by electrostatic forces of attraction (recall the attraction of oppositely charged objects pictured in Figure 2-4). The atoms of metallic elements tend to lose one or more electrons when they combine with nonmetal atoms, and the nonmetal atoms tend to gain one or more electrons. As a result of this electron transfer, the metal atom becomes a positive ion, or cation, and the nonmetal atom becomes a negative ion, or anion. We can usually deduce the charge on a main-group cation or anion from the group of the periodic table to which the element belongs. Thus the periodic table can help us to write the formulas of ionic compounds.

                                 

In the formation of sodium chloride—ordinary table salt—each sodium atom gives up one electron to become a sodium ion, Na⁺, and each chlorine atom gains one electron to become a chloride ion, Cl^-. This fact conforms to the relationship between locations of the elements in the periodic table and the charges on their simple ions. For sodium chloride to be electrically neutral, there must be one Na⁺ ion for each Cl^- ion (+1 - 1 =0). Thus, the formula of sodium chloride is NaCl, and its structure is shown in Figure 3-4.

We observe that each Na⁺ ion in sodium chloride is surrounded by six Cl^-ions, and vice versa, and we cannot say that any one of these six Cl^- ions belongs exclusively to a given Na⁺ ion. Yet, the ratio of Cl^- to Na⁺ ions in sodium chloride is 1:1, and so we arbitrarily select a combination of one Na⁺’ ion and one Cl^- ion as a formula unit. The formula unit of an ionic compound is the smallest electrically neutral collection of ions. The ratio of atoms (ions) in the formula unit is the same as in the chemical formula. Because it is buried in a vast network of ions, called a crystal, a formula unit of an ionic compound does not exist as a distinct entity. Thus it is inappropriate to call a formula unit of solid sodium chloride a molecule.

The situation with magnesium chloride is similar. In magnesium chloride, found in trace quantities in table salt, magnesium atoms lose two electrons to become magnesium ions, Mg²⁺ (Mg is in group 2). To obtain an electrically neutral formula unit, there must be two Cl^- ions, each with a charge of 1^-, for every Mg2⁺ ion. The formula of magnesium chloride is MgCl₂.

The ions Na⁺, Mg²⁺, and Cl^- are monatomic, meaning that each consists of a single ionized atom. By contrast, a polyatomic ion is made up of two or more atoms. In the nitrate ion, NO^3-, the subscripts signify that three O atoms and one N atom are joined by covalent bonds into the single ion NO^3-. Magnesium nitrate is an ionic compound made up of magnesium and nitrate ions. An electrically neutral formula unit of this compound must consist of one Mg²⁺ ion and two NO^3- ions. The formula based on this formula unit is denoted by enclosing NO3 in parentheses, followed by the subscript 2; thus, Mg(NO₃)₂. Polyatomic ions are discussed further in other blog posts.

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