Molecular Compounds 7-2 H

 Lesson Objectives  Describe the difference between an ionic compound and a molecular compound.  Name a molecular compound given its formula.  Write the correct formula for a molecular compound given its name.

 Molecular Compounds  Molecular compounds are chemical compounds that take the form of discrete molecules. Examples include such familiar substances as water (H 2 O) and carbon dioxide (CO 2 ).  These compounds are very different from ionic compounds like sodium chloride (NaCl). Ionic compounds are formed when metal atoms lose one or more of their electrons to nonmetal atoms. The resulting cations and anions are electrostatically attracted to each other. We will learn more about that later.

 So what holds the atoms of a molecule together? Rather than forming ions, the atoms of a molecule share their valence electrons in such a way that a bond forms between pairs of atoms. In a carbon dioxide molecule, there are two of these bonds, each occurring between the carbon atom and one of the two oxygen atoms.  Larger molecules can have many, many bonds that serve to keep the molecule together. In a large sample of a given molecular compound, all of the individual molecules are identical.

 Naming Binary Molecular Compounds  Recall that a molecular formula shows the number of atoms of each element that a molecule contains. A molecule of water contains two hydrogen atoms and one oxygen atom, so its formula is H 2 O. A molecule of octane, which is a component of gasoline, contains eight atoms of carbon and 18 atoms of hydrogen. The molecular formula of octane is C 8 H 18.

 A binary molecular compound is a molecular compound that is composed of two elements. In general, the elements that combine to form binary molecular compounds are both nonmetals. This contrasts with ionic compounds, which usually involve bonds between metal ions and nonmetal ions. Because ionic charges cannot be used to name them or to write their formulas, a different naming system must be used for molecular compounds. Another difference between ionic and molecular compounds is that two nonmetal atoms will frequently combine with one another in a variety of ratios.

Prefixes are used in the names of binary molecular compounds to identify the number of atoms of each element. Listed below are the prefixes up to ten. Number of AtomsPrefix 1mono- 2di- 3tri- 4tetra- 5penta- 6hexa- 7hepta- 8octa- 9nona- 10deca- Numerical Prefixes

 The rules for using the prefix system of nomenclature of binary molecular compounds can be summarized as follows.  Generally, the less electronegative element is written first in the formula, though there are a few exceptions. Carbon is always first in a formula, and hydrogen is written after nitrogen in a formula such as NH 3. The order in which common nonmetals are written in binary compound formulas is the same as their order in the following series: C, P, N, H, S, I, Br, Cl, O, F.  The first element in the formula is written first in the name of the compound, along with the appropriate prefix. No prefix is used if there is only one atom of the first element.  The second element is named after the first, but the ending of the element’s name is changed to –ide. The appropriate prefix is always used for the second element, even if there is only one atom of that element.  The a or o at the end of a prefix is usually dropped from the name when the name of the element begins with a vowel. As an example, four oxygen atoms is tetroxide instead of tetraoxide.

FormulaName NOnitrogen monoxide N2ON2Odinitrogen monoxide S2Cl2disulfur dichloride Cl 2 O 7 dichlorine heptoxide Notice that the mono- prefix is not used with the nitrogen in the first compound, but is used with the oxygen in both of the first two examples. The example S 2 Cl 2 emphasizes that the formulas for molecular compounds are not reduced to their lowest ratios. The o of mono- and the a of hepta- are dropped from the name when paired with oxide.

 Writing Formulas for Binary Molecular Compounds  When you know the name of a molecular compound, the prefixes directly tell you which subscript to place with that element in the formula. If there is no prefix, only one atom of that element is present and no subscript is used. For example, if given the name diboron hexahydride, you would realize that the molecule must contain two atoms of boron and six atoms of hydrogen. Its formula is B 2 H 6. Notice that metalloids like boron generally form molecular compounds instead of ionic compounds.

Acids and Bases 7-3 H

 Lesson Objectives  Define acids and bases in terms of the ions that are produced when each type of compound is dissolved in water.  Be able to name acids when given their chemical formulas.  Write the correct formula for an acid when given its name.  Name and write formulas for bases.

 Acids and bases are two very important classes of chemicals that you probably use every day. Many foods and beverages contain acids, such as citrus fruits and juices, soda, and vinegar. Bases are very prevalent in cleaners such as ammonia, and most drain cleaners use a strong base as their active ingredient. Acid-base chemistry will be covered in great detail in a later chapter. In this lesson, we will focus on the nomenclature used for these two types of compounds.

 Acids  An acid can be defined in several ways. For the purposes of this cLASS, the most straightforward description is that an acid is a molecular compound that contains one or more hydrogen atoms and produces hydrogen ions (H + ) when dissolved in water.  This is a different type of compound than the others we have seen. Acids are molecular, which means that in their pure state, they exist as individual molecules and do not adopt the extended three-dimensional structures exhibited by ionic compounds like NaCl.

 However, when these molecules are dissolved in water, the chemical bond between the hydrogen atom and the rest of the molecule breaks, leaving a positively-charged hydrogen ion and an anion. For example, the following chemical equation describes what happens when the acid HCl is dissolved in water:  Since acids produce H + cations upon dissolving in water, the H of an acid is written first in the formula of an inorganic acid.

 The remainder of the acid (everything except the H) constitutes the anion that is formed after the acid dissolves. Organic acids are also an important class of compounds, but the rules for naming them are different, and they will primarily be discussed in a later chapter. A binary acid is an acid that consists of hydrogen and one other element. The most common binary acids contain a halogen. An oxoacid is an acid that consists of hydrogen, oxygen, and a third element. The third element is usually a nonmetal.

 Naming Acids  Since all acids contain hydrogen, the name of an acid is based on the anion that goes with it. Recall from earlier in the chapter that anions can either be monatomic or polyatomic. The names of all monatomic ions end in –ide. The majority of polyatomic ions end in either –ate or –ite, though there are a few exceptions, such as the cyanide ion (CN − ). It is the suffix of the anion that determines how the acid is named, as described in the rules:  1.When the anion ends in –ide, the acid name begins with the prefix hydro-. The root of the anion name goes in the blank (e.g., chlor- for chloride), followed by the suffix –ic acid. HCl is named hydrochloric acid, because Cl− is a chloride ion. HCN is hydrocyanic acid because CN− is a cyanide ion.  2.When the anion ends in –ate, the name of the acid is the root of the anion followed by the suffix –ic acid. There is no prefix. H 2 SO 4 is sulfuric acid (not sulfic) because SO is the sulfate ion.  3.When the anion ends in –ite, the name of the acid is the root of the anion followed by the suffix –ous acid. Again, there is no prefix. HNO 2 is nitrous acid because NO 2 − is the nitrite ion.

Anion SuffixExampleName of acidExample -idechloride (Cl − )hydro_____ic acid hydrochloric acid (HCl) -atesulfate (SO 4 2− )_____ic acidsulfuric acid H 2 SO 4 ) -itenitrite (NO 2 − )_____ous acidnitrous acid (HNO 2 ) Note that the root for a sulfur-containing oxoacid is sulfur- instead of just sulf-. The same is true for a phosphorus containing oxoacid, which uses phosphor- as its root instead of simply phosph-.

 Writing Formulas for Acids  Like other compounds that we have studied, acids are electrically neutral. Therefore, the charge of the anion part of the formula must be exactly balanced out by the H+ ions. Since H + ions carry a single positive charge, the number of H + ions in the formula is equal to the magnitude of the negative charge on the anion.  The chloride ion carries a 1− charge, so only one H is needed in the formula of the acid (HCl). The sulfate ion carries a 2− charge, so two hydrogen atoms are needed in the formula of the acid (H 2 SO 4 ). Another way to think about writing the correct formula is to utilize the crisscross method, as shown below for sulfuric acid.

Bases  The simplest way to define a base is an ionic compound that produces hydroxide ions when dissolved in water. One of the most commonly used bases is sodium hydroxide.

 Inorganic bases are named in the same way as other ionic compounds. Since they all contain the OH − anion, the names of these bases end in hydroxide. Some examples of names and formulas for bases are shown here:

 Lesson Summary  Molecular compounds exist as a collection of individual molecules.  The formula of a binary molecular compound shows how many of each atom are present in the molecule. The less electronegative element is generally written first.  Prefixes are used in the names of molecular compounds to designate how many of each atom are in the molecule.  Acids are molecular compounds that dissolve in water to produce hydrogen ions and an anion. Bases are ionic compounds consisting of hydroxide ions and a cation.  The naming rules for acids are based on the suffix of the anion. Formulas for acids are written by balancing out the charge of the anion with the appropriate number of hydrogen ions.  Naming and formula writing for bases follows the same guidelines as for other ionic compounds.