5. Molecules And Compounds PDF

5: MOLECULES AND COMPOUNDS  CHAPTER OVERVIEW 5: Molecules and Compounds There are many substances that exist as two or more atoms connected together so strongly that they behave as a single particle. These multi-atom combinations are called molecules. A molecule is the smallest part of a substance...

5: MOLECULES AND COMPOUNDS  CHAPTER OVERVIEW 5: Molecules and Compounds There are many substances that exist as two or more atoms connected together so strongly that they behave as a single particle. These multi-atom combinations are called molecules. A molecule is the smallest part of a substance that has the physical and chemical properties of that substance. In some respects, a molecule is similar to an atom. A molecule, however, is composed of more than one atom. 5.1: Sugar and Salt 5.2: Compounds Display Constant Composition 5.3: Chemical Formulas - How to Represent Compounds 5.4: A Molecular View of Elements and Compounds 5.5: Writing Formulas for Ionic Compounds 5.6: Nomenclature- Naming Compounds 5.7: Naming Ionic Compounds 5.8: Naming Molecular Compounds 5.9: Naming Acids 5.10: Nomenclature Summary 5.11: Formula Mass - The Mass of a Molecule or Formula Unit 5: Molecules and Compounds is shared under a CK-12 license and was authored, remixed, and/or curated by Marisa Alviar-Agnew & Henry Agnew. 1 5.1: Sugar and Salt Sodium chloride, also known as table salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is commonly used as a condiment and food preservative. Salt can be created by adding two very reactive elements together: sodium (Na(s) metal and chlorine (Cl (g) gas. 2 2 Na(s) + Cl (g) → 2 NaCl(s) (5.1.1) 2 The element sodium (Figure 5.1.1a) is a very reactive metal; given the opportunity, it will react with the sweat on your hands and form sodium hydroxide, which is a very corrosive substance. The element chlorine (Figure 5.1.1b) is a pale yellow, corrosive gas that should not be inhaled due to its poisonous nature. Bring these two hazardous substances together, however, and they react to make the ionic compound sodium chloride (Figure 5.1.1c), known simply as salt. Figure 5.1.1 : Sodium + Chlorine = Sodium Chloride (a) Sodium is a very reactive metal. (b) Chlorine is a pale yellow, noxious gas. (c) Together, sodium and chlorine make sodium chloride—salt—which is necessary for our survival. Source: Photo on the left courtesy of reenhorn1 and photo in the center courtesy of Benjah-bmm27. Photo on the right © Thinkstock. Salt is necessary for life. Na ions are one of the main ions in the human body and are necessary to regulate the fluid balance in + the body. Cl ions are necessary for proper nerve function and respiration. Both of these ions are supplied by salt. The taste of salt − is one of the fundamental tastes; salt is probably the most ancient flavoring known, and one of the few rocks we eat. Clearly when the elemental sodium and chlorine combine (Equation 5.1.1), the resulting salt product has radically different properties (both physical and chemical). This reaction is spectacular to observe (Video 5.1.1). Making table salt using sodium metal a… a… Video 5.1.1 : Making Table Salt using Sodium Metal and Chlorine gas Another compound is sugar, which is the generic name for sweet, soluble carbohydrates, many of which are used in food. Sugar has the chemical formulate C H O and is constructed from different elements than salt: carbon, hydrogen and oxygen. While 12 22 11 sugar qualitatively resembles table salt (often confused in the kitchen), the two have distinctly different physical and chemical properties. There are various types of sugar derived from different sources. While sugar is made with carbon, hydrogen, and oxygen, it is considerably harder to synthesize from its constituent elements than table salt is (Equation 5.1.1). However, the thermal decomposition is considerably easier and can be represented as a dehydration of sucrose to pure carbon and water vapor in Equation 5.1.2, and demonstrated in Video 5.1.2. 5.1.1 https://chem.libretexts.org/@go/page/47472  C H O (s) + heat → 12 C(s) + 11 H O(g) (5.1.2) 12 22 11 2 Super Sugar Science Video 5.1.2 : A science experiment in the kitchen shows what happens to sugar molecules when they are heated. The experiment did not disappoint! As with salt, sugar has radically different properties (both physical and chemical) than its constituent elements. This difference in properties, of constituent elements and compounds, is a central feature of chemical reactions. Contributions & Attributions Wikipedia 5.1: Sugar and Salt is shared under a CK-12 license and was authored, remixed, and/or curated by Marisa Alviar-Agnew & Henry Agnew. 5.1.2 https://chem.libretexts.org/@go/page/47472 5.2: Compounds Display Constant Composition When building a house, the starting point is a blueprint of what the house will look like. The plan states how many windows and what kind, how many doors and what style, how many rooms and what type (bedroom, kitchen, other). The blueprint shows how the different pieces will go together to make the house. As long as the blueprint is followed and exactly the same items are used, the house will be identical to its blueprint. Compounds A compound is a substance that contains two or more elements chemically combined in a fixed proportion. The elements carbon and hydrogen combine to form many different compounds. One of the simplest is called methane, in which there are always four times as many hydrogen particles as carbon particles. Methane is a pure substance because it always has the same composition. However, it is not an element because it can be broken down into simpler substances—carbon and hydrogen. Recall that the components of a mixture can be separated from one another by physical means. This is not true for a compound. Table salt is a compound consisting of equal parts of the elements sodium and chlorine. Salt cannot be separated into its two elements by filtering, distillation, or any other physical process. Salt and other compounds can only be decomposed into their elements by a chemical process. A chemical change is a change that produces matter with a different composition. Many compounds can be decomposed into their elements by heating. When sugar is heated, it decomposes into carbon and water. Water is still a compound, but one which cannot be broken down into hydrogen and oxygen by heating. Instead, the passage of an electrical current through water will produce hydrogen and oxygen gases. The properties of compounds are generally very different than the properties of the elements from which the compound is formed. Sodium is an extremely reactive soft metal that cannot be exposed to air or water. Chlorine is a deadly gas. The compound sodium chloride is a white solid which is essential for all living things (see below). Figure 5.2.1 : (A) Sodium is so reactive that it must be stored under oil. (B) Chlorine is a poisonous yellow-green gas. (C) Salt crystals, a compound of sodium and chlorine. Summary A compound is a substance that contains two or more elements chemically combined in a fixed proportion. A chemical change is a change that produces matter with a different composition. 5.2: Compounds Display Constant Composition is shared under a CK-12 license and was authored, remixed, and/or curated by Marisa Alviar- Agnew & Henry Agnew. 5.2.1 https://chem.libretexts.org/@go/page/47474 5.3: Chemical Formulas - How to Represent Compounds  Learning Objectives Determine the number of different atoms in a formula. Define chemical formula, molecular formula, and empirical formula. A chemical formula is an expression that shows the elements in a compound and the relative proportions of those elements. Water is composed of hydrogen and oxygen in a 2:1 ratio. The chemical formula for water is H O. Sulfuric acid is one of the most widely 2 produced chemicals in the United States and is composed of the elements hydrogen, sulfur, and oxygen. The chemical formula for sulfuric acid is H SO. 2 4 Certain groups of atoms are bonded together to form what is called a polyatomic ion that acts as a single unit. Polyatomic ions are discussed in more detail in Section 5.5. Polyatomic ions are enclosed in parenthesis followed by a subscript if more than one of the same ion exist in a chemical formula. The formula Ca (PO ) represents a compound with the following: 3 4 2 3 Ca atoms + 2 PO43- ions To count the total number of atoms for formulas with polyatomic ions enclosed in parenthesis, use the subscript as a multiplier for each atom or number of atoms. Ca3(PO4)2 3 Ca + 2 x1 P + 2 x 4 O = 3 Ca atoms + 2 P atoms + 8 O atoms Molecular Formula A molecular formula is a chemical formula of a molecular compound that shows the kinds and numbers of atoms present in a molecule of the compound. Ammonia is a compound of nitrogen and hydrogen as shown below: Figure 5.3.1 : The molecular formula for ammonia. NH3. There is one atom of nitrogen and 3 atoms of hydrogen in a molecule of ammonia. Note from the example that there are some standard rules to follow in writing molecular formulas. The arrangements of the elements depend on the particular structure, which is not of concern at this point. The number of atoms of each kind is indicated by a subscript following the atom. If there is only one atom, no number is written. If there is more than one atom of a specific kind, the number is written as a subscript following the atom. We would not write N H for ammonia, because that would mean that there are 3 three nitrogen atoms and one hydrogen atom in the molecule, which is incorrect. Empirical Formula An empirical formula is a formula that shows the elements in a compound in their lowest whole-number ratio. Glucose is an important simple sugar that cells use as their primary source of energy. Its molecular formula is C H O. Since each of the 6 12 6 subscripts is divisible by 6, the empirical formula for glucose is CH O. When chemists analyze an unknown compound, often the 2 first step is to determine its empirical formula. molecular formula: C H O 6 12 6 empirical formula: CH O 2 There are a great many compounds whose molecular and empirical formulas are the same. If the molecular formula cannot be simplified into a smaller whole-number ratio, as in the case of H O or P O , then the empirical formula is also the molecular 2 2 5 formula. 5.3.1 https://chem.libretexts.org/@go/page/47476 Summary A chemical formula is an expression that shows the elements in a compound and the relative proportions of those elements. If only one atom of a specific type is present, no subscript is used. For atoms that have two or more of a specific type of atom present, a subscript is written after the symbol for that atom. Polyatomic ions in chemical formulas are enclosed in parentheses followed by a subscript if more than one of the same type of polyatomic ion exist. Molecular formulas do not indicate how the atoms are arranged in the molecule. The empirical formula tells the lowest whole-number ratio of elements in a compound. The empirical formula does not show the actual number of atoms. 5.3: Chemical Formulas - How to Represent Compounds is shared under a CK-12 license and was authored, remixed, and/or curated by Marisa Alviar-Agnew & Henry Agnew. 5.3.2 https://chem.libretexts.org/@go/page/47476 5.4: A Molecular View of Elements and Compounds  Learning Objectives Classify substances as atomic elements, molecular elements, molecular compounds, or ionic compounds. Atomic Elements Most elements exist with individual atoms as their basic unit. It is assumed that there is only one atom in a formula if there is no numerical subscript on the right side of an element’s symbol. Molecular Elements There are many substances that exist as two or more atoms connected together so strongly that they behave as a single particle. These multi-atom combinations are called molecules. A molecule is the smallest part of a substance that has the physical and chemical properties of that substance. In some respects, a molecule is similar to an atom. A molecule, however, is composed of more than one atom. Table 5.4.1 : Elements That Exist as Diatomic Molecules Hydrogen, H Oxygen Nitrogen Fluorine Chlorine Bromine Iodine Some elements exist naturally as molecules. For example, hydrogen and oxygen exist as two-atom molecules. Other elements also exist naturally as diatomic molecules—a molecule with only two atoms (Table 5.4.1). As with any molecule, these elements are labeled with a molecular formula, a formal listing of what and how many atoms are in a molecule. (Sometimes only the word formula is used, and its meaning is inferred from the context.) For example, the molecular formula for elemental hydrogen is H2, with H being the symbol for hydrogen and the subscript 2 implying that there are two atoms of this element in the molecule. Other diatomic elements have similar formulas: O2, N2, and so forth. Other elements exist as molecules—for example, sulfur normally exists as an eight-atom molecule, S8, while phosphorus exists as a four-atom molecule, P4 (Figure 5.4.1). Figure 5.4.1 : Molecular Art of S8 and P4 Molecules. If each green ball represents a sulfur atom, then the diagram on the left represents an S8 molecule. The molecule on the right shows that one form of elemental phosphorus exists, as a four-atom molecule. Figure 5.4.1 shows two examples of how molecules will be represented in this text. An atom is represented by a small ball or sphere, which generally indicates where the nucleus is in the molecule. A cylindrical line connecting the balls represents the connection between the atoms that make this collection of atoms a molecule. This connection is called a chemical bond. Ionic Compounds The elements in the periodic table are divided into specific groupings; the metals, the non-metals, the semi-metals, and so on. These groupings are largely based on physical properties and on the tendency of the various elements to bond with other elements by forming either an ionic or a covalent bond. As a general rule of thumb, compounds that involve a metal binding with either a non- metal or a semi-metal will display ionic bonding. Thus, the compound formed from sodium and chlorine will be ionic (a metal and a non-metal). The basic unit of ionic compounds is the formula unit. Molecular Compounds Compounds that are composed of only non-metals or semi-metals with non-metals will display covalent bonding and will be classified as molecular compounds. Nitrogen monoxide (NO) will be a covalently bound molecule (two non-metals) and silicon 5.4.1 https://chem.libretexts.org/@go/page/47479 dioxide (SiO2) will also be a covalently bound molecule (a semi-metal and a non-metal). The basic unit of molecular compounds is the molecule.  Example 5.4.1 Provide the classification (i.e. atomic element, molecular element, molecular compound, or ionic compound) of each substance. a. Fe b. PCl3 c. LiBr d. P4 e. oxygen gas Solution a. Fe (iron) is an element that is represented with no subscript, so it is an atomic element. b. PCl3 is made up of two nonmetals, so it is a molecular compound. c. LiBr is made up of lithium, a metal, and bromine, a nonmetal, so it is an ionic compound. d. P4 is a substance that is made up of four atoms of the same element, so it is a molecular element. e. The formula for oxygen gas is O2 so it is a molecular element.  Exercise 5.4.1 Provide the classification (i.e. atomic element, molecular element, molecular compound, or ionic compound) of each substance. a. I2 b. He c. H2O d. Al e. CuCl Answer a: molecular element Answer b: atomic element Answer c: molecular compound Answer d: atomic element Answer e: ionic compound 5.4: A Molecular View of Elements and Compounds is shared under a CK-12 license and was authored, remixed, and/or curated by Marisa Alviar-Agnew & Henry Agnew. 5.4.2 https://chem.libretexts.org/@go/page/47479 5.5: Writing Formulas for Ionic Compounds  Learning Objectives Write the correct formula for an ionic compound. Recognize polyatomic ions in chemical formulas. Ionic compounds do not exist as molecules. In the solid state, ionic compounds are in crystal lattice containing many ions each of the cation and anion. An ionic formula, like NaCl, is an empirical formula. This formula merely indicates that sodium chloride is made of an equal number of sodium and chloride ions. Sodium sulfide, another ionic compound, has the formula Na S. This 2 formula indicates that this compound is made up of twice as many sodium ions as sulfide ions. This section will teach you how to find the correct ratio of ions, so that you can write a correct formula. If you know the name of a binary ionic compound, you can write its chemical formula. Start by writing the metal ion with its charge, followed by the nonmetal ion with its charge. Because the overall compound must be electrically neutral, decide how many of each ion is needed in order for the positive and negative charges to cancel each other out.  Example 5.5.1: Aluminum Nitride and Lithium Oxide Write the formulas for aluminum nitride and lithium oxide. Solution Solution to Example 5.5.1 Write the formula for aluminum nitride Write the formula for lithium oxide 1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) Al 3 + N 3 − Li + 2 − O second. total charge of cations = total charge of total charge of cations = total charge of 2. Use a multiplier to make the total charge anions anions of the cations and anions equal to each other. 1(3+) = 1(3-) 2(1+) = 1(2-) +3 = -3 +2 = -2 3. Use the multipliers as subscript for each Al N Li O ion. 1 1 2 1 4. Write the final formula. Leave out all AlN Li O charges and all subscripts that are 1. 2 An alternative way to writing a correct formula for an ionic compound is to use the crisscross method. In this method, the numerical value of each of the ion charges is crossed over to become the subscript of the other ion. Signs of the charges are dropped.  Example 5.5.2: The Crisscross Method for Lead (IV) Oxide Write the formula for lead (IV) oxide. Solution Solution to Example 5.5.2 Crisscross Method Write the formula for lead (IV) oxide 1. Write the symbol and charge of the cation (metal) first and the 4 + 2 − Pb O anion (nonmetal) second. 2. Transpose only the number of the positive charge to become the subscript of the anion and the number only of the negative charge to become the subscript of the cation. 3. Reduce to the lowest ratio. Pb O 2 4 5.5.1 https://chem.libretexts.org/@go/page/47480  Crisscross Method Write the formula for lead (IV) oxide 4. Write the final formula. Leave out all subscripts that are 1. PbO 2  Exercise 5.5.2 Write the chemical formula for an ionic compound composed of each pair of ions. a. the calcium ion and the oxygen ion b. the 2+ copper ion and the sulfur ion c. the 1+ copper ion and the sulfur ion Answer a: CaO Answer b: CuS Answer c: Cu2S Be aware that ionic compounds are empirical formulas and so must be written as the lowest ratio of the ions.  Example 5.5.3: Sulfur Compound Write the formula for sodium combined with sulfur. Solution Solution to Example 5.5.3 Crisscross Method Write the formula for sodium combined with sulfur 1. Write the symbol and charge of the cation (metal) first and the + 2 − Na S anion (nonmetal) second. 2. Transpose only the number of the positive charge to become the subscript of the anion and the number only of the negative charge to become the subscript of the cation. 3. Reduce to the lowest ratio. This step is not necessary. 4. Write the final formula. Leave out all subscripts that are 1. Na S 2  Exercise 5.5.3 Write the formula for each ionic compound. a. sodium bromide b. lithium chloride c. magnesium oxide Answer a: NaBr Answer b: LiCl Answer c: MgO 5.5.2 https://chem.libretexts.org/@go/page/47480 Polyatomic Ions Some ions consist of groups of atoms bonded together and have an overall electric charge. Because these ions contain more than one atom, they are called polyatomic ions. Polyatomic ions have characteristic formulas, names, and charges that should be memorized. For example, NO3− is the nitrate ion; it has one nitrogen atom and three oxygen atoms and an overall 1− charge. Table 5.5.1 lists the most common polyatomic ions. Table 5.5.1 : Some Polyatomic Ions Name Formula ammonium ion NH4+ acetate ion C2H3O2− (also written CH3CO2−) carbonate ion CO32− chromate ion CrO42− dichromate ion Cr2O72− hydrogen carbonate ion (bicarbonate ion) HCO3− cyanide ion CN− hydroxide ion OH− nitrate ion NO3− nitrite ion NO2− permanganate ion MnO4− phosphate ion PO43− hydrogen phosphate ion HPO42− dihydrogen phosphate ion H2PO4− sulfate ion SO42− hydrogen sulfate ion (bisulfate ion) HSO4− sulfite ion SO32− The rule for constructing formulas for ionic compounds containing polyatomic ions is the same as for formulas containing monatomic (single-atom) ions: the positive and negative charges must balance. If more than one of a particular polyatomic ion is needed to balance the charge, the entire formula for the polyatomic ion must be enclosed in parentheses, and the numerical subscript is placed outside the parentheses. This is to show that the subscript applies to the entire polyatomic ion. An example is Ba(NO3)2. Writing Formulas for Ionic Compounds Containing Polyatomic Ions Writing a formula for ionic compounds containing polyatomic ions also involves the same steps as for a binary ionic compound. Write the symbol and charge of the cation followed by the symbol and charge of the anion.  Example 5.5.4: Calcium Nitrate Write the formula for calcium nitrate. Solution Solution to Example 5.5.4 Crisscross Method Write the formula for calcium nitrate 1. Write the symbol and charge of the cation (metal) first and the 2 + − Ca NO anion (nonmetal) second. 3 5.5.3 https://chem.libretexts.org/@go/page/47480  Crisscross Method Write the formula for calcium nitrate 2. Transpose only the number of the positive charge to become the The 2+ charge on Ca becomes the subscript of NO3 and the 1- charge subscript of the anion and the number only of the negative charge to on NO3 becomes the subscript of Ca. become the subscript of the cation. 3. Reduce to the lowest ratio. Ca (NO ) 1 3 2 4. Write the final formula. Leave out all subscripts that are 1. If there Ca(NO ) is only 1 of the polyatomic ion, leave off parentheses. 3 2  Example 5.5.5 Write the chemical formula for an ionic compound composed of the potassium ion and the sulfate ion. Solution Solution to Example 5.5.5 Explanation Answer Potassium ions have a charge of 1+, while sulfate ions have a charge of 2−. We will need two potassium ions to balance the charge on the K SO 2 4 sulfate ion, so the proper chemical formula is K SO. 2 4  Exercise 5.5.5 Write the chemical formula for an ionic compound composed of each pair of ions. a. the magnesium ion and the carbonate ion b. the aluminum ion and the acetate ion Answer a: MgCO 3 Answer b: Al(CH COO) 3 3 Recognizing Ionic Compounds There are two ways to recognize ionic compounds. Method 1 Compounds between metal and nonmetal elements are usually ionic. For example, CaBr contains a metallic element 2 (calcium, a group 2 [or 2A] metal) and a nonmetallic element (bromine, a group 17 [or 7A] nonmetal). Therefore, it is most likely an ionic compound (in fact, it is ionic). In contrast, the compound NO contains two elements that are both nonmetals 2 (nitrogen, from group 15 [or 5A], and oxygen, from group 16 [or 6A]. It is not an ionic compound; it belongs to the category of covalent compounds discussed elsewhere. Also note that this combination of nitrogen and oxygen has no electric charge specified, so it is not the nitrite ion. Method 2 Second, if you recognize the formula of a polyatomic ion in a compound, the compound is ionic. For example, if you see the formula Ba(NO ) , you may recognize the “NO ” part as the nitrate ion, NO. (Remember that the convention for writing 3 2 3 − 3 formulas for ionic compounds is not to include the ionic charge.) This is a clue that the other part of the formula, Ba , is 5.5.4 https://chem.libretexts.org/@go/page/47480  actually the Ba 2 + ion, with the 2+ charge balancing the overall 2− charge from the two nitrate ions. Thus, this compound is also ionic.  Example 5.5.6 Identify each compound as ionic or not ionic. a. Na O 2 b. PCl 3 c. NH Cl 4 d. OF 2 Solution Solution to Example 5.5.6 Explanation Answer a. Sodium is a metal, and oxygen is a nonmetal. Therefore, Na O is 2 Na O , ionic expected to be ionic via method 1. 2 b. Both phosphorus and chlorine are nonmetals. Therefore, PCl is 3 PCl , not ionic not ionic via method 1 3 c. The NH in the formula represents the ammonium ion, NH , + 4 4 NH Cl , ionic which indicates that this compound is ionic via method 2 4 d. Both oxygen and fluorine are nonmetals. Therefore, OF is not 2 OF , not ionic ionic via method 1 2  Exercise 5.5.6 Identify each compound as ionic or not ionic. a. N O 2 b. FeCl 3 c. (NH ) 4 3 PO 4 d. SOCl 2 Answer a: not ionic Answer b: ionic Answer c: ionic Answer d: not ionic Summary Formulas for ionic compounds contain the symbols and number of each atom present in a compound in the lowest whole number ratio. 5.5: Writing Formulas for Ionic Compounds is shared under a CK-12 license and was authored, remixed, and/or curated by Marisa Alviar-Agnew & Henry Agnew. 5.5.5 https://chem.libretexts.org/@go/page/47480 5.6: Nomenclature- Naming Compounds Nomenclature is the process of naming chemical compounds so that they can be easily identified as separate chemicals. The primary function of chemical nomenclature is to ensure that a spoken or written chemical name leaves no ambiguity concerning which chemical compound the name refers to—each chemical name should refer to a single substance. A less important aim is to ensure that each substance has a single name, although a limited number of alternative names is acceptable in some cases. Preferably, the name also conveys some information about the structure or chemistry of a compound. A common name will often suffice to identify a chemical compound in a particular set of circumstances. To be more generally applicable, the name should indicate at least the chemical formula. To be more specific still, the three-dimensional arrangement of the atoms may need to be specified. Contributions & Attributions Wikipedia (CC-BY-SA-3.0) 5.6: Nomenclature- Naming Compounds is shared under a CK-12 license and was authored, remixed, and/or curated by Marisa Alviar-Agnew & Henry Agnew. 5.6.1 https://chem.libretexts.org/@go/page/47481 5.7: Naming Ionic Compounds  Learning Objectives To use the rules for naming ionic compounds. After learning a few more details about the names of individual ions, you will be one step away from knowing how to name ionic compounds. This section begins the formal study of nomenclature, the systematic naming of chemical compounds. Naming Ions The name of a monatomic cation is simply the name of the element followed by the word ion. Thus, Na+ is the sodium ion, Al3+ is the aluminum ion, Ca2+ is the calcium ion, and so forth. We have seen that some elements lose different numbers of electrons, producing ions of different charges (Figure 3.3). Iron, for example, can form two cations, each of which, when combined with the same anion, makes a different compound with unique physical and chemical properties. Thus, we need a different name for each iron ion to distinguish Fe2+ from Fe3+. The same issue arises for other ions with more than one possible charge. There are two ways to make this distinction. In the simpler, more modern approach, called the Stock system, an ion’s positive charge is indicated by a roman numeral in parentheses after the element name, followed by the word ion. Thus, Fe2+ is called the iron(II) ion, while Fe3+ is called the iron(III) ion. This system is used only for elements that form more than one common positive ion. We do not call the Na+ ion the sodium(I) ion because (I) is unnecessary. Sodium forms only a 1+ ion, so there is no ambiguity about the name sodium ion. Table 5.7.1 : The Modern and Common System of Cation Names Element Stem Charge Modern Name Common Name 2+ iron(II) ion ferrous ion iron ferr- 3+ iron(III) ion ferric ion 1+ copper(I) ion cuprous ion copper cupr- 2+ copper(II) ion cupric ion 2+ tin(II) ion stannous ion tin stann- 4+ tin(IV) ion stannic ion 2+ lead(II) ion plumbous ion lead plumb- 4+ lead(IV) ion plumbic ion 2+ chromium(II) ion chromous ion chromium chrom- 3+ chromium(III) ion chromic ion 1+ gold(I) ion aurous ion gold aur- 3+ gold(III) ion auric ion The second system, called the common system, is not conventional but is still prevalent and used in the health sciences. This system recognizes that many metals have two common cations. The common system uses two suffixes (-ic and -ous) that are appended to the stem of the element name. The -ic suffix represents the greater of the two cation charges, and the -ous suffix represents the lower one. In many cases, the stem of the element name comes from the Latin name of the element. Table 5.7.1 lists the elements that use the common system, along with their respective cation names. Table 5.7.2 : Some Monatomic Anions Ion Name F− fluoride ion Cl− chloride ion 5.7.1 https://chem.libretexts.org/@go/page/47482  Ion Name Br− bromide ion I− iodide ion O2− oxide ion S2− sulfide ion P3− phosphide ion N3− nitride ion The name of a monatomic anion consists of the stem of the element name, the suffix -ide, and then the word ion. Thus, as we have already seen, Cl− is “chlor-” + “-ide ion,” or the chloride ion. Similarly, O2− is the oxide ion, Se2− is the selenide ion, and so forth. Table 5.7.2 lists the names of some common monatomic ions. The polyatomic ions have their own characteristic names, as discussed earlier.  Example 5.7.1 Name each ion. a. Ca2+ b. S2− c. SO32− d. NH4+ e. Cu+ Solution a. the calcium ion b. the sulfide ion c. the sulfite ion d. the ammonium ion e. the copper(I) ion or the cuprous ion  Exercise 5.7.1 Name each ion. a. Fe2+ b. Fe3+ c. SO42− d. Ba2+ e. HCO3− Answer a: iron(II) ion Answer b: iron(III) ion Answer c: sulfate ion Answer d: barium ion Answer e: hydrogen carbonate ion or bicarbonate ion 5.7.2 https://chem.libretexts.org/@go/page/47482   Example 5.7.2 Write the formula for each ion. a. the bromide ion b. the phosphate ion c. the cupric ion d. the magnesium ion Solution a. Br− b. PO43− c. Cu2+ d. Mg2+  Exercise 5.7.2 Write the formula for each ion. a. the fluoride ion b. the carbonate ion c. the stannous ion d. the potassium ion Answer a: F- Answer b: CO32- Answer c: Sn 2+ Answer d: K+ Naming Binary Ionic Compounds with a Metal that Forms Only One Type of Cation A binary ionic compound is a compound composed of a monatomic metal cation and a monatomic nonmetal anion. The metal cation is named first, followed by the nonmetal anion as illustrated in Figure 5.7.1 for the compound BaCl2. The word ion is dropped from both parts. 5.7.3 https://chem.libretexts.org/@go/page/47482  Figure 5.7.1 : Naming BaC l2 Naming formula: Name of metal cation + base name of nonmetal anion + suffix -ide. BaCl2 is named as barium chloride. Subscripts in the formula do not affect the name.  Example 5.7.3: Naming Ionic Compounds Name each ionic compound. a. CaCl2 b. AlF3 c. KCl Solution a. Using the names of the ions, this ionic compound is named calcium chloride. b. The name of this ionic compound is aluminum fluoride. c. The name of this ionic compound is potassium chloride  Exercise 5.7.3 Name each ionic compound. a. AgI b. MgO c. Ca3P2 Answer a: silver iodide Answer b: magnesium oxide Answer c: calcium phosphide Naming Binary Ionic Compounds with a Metal That Forms More Than One Type of Cation If you are given a formula for an ionic compound whose cation can have more than one possible charge, you must first determine the charge on the cation before identifying its correct name. For example, consider FeCl2 and FeCl3. In the first compound, the iron ion has a 2+ charge because there are two Cl− ions in the formula (1− charge on each chloride ion). In the second compound, the 5.7.4 https://chem.libretexts.org/@go/page/47482 iron ion has a 3+ charge, as indicated by the three Cl− ions in the formula. These are two different compounds that need two different names. By the Stock system, the names are iron(II) chloride and iron(III) chloride (Figure 5.7.2). Table 5.7.3 : Naming the F eC l and F eC l Compounds in the Modern/Stock System. 2 3 Name of cation (metal) + (Roman Numeral in parenthesis) + Base name of anion (nonmetal) and -ide If we were to use the stems and suffixes of the common system, the names would be ferrous chloride and ferric chloride, respectively (Figure 5.7.3). Table 5.7.4 : Naming the F eC l and F eC l Compounds in the Old/Common System. 2 3 "Old" base name of cation (metal) and -ic or -ous + Base name of anion (nonmetal) and -ide -ous (for ions with lower charge) -ic (for ions with higher charge) 5.7.5 https://chem.libretexts.org/@go/page/47482   Example 5.7.4: Name each ionic compound. a. Co2O3 b. FeCl2 Solution Solutions to Example 5.7.4 Explanation Answer We know that cobalt can have more than one possible charge; we just need to determine what it is. Oxide always has a 2− charge, so with three oxide ions, we have a total negative a charge of 6−. cobalt(III) oxide This means that the two cobalt ions have to contribute 6+, which for two cobalt ions means that each one is 3+. Therefore, the proper name for this ionic compound is cobalt(III) oxide. Iron can also have more than one possible charge. Chloride always has a 1− charge, so with two chloride ions, we have a total b negative charge of 2−. iron(II) chloride This means that the one iron ion must have a 2+ charge. Therefore, the proper name for this ionic compound is iron(II) chloride.  Exercise 5.7.4 Name each ionic compound. a. AuCl3 b. PbO2 c. CuO Answer a: gold(III) chloride Answer b: lead(IV) oxide Answer c: copper(II) oxide Naming Ionic Compounds with Polyatomic Ions The process of naming ionic compounds with polyatomic ions is the same as naming binary ionic compounds. The cation is named first, followed by the anion. One example is the ammonium sulfate compound in Figure 5.7.6. 5.7.6 https://chem.libretexts.org/@go/page/47482  Figure 5.7.2 : Naming Ionic Compounds with Polyatomic Ions  Example 5.7.5: Naming Ionic Compounds Write the proper name for each ionic compound. a. (NH4)2S b. AlPO4, c. Fe3(PO4)2 Solution Solutions to Example 5.7.5 Explanation Answer a. The ammonium ion has a 1+ charge and the sulfide ion has a 2− charge. Two ammonium ions need to balance the charge on a single sulfide ammonium sulfide ion. The compound’s name is ammonium sulfide. b. The ions have the same magnitude of charge, one of each (ion) is needed to balance the charges. aluminum phosphate The name of the compound is aluminum phosphate. c. Neither charge is an exact multiple of the other, so we have to go to the least common multiple of 6. To get 6+, three iron(II) ions are needed, and to get 6−, two iron(II) phosphate phosphate ions are needed. The compound’s name is iron(II) phosphate.  Exercise 5.7.5A Write the proper name for each ionic compound. a. (NH4)3PO4 b. Co(NO2)3 Answer a: ammonium phosphate Answer b: cobalt(III) nitrite Figure 5.7.1 is a synopsis of how to name simple ionic compounds. 5.7.7 https://chem.libretexts.org/@go/page/47482  Figure 5.7.3 : A Guide to Naming Simple Ionic Compounds.  Exercise 5.7.5B Name each ionic compound. a. ZnBr2 b. Al2O3 c. (NH4)3PO4 d. AuF3 e. AgF Answer a: zinc bromide Answer b: aluminum oxide Answer c: ammonium phosphate Answer d: gold(III) fluoride or auric fluoride Answer e: silver fluoride Summary Ionic compounds are named by stating the cation first, followed by the anion. Positive and negative charges must balance. Some anions have multiple forms and are named accordingly with the use of roman numerals in parentheses. Ternary compounds are composed of three or more elements. 5.7: Naming Ionic Compounds is shared under a CK-12 license and was authored, remixed, and/or curated by Marisa Alviar-Agnew & Henry Agnew. 5.7.8 https://chem.libretexts.org/@go/page/47482 5.8: Naming Molecular Compounds  Learning Objectives Determine the name of a simple molecular compound from its chemical formula. Molecular Compounds Molecular compounds are inorganic compounds that take the form of discrete molecules. Examples include such familiar substances as water (H O) and carbon dioxide (CO ). These compounds are very different from ionic compounds like sodium 2 2 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. So what holds the atoms of a molecule together? Rather than forming ions, the atoms of a molecule share their electrons in such a way that a bond forms between a pair 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. Figure 5.8.1 : Carbon dioxide molecules consist of a central carbon atom bonded to 2 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 O. A molecule of octane, which is a component of gasoline, 2 contains 8 atoms of carbon and 18 atoms of hydrogen. The molecular formula of octane is C H. 8 18 Figure 5.8.2 : Nitrogen dioxide ( NO ) 2 is a reddish-brown toxic gas that is a prominent air pollutant produced by internal combustion engines. Naming binary (two-element) molecular compounds is similar to naming simple ionic compounds. The first element in the formula is simply listed using the name of the element. The second element is named by taking the stem of the element name and adding the suffix -ide. A system of numerical prefixes is used to specify the number of atoms in a molecule. Table 5.8.1 lists these numerical prefixes. Table 5.8.1 : Numerical Prefixes for Naming Binary Covalent Compounds Number of Atoms in Compound Prefix on the Name of the Element 1 mono-* 2 di- 3 tri- 4 tetra- 5 penta- 6 hexa- 5.8.1 https://chem.libretexts.org/@go/page/47483  Number of Atoms in Compound Prefix on the Name of the Element 7 hepta- 8 octa- 9 nona- 10 deca- *This prefix is not used for the first element’s name.  Note 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 after nitrogen in a formula such as NH. The order of common nonmetals in 3 binary compound formulas is C, P, N , H , S, I , Br , Cl, O, F. 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. The prefix is "mono" is not added to the first element’s name if there is only one atom of the first element in a molecule. Some examples of molecular compounds are listed in Table 5.8.2. Table 5.8.2 Formula Name NO nitrogen monoxide N O 2 dinitrogen monoxide S Cl 2 2 disulfur dichloride Cl O 2 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 S Cl emphasizes that the formulas for molecular compounds are not reduced to their lowest ratios. The o of the 2 2 mono- and the a of hepta- are dropped from the name when paired with oxide.  Exercise 5.8.1 Write the name for each compound. a. CF4 b. SeCl2 c. SO3 Answer a: 5.8.2 https://chem.libretexts.org/@go/page/47483  carbon tetrafluoride Answer b: selenium dichloride Answer c: sulfur trioxide Simple molecular compounds with common names For some simple covalent compounds, we use common names rather than systematic names. We have already encountered these compounds, but we list them here explicitly: H2O: water NH3: ammonia CH4: methane H2O2: hydrogen peroxide Methane is the simplest organic compound. Organic compounds are compounds with carbon atoms and are named by a separate nomenclature system. Some Compounds Have Both Covalent and Ionic Bonds If you recall the introduction of polyatomic ions, you will remember that the bonds that hold the polyatomic ions together are covalent bonds. Once the polyatomic ion is constructed with covalent bonds, it reacts with other substances as an ion. The bond between a polyatomic ion and another ion will be ionic. An example of this type of situation is in the compound sodium nitrate. Sodium nitrate is composed of a sodium ion and a nitrate ion. The nitrate ion is held together by covalent bonds and the nitrate ion is attached to the sodium ion by an ionic bond. Summary A molecular compound is usually composed of two or more nonmetal elements. Molecular compounds are named with the first element first and then the second element by using the stem of the element name plus the suffix -ide. Numerical prefixes are used to specify the number of atoms in a molecule. 5.8: Naming Molecular Compounds is shared under a CK-12 license and was authored, remixed, and/or curated by Marisa Alviar-Agnew & Henry Agnew. 5.8.3 https://chem.libretexts.org/@go/page/47483 5.9: Naming Acids A spot test for gold has been in use for decades. The sample is first treated with nitric acid. Other metals may react or dissolve in this acid, but gold will not. Then the sample is added to a mixture of nitric acid and hydrochloric acid. Gold will only dissolve in this mixture. The term "acid test" arose from the California gold rush in the late 1840's when this combination was used to test for the presence of real gold. It has since come to mean, "tested and approved" in a number of fields. Acids An acid can be defined in several ways. The most straightforward definition is that an acid is a molecular compound that contains one or more hydrogen atoms and produces hydrogen ions (H ) when dissolved in water. + Figure 5.9.1 : (A) Vinegar comes in a variety of types, but all contain acetic acid. (B) Citrus fruits like grapefruit contain citric and ascorbic acids. This is a different type of compound than the others we have seen so far. Acids are molecular, which means that in their pure state they are individual molecules and do not adopt the extended three-dimensional structures of 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. This can be symbolized in a chemical equation: + − HCl → H + Cl 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 (other than the H ) is the anion after the acid dissolves. Organic acids are also an important class of compounds, but will not be discussed here. Naming Acids Since all acids contain hydrogen, the name of an acid is based on the anion that goes with it. These anions can either be monatomic or polyatomic. Naming Binary acids (in aqueous form) A binary acid is an acid that consists of hydrogen and one other element. The most common binary acids contain a halogen. The acid name begins with the prefix hydro-. followed by the base name of the anion, followed by the suffix -ic. Formula for naming acids: Hydro- and Base name of nonmetal and -ic + acid. Example: HCl is hydrochloric acid. 5.9.1 https://chem.libretexts.org/@go/page/47484 Naming Oxyacids An oxyacid is an acid that consists of hydrogen, oxygen, and a third element. The third element is usually a nonmetal. a. Oxyanions with -ite ending. The name of the acid is the root of the anion followed by the suffix -ous. There is no prefix. Formula for naming oxyanions with -ite ending: Base name of oxyanion and -ous + acid. Example: H2SO3 is sulfurous acid. b. Oxyanions with -ate ending. The name of the acid is the root of the anion followed by the suffix -ic. There is no prefix. Formula for naming oxyanions with -ate ending: Base name of oxyanion and -ic + acid. Example: H3PO4 is phosphoric acid.  Note The base name for sulfur containing oxyacid is sulfur- instead of just sulf-. The same is true for a phosphorus containing oxyacid. The base name is phosphor- 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. Another way to think about writing the correct formula is to utilize the crisscross + method, shown below for sulfuric acid. H2SO4: H has +1 charge and SO4 has -2 charge, so there must be 2 H+ and 1 SO4 for the charges to balance out. Formula: H2SO4 Figure 5.9.2 : Crisscross approach to writing formula for sulfuric acid. Summary Acids are molecular compounds that release hydrogen ions. A binary acid consists of hydrogen and one other element. Oxyacids contain hydrogen, oxygen, and one other element. The name of the acid is based on the anion attached to the hydrogen. 5.9.2 https://chem.libretexts.org/@go/page/47484 5.9: Naming Acids is shared under a CK-12 license and was authored, remixed, and/or curated by Marisa Alviar-Agnew & Henry Agnew. 5.9.3 https://chem.libretexts.org/@go/page/47484 5.10: Nomenclature Summary Contributions & Attributions 5.10: Nomenclature Summary is shared under a CK-12 license and was authored, remixed, and/or curated by Marisa Alviar-Agnew & Henry Agnew. 5.10.1 https://chem.libretexts.org/@go/page/47486 5.11: FORMULA MASS - THE MASS OF A MOLECULE OR FORMULA UNIT  LEARNING OBJECTIVES To determine the formula mass of an ionic or molecular compound. A necessary skill for future chapters is the ability to determine the mass of the formula of an ionic compound. This quantity is called the formula mass. The formula mass is obtained by adding the masses of each individual atom in the formula of the compound. Because a proper formula is electrically neutral (with no net electrons gained or lost), the ions can be considered atoms for the purpose of calculating the formula mass. Let us start by calculating the formula mass of sodium chloride (NaCl). This formula mass is the sum of the atomic masses of one sodium atom and one chlorine atom, which we find from the periodic table; here, we use the masses to two decimal places: Na: 22.99 amu Cl: +35.34 amu Total: 58.44 amu To two decimal places, the formula mass of NaCl is 58.44 amu. When an ionic compound has more than one anion or cation, you must remember to use the proper multiple of the atomic mass for the element in question. For the formula mass of calcium fluoride (CaF2), we must multiply the mass of the fluorine atom by 2 to account for the two fluorine atoms in the chemical formula: Ca: 1 x 40.08 = 40.08 amu F: 2 x 19.00 = +38.00 amu Total = 78.08 amu The formula mass of CaF2 is 78.08 amu. For ionic compounds with polyatomic ions, the sum must include the number and mass of each atom in the formula for the polyatomic ion. For example, potassium nitrate (KNO3) has one potassium atom, one nitrogen atom, and three oxygen atoms: K: 1 x 39.10 = 39.10 amu N: 1 x 14.00 = +14.00 amu O: 3 x 16.00 = +48.00 amu Total = 101.10 amu The formula mass of KNO3 is 101.10 amu. Potassium nitrate is a key ingredient in gunpowder and has been used clinically as a diuretic. When a formula contains more than one polyatomic unit in the chemical formula, as in Ca(NO3)2, do not forget to multiply the atomic mass of every atom inside of the parentheses by the subscript outside of the parentheses. This is necessary because the subscript refers to the entire polyatomic ion. Thus, for Ca(NO3)2, the subscript 2 implies two complete nitrate ions, so we must sum the masses of two (1 × 2) nitrogen atoms and six (3 × 2) oxygen atoms, along with the mass of a single calcium atom: Ca: 1 x 40.08 = 40.08 amu N: 2 x 14.00 = +28.00 amu O: 6 x 16.00 = +96.00 amu Total = 164.08 amu The key to calculating the formula mass of an ionic compound is to correctly count each atom in the formula and multiply the atomic masses of its atoms accordingly.  EXAMPLE 5.11.1 Use the atomic masses (rounded to two decimal places) to determine the formula mass for each ionic compound. a. FeCl3 b. (NH4)3PO4 5.11.1 https://chem.libretexts.org/@go/page/47488 Solution a. Fe: 1 x 55.85 = 55.85 amu Cl: 1 x 35.45 = +106.35 amu ________________________ Total = 162.20 amu The formula mass of FeCl3 is 162.2 amu. b. When we distribute the subscript 3 through the parentheses containing the formula for the ammonium ion, we see that we have 3 nitrogen atoms and 12 hydrogen atoms. Thus, we set up the sum as follows: N: 3 x 14.00 = 42.00 amu H: 12 x 1.00 = +12.00 amu P: 1 x 30.97 = +30.97 amu O: 4 x 16.00 = +64.00 amu Total = 148.97 amu The formula mass for (NH4)3PO4 is 149.0 amu.  EXERCISE 5.11.1 Use the atomic masses (rounded to two decimal places) to determine the formula mass for each ionic compound. a. TiO2 b. AgBr c. Au(NO3)3 d. Fe3(PO4)2 Answer a. 79.87 amu b. 187.77 amu c. 383.0 amu  TO YOUR HEALTH: HYDRATES Some ionic compounds have water (H O ) incorporated within their formula unit. These compounds, called hydrates, have a 2 characteristic number of water units associated with each formula unit of the compound. Hydrates are solids, not liquids or solutions, despite the water they contain. To write the chemical formula of a hydrate, write the number of water units per formula unit of compound after its chemical formula. The two chemical formulas are separated by a vertically centered dot. The hydrate of copper(II) sulfate has five water units associated with each formula unit, so it is written as CuSO ⋅ 5 H O. The name of this compound is copper(II) sulfate pentahydrate, with the 4 2 penta- prefix indicating the presence of five water units per formula unit of copper(II) sulfate. Cast (CC BY-SA 4.0; JanSLWC via Wikipedia). Hydrates have various uses in the health industry. Calcium sulfate hemihydrate (CaSO ⋅ H O ), known as plaster of Paris, is used 4 1 2 2 to make casts for broken bones. Epsom salt (MgSO ⋅ 7 H O ) is used as a bathing salt and a laxative. Aluminum chloride hexahydrate 4 2 5.11.2 https://chem.libretexts.org/@go/page/47488  is an active ingredient in antiperspirants. Table 5.11.1 lists some useful hydrates. Table 5.11.1: Names and Formulas of Some Widely Used Hydrates Formula Name Uses AlCl3 6H2O aluminum chloride hexahydrate antiperspirant CaSO4 ½H2O calcium sulfate hemihydrate (plaster of Paris) casts (for broken bones and castings) CaSO4 2H2O calcium sulfate dihydrate (gypsum) drywall component CoCl2 6H2O cobalt(II) chloride hexahydrate drying agent, humidity indicator CuSO4 5H2O copper(II) sulfate pentahydrate fungicide, algicide, herbicide MgSO4 7H2O magnesium sulfate heptahydrate (Epsom salts) laxative, bathing salt Na2CO3 10H2O sodium carbonate decahydrate (washing soda) laundry additive/cleaner KEY TAKEAWAY Formula masses of ionic compounds can be determined from the masses of the atoms in their formulas. This page titled 5.11: Formula Mass - The Mass of a Molecule or Formula Unit is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Marisa Alviar-Agnew & Henry Agnew. 5.11.3 https://chem.libretexts.org/@go/page/47488