AP Chemistry Ch. 2 Section 2.8 Notes Naming Simple Compounds Naming Binary Compounds Binary Compounds: compounds composed of two elements. Binary Ionic Compounds (Type I) Binary ionic compounds contain a positive ion (cation) always written first in the formula and a negative ion (anion). In naming binary ionic compounds, the following rules apply: 1. The cation is always named first and the anion second. 2. A monatomic (meaning one-atom ) cation takes its name from the name of the element. For example, Na +, is called sodium in the names of compounds containing this ion. 3. A monatomic anion is named by taking the root of the element name and adding ide. Thus the Cl - ion is called chloride. In formulas of ionic compounds, simple ions are represented by the element symbol: Cl means Cl -, Na means Na +, and so on. The rules for naming binary ionic compounds are illustrated by the following examples: Compound Ions Present Name NaCl Na +, Cl - Sodium chloride KI K +, I - Potassium iodide CaS Ca 2+, S 2- Calcium sulfide Li 3 N Li +, N 3- Lithium nitride CsBr Cs +, Br - Cesium bromide MgO Mg 2+, O 2- Magnesium oxide Formulas from names: the reverse process is also important. Rules for Writing Formulas of Ionic Compounds 1. The positive ion is given first in the formula. 2. The subscripts in the formula must produce an electrically neutral formula unit. (Nature does require electrical neutrality.) Formula unit: smallest unit of an ionic compound. 3. The subscripts should be the smallest set of whole numbers possible. Example: Write the formulas for the ionic compounds. 1. Barium sulfide The ions are Ba 2+ and S 2-. The charges are equal but opposite, so a 1:1 ratio will give a neutral formula unit. (2+) + (2-) = 0 (electrical neutrality) BaS 2. Aluminum chloride The ions are Al 3+ and Cl - (the charge on Cl is 1-; the one is understood). We can obtain a neutral formula unit by combining one Al 3+ with three Cl -. 1(3+) + 3(1-) = (3+) + (3-) = 0 AlCl 3 3. Aluminum oxide The ions are Al 3+ and O 2-. In the formula there must be the same number of positive charges as negative charges for the compound to be electrically neutral. This number must be a whole-number multiple of both 3 and 2.
The smallest whole number-multiple of 3 and 2 is 6, so there must be two Al 3+ and three O 2- in the formula. 2(3+) + 3(2-) = (6+) + (6-) = 0 Al 2 O 3 Binary Ionic Compounds Type II There are many metals that form more than one type of positive ion and thus form more than one type of ionic compound with a given anion. For example, the compound FeCl 2 contains Fe 2+ ions, and the compound FeCl 3 contains Fe 3+ ions. In this case, the charge on the metal ion must be specified in the name. The systematic names for these two iron compounds are iron(ii) chloride and iron(iii) chloride, respectively. The Roman numeral indicates the charge of the cation. Another system for naming these compounds is seen in older literature and still on some reagent bottles. The ion with the higher charge has a name ending in ic, and the one with the lower charge has a name ending in ous. For example, Fe 3+ is called the ferric iron, and Fe 2+ is called the ferrous ion. The names for FeCl 3 and FeCl 2 are then ferric chloride and ferrous chloride, respectively. Example: Give the systematic name for each of the following compounds. 1. CuCl Because the anion is Cl -, the cation must be Cu + (for charge balance), which requires a Roman numeral I. Copper(I) chloride 2. HgO Because the anion is O 2-, the cation must be Hg 2+ [mercury(ii)]. Mercury(II) oxide 3. Fe 2 O 3 The three O 2- ions carry a total charge of 6-, so the total positive charge must be 6+ for electrical neutrality. There are two Fe ions so 6+ divided by 2 = 3+. The Fe ion is Fe 3+ [iron(iii)]. Iron(III) oxide Example: Given the following systematic names, write the formula for each compound. 1. Manganese(IV) oxide The manganese ion has a 4+ charge. There is only one so the total positive charge is 4+. Thus, the total negative charge must be 4-. (4+) + (4-) = 0 (electrical neutrality) Oxygen ions (oxide) carry a 2- charge; two O 2- ions (total charge 4-) are required by the Mn 4+ ion. MnO 2 2. Lead(II) chloride The lead ion is Pb 2+. Chloride ion has a 1- charge (Cl - ). For electrical neutrality two Cl - ions would be required. (2+) + (2 x 1-) = (2+) + (2-) = 0 PbCl 2
Example: Name the following using the old nomenclature system. 1. CrCl 3 There are three Cl - ions giving a total 3- charge which must be balanced by a 3+ charge. Only one chromium ion is given in the formula so it must have a 3+ charge. Remember the ion with the higher charge has a name ending in ic and the one with the lower charge has a name ending in ous. Chromium forms the Cr 2+ ion and the Cr 3+ ion. Chromic chloride 2. PbS Sulfide ion has a 2- charge (S 2- ) so to balance the charges the lead ion must have a 2+ charge (Pb 2+ ). Remember lead forms two ions: Pb 2+ and Pb 4+. Since we are using the ion with the lower charge it will have the ending ous. Plumbous sulfide Refer to Figure 1 for a flowchart on naming Type I and Type II binary ionic compounds. Ionic Compounds with Polyatomic Ions Polyatomic ions are assigned special names that must be memorized to name the compounds containing them. Several series of polyatomic anions contain an atom of a given element and different numbers of oxygen atoms. These anions are called oxyanions. The name of the one with the smaller number of oxygen atoms ends in ite and the name of the one with the larger number ends in ate. For example, sulfite (SO 3 2- ) and sulfate (SO 4 2- ). When more than two oxyanions make up a series, hypo- (less than) and per- (more than) are used as prefixes to name the oxyanions with the fewest and most oxygen atoms. For example, hypochlorite (ClO - ), chlorite (ClO 2 - ), chlorate (ClO 3 - ), and perchlorate (ClO 4 - ). Example: Give the systematic name for the each of the following compounds. 1. Ca 3 (PO 4 ) 2 Calcium phosphate 2. Na 2 CO 3 Sodium carbonate 3. (NH 4 ) 2 SO 4 Ammonium sulfate 4. Fe(NO 3 ) 3 Iron is a transition metal requires a Roman numeral. Fe 3+ balances three NO - 3 ions. Iron(III) nitrate Example: Given the following systematic names, write the formula for each compound. 1. Calcium sulfate Calcium forms an ion with a 2+ charge (Ca 2+ ) and the sulfate ion has a 2 - charge (SO 4 2- ). One Ca 2+ ion balances charge with one SO 4 2+ ion. CaSO 4 2. Iron(III) acetate The iron ion has a 3+ charge (Fe 3+ ) and the acetate ion has a 1- charge (C 2 H 3 O 2 - ). Three acetate ions are needed to balance charge. Fe(C 2 H 3 O 2 ) 3 Note: the formula for the acetate is written with parentheses to show it appears three times.
Binary Covalent Compounds (Type III) Binary covalent compounds are formed between two nonmetals. In the naming of binary covalent compounds, the following rules apply: 1. The first element in the formula is named first, using the full element name. 2. The second element is named as if it were an anion. 3. Prefixes are used to denote the numbers of atoms present. 4. The prefix mono- is never used for naming the first element. For example, CO is called carbon monoxide, not monocarbon monoxide. Prefixes Example: Name each of the following compounds. 1. P 4 S 7 Tetraphosphorus heptasulfide 2. I 2 O 5 Diiodine pentoxide 3. S 2 F 10 Disulfur decafluoride 4. OF 2 Oxygen difluoride Example: From the following systematic names, write the formula for each compound. 1. Diiodine tetroxide I 2 O 4 2. Tetraphosphorus triselenide P 4 Se 3 3. Antimony pentafluoride SbF 5 Refer to Figure 2 for a flowchart for naming binary compounds. Refer to Figure 3 for an overall strategy for naming chemical compounds. Acids An acid can be viewed as a molecule with one or more H + ions attached to an anion. The rules for naming acids depend on whether the anion contains oxygen. If the anion does not contain oxygen, the acid is named with the prefix hydro- and the suffix ic. For example, when gaseous HCl is dissolved in water, it forms hydrochloric acid. When the anion contains oxygen, the acidic name is formed from the root name of the anion with a suffix of ic or ous, depending on the name of the anion. 1. If the anion name ends in ate, the suffix ic is added to the root name. For example, H 2 SO 4 contains the sulfate anion (SO 4 2- ) and is called sulfuric acid. 2. If the anion has an ite ending, the ite is replaced by ous. For example, H 2 SO 3, which contains sulfite (SO 3 2- ), is named sulfurous acid. Example: name the following acids. 1. HI(g) dissolved in water The anion does not contain oxygen so the prefix hydro- and the suffix ic is added. Hydroiodic acid 2. HClO 4 The anion contains oxygen and is called perchlorate so the suffix ic is added to the name. Perchloric acid Refer to Figure 4 for a flowchart for naming acids.
Figure 1. Flowchart for Naming Type I and Type II Binary Ionic Compounds Does the compound contain Type I or Type II cations? Type I Type II Name the cation using the element name. Using the principle of charge balance, determine the cation charge. Include in the cation name a Roman numeral indicating the charge. Figure 2. Flowchart for Naming Binary Compounds Binary Compound? Metal present? No Type III: Use prefixes. Does the metal form more than one cation? No Type I: Use the element name for the cation. Type II: Determine the charge of the cation; use a Roman numeral after the element name for the cation.