CHAPTER 5: MOLECULES AND COMPOUNDS

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1 CHAPTER 5: MOLECULES AND COMPOUNDS Problems: 1-6, 9-13, 16, 20, 31-40, 43-64, 65 (a,b,c,e), 66(a-d,f), 69(a-d,f), 70(a-e), 71-78, 81-82, A compound will display the same properties (e.g. melting and boiling point, solubility, etc.) because it consists of the same molecules or ions. 5.2 COMPOUNDS DISPLAY CONSTANT COMPOSITION law of constant composition (or definite composition): A compound will always have the same formula. i.e., water s formula is always H 2 O Compounds always contain the same elements in the same proportion by mass e.g., water always consists of 11.2% hydrogen and 88.8% oxygen by mass 5.4 A MOLECULAR VIEW OF ELEMENTS AND COMPOUNDS Elements exist as individual atoms (atomic elements) or as molecules (molecular elements). Compounds are either molecules (molecular compounds of covalently bonded atoms) or ionic compounds (ions held together by ionic bonds). CHEM 121: Tro Chapter 5 page 1 of 9

2 Diatomic Molecules: Know these 7 elements exist as diatomic (two-atom) molecules: H 2 N 2 O 2 F 2 Cl 2 Br 2 I 2 Consider them the diatomic seven since there are seven of them, and six of them form a 7 on the Periodic Table. molecule: compound of nonmetal atoms held together by covalent bonds (e.g., H 2 O, CO 2, etc.) ionic compound: a compound consisting of ions usually metal cations with nonmetal or polyatomic anions held together by ionic bonds (e.g., NaCl, Al 2 O 3, CaBr 2, KMnO 4, BaSO 4 ) 5.5 WRITING FORMULAS FOR IONIC COMPOUNDS 5.7 NAMING IONIC COMPOUNDS monoatomic ions: from a single atom (eg Na +, Cl, O 2 ) polyatomic ions: consist of 2 or more atoms (eg. OH, MnO 4, SO 4 2 ) cations: positively charged ions Metal atoms and hydrogen lose electrons to form cations. I. Some metals always form the same charge: Group IA elements form ions with a +1 charge: H +, Li +, Na +, K + Group IIA elements form ions with a +2 charge: Mg 2+, Ca 2+, Sr 2+, Ba 2+ aluminum ion = Al 3+, silver ion = Ag + ; zinc ion = Zn 2+ ; cadmium ion = Cd 2+ element name + ion Na + =sodium ion or Ba 2+ =barium ion II. The Stock system is used to name most transition metals and other metals that can form more than one charge: e.g., iron (Fe), a transition metal, forms 2 different ions: Fe 2+ and Fe 3+ e.g., lead (Pb), in Group IVA, forms 2 different ions: Pb 2+ and Pb 4+ element name (charge in Roman numerals) + ion Sn 2+ =tin (II) ion and Sn 4+ =tin (IV) ion CHEM 121: Tro Chapter 5 page 2 of 9

3 Example: Write the name for each of the cations below: Ag + = Hg 2+ = Li + = Pb 4+ = K + = Cd 2+ = Zn 2+ = Al 3+ = Ni 3+ = Sr +2 = H + = Cr 3+ = ANIONS: negatively charged ions Nonmetal atoms gain electrons to form anions Group VIIA elements form 1 ions: F, Cl, Br, I Group VIA elements form 2 ions: O 2, S 2 Group VA elements form 3 ions: N 3, P 3 element stem name + ide + ion: O=oxygen O 2 = oxide ion F = N 3 = S 2 = Br = Cl = P 3 = Know the monatomic ions formed by the following Representative Elements and the four elements that form only one charge! Example: Give the formulas for each of the following ions: calcium ion = sulfide ion = nitride ion = potassium ion = phosphide ion = fluoride ion = silver ion = aluminum ion = zinc ion = CHEM 121: Tro Chapter 5 page 3 of 9

4 POLYATOMIC IONS Know the formulas and names of the following polyatomic ions: + NH 4 = ammonium ion CrO 2 4 = chromate ion CO 2 3 = carbonate ion Hg 2+ 2 = mercury (I) ion Cr 2 O 2 7 = dichromate ion HCO 3 = bicarbonate or SO 2 4 = sulfate ion hydrogen carbonate ion MnO 4 = permanganate ion SO 2 3 = sulfite ion ClO = hypochlorite ion C 2 H 3 O 2 = acetate ion NO 3 = nitrate ion ClO 2 = chlorite ion PO 3 4 = phosphate ion NO 2 = nitrite ion ClO 3 = chlorate ion CN = cyanide ion OH = hydroxide ion ClO 4 = perchlorate ion SCN = thiocyanate ion 2 O 2 = peroxide ion FORMULAS OF IONIC COMPOUNDS indicate the number of cations and anions present using symbols and subscripts Compounds must be neutral positive (+ve) charges = negative (-ve) charges How to write chemical formulas of ionic compounds: 1. If both ions have charges that are exactly opposite (+1 & -1, +2 & -2, etc.), the compound s formula contains one of each ion This also applies for polyatomic ions. Na + + Cl NaCl and Na + + CN NaCN K + + Cl K + + NO 3 Mg 2+ + O 2 Mg 2+ + SO 4 2 Al 3+ + N 3 Al 3+ + PO For monatomic ions with different charges, use the crossover method: Make the number on the negative charge the subscript of the cation and the number on the positive charge the subscript of the anion. Do NOT bring down the signs (+ or ), just the numbers! Ba 2+ + Cl Al 3+ + O 2 K + + P 3 CHEM 139: Tro Chapter 5 page 4 of 9

5 3. For polyatomic ions, where ions have different charges, also use the crossover method: Express more than one polyatomic ion with parentheses and subscripts. Sr 2+ + NO 3 Fe 3+ + CO 3 2 Zn 2+ + PO 4 3 EXCEPTION FOR CROSSOVER METHOD: Ions with 4+ and 2 charges! Pb 4+ + O 2 Also applies to polyatomic ions! Sn 4+ + SO 4 2 Ionic compound formulas have the lowest ratio of elements NOT Pb 2 O 4 but PbO 2! NAMING IONIC COMPOUNDS Given known charges for some elements get charge on transition metals! I. If one of each (cation & anion) present opposite but equal charges! Example: For each of the following compounds: 1. Indicate the charge on the anion, then determine the charge on the cation given that the compound must be neutral. 2. Name each ion then name the compound. CuCl FeS Name of CuCl: Name of FeS: CHEM 139: Tro Chapter 5 page 5 of 9

6 CuSO 4 FePO 4 Name of CuSO 4 : Name of FePO 4 : II. If more than one of each ion is present in the compound, Remember the overall negative charge = overall positive charge to determine the charge for each ion Example: For each of the following compounds: 1. Determine the overall negative charge on the anion, then determine the overall positive charge on the cation given that the compound must be neutral. If more than one of the cation is present, divide the overall positive charge by the number of cations present. 2. Name each ion then name the compound. SnCl 4 Cr 2 (CO 3 ) 3 Name: Name: PbO 2 Ni 3 (PO 4 ) 2 Name: Name: CHEM 139: Tro Chapter 5 page 6 of 9

7 Example: Determine the individual ions in each compound, then name the compound. CATION NAME + ANION NAME minus ion Name of compound CoCl 3 = individual ions name of compound Sr(OH) 2 = HgCrO 4 = MgF 2 = Ag 3 PO 4 = KNO 3 = LiMnO 4 = TiO 2 = Cu(HCO 3 ) 2 = Example: Given the name of a compound, determine its formula: For cations that can form more than one charge, knowing the charges on the monatomic and polyatomic anions you can determine charge on the cation. sodium nitride: individual ions formula of compound zinc phosphate: strontium chloride: lead (IV) sulfate: iron (III) carbonate: silver phosphide: calcium iodide: cadmium acetate: CHEM 139: Tro Chapter 5 page 7 of 9

8 5.8 NAMING MOLECULAR COMPOUNDS (OR MOLECULES) Binary molecular compound: consists of 2 types of nonmetal atoms held together by covalent bonds When naming, # of atoms of element indicated by Greek prefix before element name. 1. For first element, Greek prefix + element name 2. For second element, Greek prefix + element name stem + "ide" When only one atom, mono- is generally omitted, except in CO=carbon monoxide. # of atoms Greek prefix # of atoms Greek prefix 1 mono 6 hexa 2 di 7 hepta 3 tri 8 octa 4 tetra 9 nona 5 penta 10 deca CO 2 = carbon dioxide SF 6 = PCl 3 = Cl 2 O 7 = N 2 S 5 = Note: Greek prefixes are only used for binary molecular compounds NEVER for ionic compounds! NEVER use a Greek prefix to name metal cations or any anions! Some binary molecular compounds also have common names. e.g., everyone knows (or should know) H 2 O is water. Molecules with common names that you must know: NH 3 = ammonia CH 4 = methane H 2 O 2 = hydrogen peroxide 5.9 NAMING ACIDS aqueous solution (aq): a substance consisting of ions or compounds dissolved in water acid: a substance that releases hydrogen (H + ) ions in water easy to recognize because the chemical formula usually starts with H binary acid: consists of hydrogen and another nonmetal e.g. HCl(aq), H 2 S(aq) ternary oxyacid: consists of hydrogen and a polyatomic ion with oxygen e.g. H 2 SO 4 (aq), HNO 3 (aq), H 3 PO 4 (aq) CHEM 139: Tro Chapter 5 page 8 of 9

9 Some acids are monoprotic (release only H + per molecule) e.g. HCl, HBr, HI, HNO 3, HClO 4 Some acids are polyprotic (release more than on H + per molecule) e.g. H 2 SO 4 and H 2 CO 3 are both diprotic; H 3 PO 4 is triprotic. The name of an acid depends on the anion from which the acid forms. F = fluoride ion NO 2 = nitrite ion NO 3 = nitrate ion HF(aq) = hydrofluoric acid HNO 2 (aq) = nitrous acid HNO 3 (aq) = nitric acid For some acids, the stem name changes: PO 4 3 = phosphate ion H 3 PO 4 (aq) = phosphoric acid Examples: Name the following ions, then give the chemical formula (including the physical state), and name the acid. Br = = SO 2 4 = = Cl = = C 2 H 3 O 2 = = SO 2 3 = = CO 2 3 = = CN = = Note: The physical state aqueous, (aq), must be included in the chemical formula of an acid to distinguish the acid from another form of a compound. For example, HCl can exist as hydrochloric acid, HCl(aq), or as hydrogen chloride gas, HCl(g). CHEM 139: Tro Chapter 5 page 9 of 9