1 POLYATOMIC IONS We have seen that atoms can lose or gain electrons to become ions. Groups of atoms can also become ions. These groups of atoms are called polyatomic ions. Examples: O hydroxide ion NO 3 nitrate ion CO 2 3 carbonate ion N + 4 ammonium ion Polyatomic ions can form ionic compounds just as monatomic ions. Example: What is the chemical formula for magnesium hydroxide? magnesium Mg 2+ hydroxide O Mg(O) 2 Note parenthesis indicates we need two of the entire O group. MgO 2 is wrong. NOMENCLATURE: Refer to handout Nomenclature of ions Cations (positive) name of ion is same as metal with main group metals, Roman numerals are used to indicate the charge of the ion only if the metal can have more than one charge. Ex: All alkali metals have only +1 charge K + potassium ion Rb + rubidium ion Ex: All alkaline earth metals have only +2 charge Mg 2+ magnesium ion Ca 2+ calcium ion Ex: Al 3+ aluminum Sn 4+ tin(iv) Pb 2+ lead(ii) with transition metals, the charge of cation is indicated with Roman numerals Ex: Fe 2+ iron(ii) many transition metals have only one common charge; thus, using the roman numeral is optional. Ni 2+, Zn 2+, Ag + polyatomic cations are given ium suffix Ex: N 4 + ammonium
2 Anions (negative) monatomic anions have ide suffix Ex: Cl chloride S 2 sulfide As 3 arsenide two polyatomic ions are exceptions and also have ide suffix O CN hydroxide cyanide all other polyatomic ions (oxyanions) have ite or ate suffix ite is always one less oxygen than ate Ex: 2 SO 4 sulfate sulfite SO 3 2 Ex: ClO 4 ClO 3 ClO 2 ClO perchlorate chlorate chlorite hypochlorite Ex: 2 SO 4 sulfate 2 SO 3 sulfite note sulfite anion has 3 oxygen atoms and sulfate anion has 4 oxygen atoms Ex: ClO 3 ClO 2 chlorate chlorite note chlorite anion has 2 oxygen atoms and chlorate anion has 3 oxygen atoms
3 CONSTRUCTING CORRECT IONIC CEMICAL FORMULAS When the charges of ions are known, correct chemical formulas are found by combining the correct number of ions to make the total charge of the compound zero. Example: What is the chemical formula for K + and Br? One + charge and one charge make zero charge. 1 (+1) + 1 (1) = 0 Answer is KBr. Example: What is the chemical formula for Ca 2+ and Cl? Calcium ion has +2 charge, therefore, two 1 chlorine ions are needed for the compound to equal zero charge. Example: What is chemical formula for Na + and O 2? Example: What is chemical formula for Fe 3+ and S 2?
4 NAMING IONIC COMPOUNDS 1. Write name of cation first (include Roman numeral, if necessary). 2. Write name of anion. Binary Compounds Example: NaI Na + is metal ion and I is nonmetal ion NaI sodium iodide Example: SrBr 2 Sr 2+ is metal ion and Br is nonmetal ion SrBr 2 strontium bromide Example: FeCl 3 Fe 3+ is metal ion and Cl is nonmetal ion FeCl 3 Example: potassium oxide potassium ion is K + and oxide ion is O 2 potassium oxide Example: zinc nitride CationPolyatomic Anion Compounds 1. Write name of metal ion (or ammonium ion) first 2. Write name of polyatomic anion Examples: NaNO 3 Na + and NO 3 Fe(C 2 3 O 2 ) 2 aluminum cyanide barium sulfite ammonium oxalate
5 NAMING MOLECULAR COMPOUNDS 1. Write the name of the element that is farthest from upper righthand corner first. 2. Indicate number of atoms with numerical prefix. 1 mono ** 6 hexa 2 di 7 hepta 3 tri 8 octa 4 tetra 9 nono 5 penta 10 deca ** use of the mono prefix is not preferred, except for carbon monoxide. 3. Add name of second element with ide suffix. 4. Indicate number of atoms with numerical prefix. 5. Note: No numerical prefixes with hydrogen. 6. Important exceptions to rules a) 2 O water b) N 3 ammonia c) C 4 methane ydrocarbons and their derivatives have their own nomenclature system. Examples N 2 O P 2 S 3 boron trifluoride carbon tetrachloride
6 YDROCARBONS compounds with carbon and hydrogen simplest hydrocarbons are called alkanes. general formula of an alkane is C n 2n+2 Nomenclature of Alkanes 1. Need to count longest continuous chain of carbons 2. Number of carbons gives compound s prefix 1 meth 6 hex 2 eth 7 hept 3 prop 8 oct 4 but 9 non 5 pent 10 dec 3. All alkanes have the suffix ane. Examples: butane C 4 10 C C C C Examples: heptane C 7 16 C C C C C C C