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1. NAMING A COMPOUND GIVEN ITS FORMULA Ionic Compounds: Covalent/Molecular Compounds: contain a METAL and NONMETAL contain NONMETALS (and/or metalloids) The following elements are nonmetals or metalloids: H, B, C, N, O, F, Si, P, S, Cl, Ge, As, Se, Br, Sb, Te, I, At (you don’t have to memorize these, just look at or a polyatomic ion (listed on Table E) (metals are on the left side of the staircase, nonmetals are on the right) Steps: Steps: 1. Write the name of the first element (metal) 2. Write the name of the second element (nonmetal) with an “ide” ending OR the full name for a polyatomic ion Ref. Table E 3. 4. where they are located on the periodic table) Only put a roman numeral after the name of the metal if there is more than one oxidation state to choose from for that metal. The Roman numeral goes right after the name of the metal because it stands for the charge of the METAL ION. Put the Roman numeral in parentheses. To determine the charge of the metal ion, first look up the charge on the NONmetal ion and write it above the NONmetal symbol. The NONmetal will ALWAYS have a NEGATIVE charge, the metal will ALWAYS have a positive charge. Balance the negative charges on the nonmetal by choosing a positive charge for the metal ion that will cancel out the total negative charges on the NONmetal. 1. Write the full name of the first element. If it has a subscript, turn it into the Greek prefix on the element’s name 2. Write the name of the second element with an “ide” ending. If it has a subscript, turn the subscript into the Greek prefix on the element's name *NEVER put “mono” in front of the 1st element *Drop the last vowel of the prefix if the element begins with the SAME vowel mono = 1 di = 2 tri = 3 tetra = 4 nono = 9 Examples: Greek prefixes penta = 5 hexa = 6 hepta = 7 octo = 8 deca = 10 Formula NaCl Name sodium chloride Examples: Formula Name CuCO3 copper(II) carbonate CO carbon monoxide SO2 sulfur dioxide PCl3 phosphorus trichloride As2S3 diarsenic trisulfide (because the carbonate ion has a 2- charge and there is one carbonate ion and one copper ion in the formula, the charge on the copper ion must be 2+ because the compound overall is neutral.) Cu2CO3 copper(I) carbonate (because the carbonate ion has a 2- charge and there is one carbonate ion and two copper ions in the formula, the charge on each copper ion must be 1+ because the compound overall is neutral: two 1+ charges cancel one 2- charge) Al2O3 aluminum oxide Sr(NO3)2 strontium nitrate 2. DETERMINING THE FORMULA OF A COMPOUND FROM ITS NAME For IONIC Compounds (metal + nonmetal elements) Steps 1. Write the symbols for the elements 2. Write the oxidation numbers in the upper right corner of each symbol * Remember to put polyatomic ions in parentheses ( )! Treat them as if they were one element. * If there is a Roman numeral in parenthesis after the name of the metal, that is the charge on the metal ion, so write that above the metal symbol. Example: in the compound iron(III) oxide, the iron ion has a charge of 3+, which we would write as Fe3+ 3. If the positive charge on the metal ion equals the negative charge on the nonmetal ion, that is the formula for the compound 4. If the positive charge on the metal ion does not equal the negative charge on the nonmetal ion, Crisscross the numbers and put as subscripts for the opposite symbol – put only the NUMBER as the subscript, do not bring the sign with it. ***Remember, IONIC formulas are ALWAYS written as empirical (simplest ratio), so if you can simplify the formula by dividing both subscripts by the same number, do so. 5. Check: multiply each ion’s charge by its subscript overall charge should be zero Examples: final answers in bold a. b. c. d. e. f. g. What is the formula for sodium nitride? What is the formula for barium hydroxide? What is the formula for potassium thiosulfate? What is the formula for ammonium chloride? What is the formula for chromium (II) oxide? What is the formula for chromium (III) oxide? What is the formula for chromium (VI) oxide? Na1+ N3Ba2+(OH)1K1+ (S2O3)2(NH4)1+ Cl1Cr2+O2Cr3+ O2Cr6+O2- (criss-cross) Na3N (criss-cross) Ba(OH)2 (criss-cross) K2S2O3 NH4Cl CrO (criss-cross) Cr2O3 (criss-cross) Cr2O6 ***simplifies to CrO3 For Covalent/Molecular Compounds (nonmetal elements and/or metalloids) Steps 1. Write the symbol for the first element given. If it has a Greek prefix, it becomes the subscript on the symbol. If it does not have a Greek prefix, there is only one of that atom in the formula, so it does not have a subscript. 2. Write the symbol for the second element given. The Greek prefix becomes the subscript on the symbol. Examples: final answers in bold a. What is the molecular formula for carbon tetrachloride? b. What is the molecular formula for sulfur pentafluoride? c. What is the molecular formula for arsenic tribromide? d. What is the molecular formula for dihydrogen monoxide? e. What is the molecular formula for diphosphorus pentoxide? f. What is the molecular formula for antimony trichloride? g. What is the molecular formula for tellurium difluoride? CCl4 SF5 AsBr3 H2O P2O5 SbCl3 TeF2