Formula Construct Activity GOHS Chemistry 4.6 – Writing Formulas

How do you write formulas for binary ionic compounds given the name? Two simple steps: Write the symbol and charge of each ion Balance the charges by providing subscripts Magnesium chloride Write the symbol and charge of each ion. Mg+2 Cl-1 Cl-1 Balance the charges by supplying subscripts. Subscripts tell how many of each atom is present. MgCl2 You need a second Cl-1 to balance the charges

FeBr3 More examples: Iron III bromide Write the symbol and charge of each ion. The charge of the iron is provided by the Roman Numeral. Fe+3 Br -1 Br -1 Br -1 FeBr3 Balance the charges by supplying subscripts. The subscripts tell how many of each ion is needed to balance the compound. You’ll need three bromine ions to balance the one iron.

Al2S3 Aluminum Sulfide Write the symbol and charge of each ion. Balance the charges by supplying subscripts. S-2 In this case the charges do not evenly divide into each other. You must find the least common multiple. SIX Al2S3 How many aluminums are needed to arrive at a +6 charge? 2 How many sulfurs are needed to arrive at a -6 charge? 3

How do you write formulas for Polyatomic Ion Compounds? Very much like writing formulas for Binary Ionic Compounds. Two steps: Write the formulas/symbols of each ion. Balance the charges by supplying subscripts. If a subscript is needed for a polyatomic ion, it must be put in parentheses with the subscript on the outside. Let’s do some!!

Al(NO3)3 Balance the charges by supplying subscripts. Aluminum nitrate First, you can tell from the name that there is a polyatomic ion present (nitrate). All binary ionic compounds have suffixes of –ide. Al+3 NO3-1 NO3-1 NO3-1 Write the formula/symbol and charge of each ion. Balance the charges by supplying subscripts. Since Al is +3 and NO3 is -1, you need a total of 3NO3-1 to balance one Al+3 Since you will need a subscript of 3 for NO3-1, you need to put this in parentheses with the 3 outside. Al(NO3)3

Ca3(PO4)2 Pb(C2H3O2)4 Lead (IV) acetate Write the formula/symbol and charge of each ion. Pb+4 C2H3O2-1 C2H3O2-1 Balance the charges by supplying subscripts. Since Pb is +4 and C2H3O2 is -1, you will need 4 C2H3O2-1 to balance 1Pb+4 Pb(C2H3O2)4 Write the formula/symbol and charge of each ion. Calcium phosphate Ca+2 PO4-3 Balance the charges by supplying subscripts. Since Ca is +2 and PO4 is -3, you will need 3 Ca+2 to balance 2 PO4-3 Ca+2 PO4-3 Ca+2 Ca3(PO4)2

“Cross-Over Method” Al3+ O2- example: aluminum oxide The correct formula is Al2O3 Always make sure the combined charges add up to zero (2 × 3+) + (3 × 2–) = 0

Let’s practice Write the formulas: H+ Cl- Cu2+ Br- K+ O2- Co3+ CO32- Pb2+ O2- Pb4+ O2- Answers HCl CuBr2 K2O Co2(CO3)3 PbO PbO2