2The Criss-Cross Method The criss-cross method is an easy way to determine the formula of any ionic compound.The ionic compound can be the simple (binary) ionic compound with just two elements, or one that has a polyatomic ion as part of it.All you need is the “combining capacity” of the elements/ions with which you will be working.You can get these combining capacity values directly off of the periodic table for most elements or they are part of the list for either the “shifty” metals and polyatomic ions.
3How to Criss-Cross Steps (in order) Write the chemical symbols. Write the combining capacities over the symbols.Criss-cross and drop.Reduce – if necessary – the same numbers or a 2 & 4 together.Example: Calcium nitrideCa NCa2 N3Ca3N2
4Try These Formulas Use the criss-cross method to find the formulas for the following. Name of CompoundBeryllium bromideLithium oxidePotassium nitrideCalcium sulfideMagnesium nitrideAluminum phosphideLithium chlorideSodium sulfideMagnesium oxideCalcium iodideChemical FormulaBeBr2Li2OK3NCaSMg3N2AlPLiClNa2SMgOCaI2
5Formulas With “Shifty” Metals The criss-cross method is also used when dealing with metals that have more than one combining capacity value. (Copper, Iron , Lead or Tin)The combining capacity value for these metals can be found in the name of the compound that contains them.Examples:Iron (II) oxide – the Iron (Fe) used here has a combining capacity of 2.Lead (IV) sulfide – the Lead (Pb) used here has a combining capacity of 4.
6Formulas With “Shifty” Metals Write the formulas for each of the following ionic compounds that feature one of the transition metals.Name of CompoundIron (III) oxideTin (II) sulfideLead (II) chlorideCopper (II) bromideIron (II) nitrideLead (IV) oxideCopper (I) phosphideTin (IV) fluorideChemical FormulaFe2O3SnSPbCl2CuBr2Fe3N2PbO2Cu3PSnF4
7Polyatomic Ionic Compounds Polyatomic ions are clusters of atoms that stay together as one unit and carry an overall charge. Most polyatomic ions are negatively charged.The charge of the polyatomic ion is the same as its combining capacity so that is the number you will use in the criss-cross method.BracketsYou may need more than one polyatomic ion in your formula…This means that you will have to use brackets. Use brackets whenever you need more than one polyatomic ion in the formula.Example: Iron (III) nitrate…Start with symbols and combining capacities…Fe3 NO31After the criss-cross, you may get one of two possibilities…FeNO33 OR Fe(NO3)3The second option is right because it says you need two nitrate clusters to go with every iron atom. The first option, which did not use brackets, reads as one iron atom with one nitrogen atom and thirty-three oxygen atoms.Fe(NO3)3 is the correct formula! Use those brackets!
8Polyatomic Ionic Compounds Write the formulas for each of the following polyatomic ionic compounds. Use brackets when necessary.Name of CompoundSodium nitrateCalcium chlorateLithium phosphateAluminum sulfateTin (IV) carbonateIron (II) hydroxideMagnesium carbonateLead (II) bicarbonatePotassium chlorateAmmonium phosphateChemical FormulaNaNO3Ca(ClO3)2Li3PO4Al2(SO4)3Sn(CO3)2Fe(OH)2MgCO3Pb(HCO3)2KClO3(NH4)3PO4
9Formulas of Covalent Compounds Covalent compounds are between two nonmetals and it involves the sharing of electrons – no bullying involved.The names of covalent compounds have prefixes (starters) that tell you how many of each element is required in the formula.You must remember the prefixes and you must remember that covalent compounds do not reduce to lowest terms like those ionic ones do:Mono – 1 Tetra – 4 Hepta - 7Di – 2 Penta – 5 Octa - 8Tri – 3 Hexa – 6 Nona - 9Examples:Carbon tetrachloride = CCl4Dinitrogen dioxide = N2O2Disulfur trihydride = S2H3
10Formulas of Covalent Compounds Write the formulas for each of the following covalent compounds. Name of CompoundCarbon dioxideNitrogen trioxideTrisulfur difluoridePhosphorous pentoxideDinitrogen triiodideDiphosphorus hexabromideDihydrogen monoxideDisulfur heptachlorideNitrogen dioxideCarbon monoxideChemical FormulaCO2NO3S3F2PO5N2I3P2Br6H2OS2Cl7NO2CO