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How to Figure Out Chemical Formulas. The 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.

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the criss cross method
The 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.
how 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 nitride

Ca N

Ca2 N3

Ca3N2

How to Criss-Cross
try these formulas use the criss cross method to find the formulas for the following
Name of Compound

Beryllium bromide

Lithium oxide

Potassium nitride

Calcium sulfide

Magnesium nitride

Aluminum phosphide

Lithium chloride

Sodium sulfide

Magnesium oxide

Calcium iodide

Chemical Formula

BeBr2

Li2O

K3N

CaS

Mg3N2

AlP

LiCl

Na2S

MgO

CaI2

Try These FormulasUse the criss-cross method to find the formulas for the following.
formulas with shifty metals
Formulas 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.
slide6
Name of Compound

Iron (III) oxide

Tin (II) sulfide

Lead (II) chloride

Copper (II) bromide

Iron (II) nitride

Lead (IV) oxide

Copper (I) phosphide

Tin (IV) fluoride

Chemical Formula

Fe2O3

SnS

PbCl2

CuBr2

Fe3N2

PbO2

Cu3P

SnF4

Formulas With “Shifty” MetalsWrite the formulas for each of the following ionic compounds that feature one of the transition metals.
polyatomic ionic compounds
Polyatomic 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.

Brackets

  • You 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 NO31

    • After the criss-cross, you may get one of two possibilities…

FeNO33 OR Fe(NO3)3

    • The 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!
slide8
Name of Compound

Sodium nitrate

Calcium chlorate

Lithium phosphate

Aluminum sulfate

Tin (IV) carbonate

Iron (II) hydroxide

Magnesium carbonate

Lead (II) bicarbonate

Potassium chlorate

Ammonium phosphate

Chemical Formula

NaNO3

Ca(ClO3)2

Li3PO4

Al2(SO4)3

Sn(CO3)2

Fe(OH)2

MgCO3

Pb(HCO3)2

KClO3

(NH4)3PO4

Polyatomic Ionic CompoundsWrite the formulas for each of the following polyatomic ionic compounds. Use brackets when necessary.
formulas of covalent compounds
Formulas 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 - 7

Di – 2 Penta – 5 Octa - 8

Tri – 3 Hexa – 6 Nona - 9

  • Examples:
    • Carbon tetrachloride = CCl4
    • Dinitrogen dioxide = N2O2
    • Disulfur trihydride = S2H3
formulas of covalent compounds write the formulas for each of the following covalent compounds
Name of Compound

Carbon dioxide

Nitrogen trioxide

Trisulfur difluoride

Phosphorous pentoxide

Dinitrogen triiodide

Diphosphorus hexabromide

Dihydrogen monoxide

Disulfur heptachloride

Nitrogen dioxide

Carbon monoxide

Chemical Formula

CO2

NO3

S3F2

PO5

N2I3

P2Br6

H2O

S2Cl7

NO2

CO

Formulas of Covalent CompoundsWrite the formulas for each of the following covalent compounds.