Ionic Compounds:Writing Formulas and Names

Formula Units

Empirical FormulasEmpirical Formulas

• Give the smallest whole-number ratio of elements in the compound.

• Ionic compounds have empirical formulas only.

Identify the empirical formulas

• H2O• H2O2

• CaF2

• NaCl• CH4

• C2H6

• NiO• FeCl3

Empirical and possibly MolecularEmpirical and possibly Molecular

MolecularMolecular

Empirical and IonicEmpirical and Ionic

Empirical and IonicEmpirical and Ionic

Empirical and possibly MolecularEmpirical and possibly Molecular

MolecularMolecular

Empirical and Ionic

Empirical and Ionic

Formula Unit

• The simplest ratio of the ions in an ionic compound.

• KBr means potassium ions and bromide ions are in a 1-to-1 ratio and is the formula unit.

• For ionic compounds, the formula mass is based on the formula unit.

Binary CompoundsBinary Compounds

• Composed of two elements–Positive monatomic metal ion

–Negative monatomic nonmetal ion

–Monoatomic ion is a one-atom ion.

Note: A binary compound may contain more than 2 ions but only 2 kinds of ions like Al2O3.

Oxidation Number or StateOxidation Number or State

• Charge of a monatomic ion.

• Given by right superscript.

• Some elements form only 1 ion.

• Some elements can form more than 1 ion.

• Try to find patterns.

Ca+2

Na+1

OO-2-2

FeFe+2+2 and Fe and Fe+3+3

Oxidation State of Selected Oxidation State of Selected ElementsElements

• Group 1 ions always +1• Group 2 ions always +2• MOST group 13 ions are +3• Groups 14, 15, 16, and 17 have multiple

oxidation states, both positive and negative. (Start with the first ox #)

• Transition metals (Group B) can have more than 1 oxidation state but always positive.

Writing Formulas for Binary Cmpds

• The first rule in writing formulas for ionic compounds is

POSITIVE ION FIRSTPOSITIVE ION FIRST

But how do you get the subscripts?But how do you get the subscripts?

We will start by writing formulas from the ions.

Writing Formulas for Binary Cmpds

• Compounds are electrically neutral – no charge.

• Take the number of each kind of ion X the charge on the ion. Sum these terms. Should get 0.

• Positive and negative charges must balance out.

Equal but Opposite Charges

• Na+1 and Cl-1: NaCl +1 + (-1) = 0

• Mg+2 and O-2: MgO +2 + (-2) = 0

• Al+3 and P-3: AlP +3 + (-3) = 0

• Rule: Just write the symbols, POSITIVE FIRSTPOSITIVE FIRST!

Try a few formulas:

• Li+1 and I-1

• Ca+2 and O-2

• Al+3 and N-3

• K+1 and F-1

• Ba+2 and S-2

LiILiICaOCaO

AlNAlN

KFKF

BaSBaS

Criss-Cross Method

• When the charges don’t cancel out immediately:

• Mg+2 and Cl-1, CROSS and DROP!CROSS and DROP!• (Numbers only, forget signs!)(Numbers only, forget signs!)

• Mg1Cl2 but if the subscript is 1, forget it!

• MgCl2 means 1 Mg+2 and 2 Cl-1

Check the Math

• MgCl2 means 1 Mg+2 and 2 Cl-1

• (1 X +2) + (2 X -1) = 2 – 2 = 0

• The charges add up to zero!

Try a few formulas:

• Ca+2 + Cl-1

• Na+1 + O-2

• Cs+1 + S-2

• Al+3 + Cl-1

• Al+3 + Se-2

• Mg+2 + F-1

• K+1 + N-3

CaClCaCl22NaNa22OO

CsCs22SSAlClAlCl33AlAl22SeSe33

MgFMgF22

KK33NN

Of course, it gets more difficult

• Potassium (K) and Fluorine (F)• Zinc (Zn) and Iodine (I)• Sodium (Na) and Oxygen (O)• Magnesium (Mg) and Oxygen (O)• Aluminum (Al) and Oxygen (O)• Calcium (Ca) and Bromine (Br)• Cesium (Cs) and Iodine (I)• Silver (Ag) and Sulfur (S)

KFKF

NaNa22OO

MgOMgOAlAl22OO33

CaBrCaBr22

CsICsI

ZnIZnI22

AgAg22SS

Naming Binary Ionic Compounds

1. Always name the metal (the cation or positive ion) first and use its full name.

2. Write the stemstem of the nonmetal (the anion or negative ion).

3. Add the ending “ide” to the nonmetal.

Stems of nonmetals

Nitr Ox Fluor

Phosph Sulf Chlor

Arsen Selen Brom

Tellur Iod

Hydr is the stem for H

The First Step in NamingThe First Step in Naming

• Look up the positive metal in the periodic table in your reference tables.

• If the metal has only one oxidation state (possible charge), it’s easy.

• If the metal has more than one oxidation state, there’s an extra step.

Metals with one oxidation state

• CaO• BaS• AlN• LiCl• Al2Se3

• Na2O• K3N• MgF2

Calcium OxideCalcium OxideBarium SulfideBarium SulfideAluminum NitrideAluminum Nitride

Lithium ChlorideLithium ChlorideAluminum SelenideAluminum Selenide

Sodium OxideSodium OxidePotassium NitridePotassium NitrideMagnesium FluorideMagnesium Fluoride

Metals with > 1 oxidation state.

• Use the formula to figure out which Use the formula to figure out which oxidation state the metal ion has.oxidation state the metal ion has.

• Ex: Fe can be Fe+2 oror Fe+3

• Name FeO and Fe2O3. These are

two differenttwo different compounds. You cannot name them iron oxide. Every formula goes with 1 name and vice-versa.

FeO and Fe2O3

• Compounds are electrically neutral.

• Oxygen is -2.

FeOFeO

Name:Name:Iron (II) oxideIron (II) oxideThe roman numeral II is The roman numeral II is the charge on the Fe.the charge on the Fe.

1 O which is -2.1 O which is -2.The Fe must be +2.The Fe must be +2.

FeO and Fe2O3

• Compounds are electrically neutral.

• Oxygen is -2.

FeFe22OO33

Each O is -2.Each O is -2.

There are 3 O’s.There are 3 O’s.3 X (-2) = -6.3 X (-2) = -6.Total negative charge.Total negative charge.

Total positive Total positive charge must charge must be +6.be +6.

Each Fe is +3.Each Fe is +3.Iron (III) oxideIron (III) oxide

Name the followingHint: Positive Always FirstPositive Always First

• TiCl3• Mn2O4

• Co2O3

• PdBr2

• AuCl3• MoN• MnO• TiO

Titanium (III) chlorideTitanium (III) chloride

Manganese (IV) oxideManganese (IV) oxide

Cobalt (III) oxideCobalt (III) oxide

Palladium (II) bromidePalladium (II) bromideGold (III) chlorideGold (III) chloride

Molybdenum (III) nitrideMolybdenum (III) nitride

Manganese (II) oxideManganese (II) oxide

Titanium (II) oxideTitanium (II) oxide

POLYATOMIC IONSPOLYATOMIC IONS

• A group of covalently bonded atoms that carries a charge.

• See Table E. May be +’ve or –’ve.

• Polyatomic ions have “names.”

• Because they are charged, polyatomic ions can form ionic bonds with oppositely-charged ions.

SOSO442-2- CO CO33

2-2- PO PO443-3- OH OH--

Ternary (or more) Compounds

• Contain 3 or more elements (uppercase letters)

• Usually have a polyatomic ion– If the polyatomic is +’ ve, it’s bonded to a

nonmetal– If the polyatomic is –’ve, it’s bonded to a

metal– Once in a while, 2 polyatomics are

bonded together.

Formulas with polyatomicsFormulas with polyatomics

• What’s the formula for the compound formed from NH4

+1 and Cl-1?

• The charges add up to zero, so just write the symbols, positive first!

NHNH44ClCl

Try a few more:

• Na+ and OH-

• K + and HCO3-1

• Mg+2 and CO3-2

• Li + and NO3-

• NH4 + and CN-

• Ca+2 and SO4-2

NaOHNaOH

KHCOKHCO33

MgCOMgCO33

LiNOLiNO33

NHNH44CNCN

CaSOCaSO44

These are more challenging:

• Mg+2 and (PO4)-3

• (NH4)+1 and S-2

• Al+3 and (NO3)-1

• Fe+2 and OH-1

• Hg2+2 and SCN-1

• Mg+2 and HCO3-

• Al+3 and C2O42-

MgMg33(PO(PO44))22

(NH(NH44))22SS

Al(NOAl(NO33))33

Fe(OH)Fe(OH)22

HgHg22(SCN)(SCN)22

Mg(HCOMg(HCO33))22

AlAl22(C(C22OO44))33

Some of the most challenging are

• Zinc + Nitrate ion• Magnesium + Hydroxide ion• Lithium + Carbonate ion• Ammonium ion + Bromine• Potassium + Sulfate ion• Calcium + Phosphate ion• Beryllium + Chlorate ion• Ammonium ion + Sulfate ion

Zn(NOZn(NO33))22

Mg(OH)Mg(OH)22

LiLi22COCO33

NHNH44BrBrKK22SOSO44

CaCa33(PO(PO44))22

Be(ClOBe(ClO33))22

(NH(NH44))22SOSO44

Naming compounds with polyatomics

• Polyatomic ions have names, given in Table E.

• Naming is parallel to binary naming.

• Positive always first.

• If +’ve ion is a metal, check to see how many oxidation states it has. If > 1, name has a roman numeral.

• If –’ve is polyatomic, 2nd part of name is name of polyatomic. Don’t modify ending.

Name the following

• NaOHNaOH• KHCOKHCO33

• LiNOLiNO33

• CaSOCaSO44

• Al(NOAl(NO33))33

• Fe(OH)Fe(OH)22

• CuSOCuSO44

• CuSCNCuSCN

Sodium hydroxideSodium hydroxidePotassium hydrogen carbonatePotassium hydrogen carbonate

Lithium nitrateLithium nitrateCalcium sulfateCalcium sulfate

Aluminum nitrateAluminum nitrateIron (II) hydroxideIron (II) hydroxide

Copper (II) sulfateCopper (II) sulfateCopper (I) thiocyanateCopper (I) thiocyanate

Summary for Ionic CompoundsSummary for Ionic Compounds

• Name = metal + stem of nonmetal + ide.

• If the metal has more than 1 oxidation state, it has a roman numeral in the name.

• Formula: positive first, always.

• Compounds are electrically neutral.

• Use oxidation states you know, like O, to figure out the oxidation states that have more than 1 possibility.