Ionic Compounds:Writing Formulas

Empirical FormulasEmpirical Formulas

formulas with smallest whole-number ratio of elements in compound

ionic compounds only written as empirical formulas

Identify Formulas: ? empirical vs molecular ? ionic

• H2O• H2O2

• CaF2

• NaCl• CH4

• C2H6

• NiO• FeCl3

empirical – empirical – NOTNOT ionic ionic

molecular – molecular – NOTNOT ionic ionic

empirical, ionicempirical, ionic

empirical, ionicempirical, ionic

empirical – empirical – NOTNOT ionic ionic

molecular – NOT ionicmolecular – NOT ionic

empirical, ionic

empirical, ionic

Formula Unit

simplest ratio of ions in ionic compound

examples:

• KBr is formula unit– potassium ions & bromide ions are in a 1-to-1 ratio

• AlCl3 is formula unit

– aluminum & chloride ions in 1-to-3 ratio

Binary Ionic CompoundsBinary Ionic Compounds

• composed of two different elements–since ionic:

• positive monatomic metal ion

• negative monatomic non-metal ion

note: binary compound may contain more than 2 ions but only 2 kinds of ions (ex: Al2O3)

Oxidation Number or StateOxidation Number or State

• charge of monatomic ion

(right superscript)

• some elements form only 1 ion

• some elements can form more than 1 ion

Ca+2

Na+1

OO-2-2

FeFe+2+2 Fe Fe+3+3

AlAl+3+3 Ag Ag+1+1

Oxidation StatesOxidation StatesA group elements:•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 (+) and (-)

B group elements (transition metals):•can have more than 1 oxidation state

– but are always (+)

Writing Formulas for Binary Ionic Compounds

first rule in writing formulas

POSITIVE ION FIRSTPOSITIVE ION FIRST

how get the subscripts?how get the subscripts?

start by writing formulas from the ions

• reminder: compounds are electrically neutral therefore sum of all charges in compound MUST be 0

(+) and (-) charges must = each other

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: write symbols, (+) FIRST 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

LiILiI

CaOCaO

AlNAlN

KFKF

BaSBaS

Criss-Cross Method

charges opposite but NOT equal:

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

• Mg1Cl2 (if subscript is 1, forget it!)

MgCl2

Check the Math

• MgCl2 means 1 Mg+2 and 2 Cl-1

(1)(+2) + (2)(-1) = +2 + -2 = 0

• charges MUST add up to zero!

try these:• 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

CaClCaCl22

NaNa22OO

CsCs22SS

AlClAlCl33

AlAl22SeSe33

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

MgOMgO

AlAl22OO33

CaBrCaBr22

CsICsI

ZnIZnI22

AgAg22SS

and more challenging• gold +1 and fluorine

• gold +3 and fluorine

• titanium +2 and oxygen

• titanium +3 and oxygen

• titanium +4 and oxygen

• manganese +4 and bromine

• manganese +7 and bromine

AuF

AuF3

TiO

Ti2O3

Ti2O4 Ti O2

MnBr4

MnBr7

POLYATOMIC IONSPOLYATOMIC IONS

• Table E: groups of covalently bonded atoms that have a charge (+) or (–)

• polyatomic ions have “names”

• (-) polyatomic ions can form ionic bonds with (+) metal ions

SOSO442-2- CO CO33

2-2- PO PO443-3- OH OH1-1-

Ternary Compounds

• contain 3 or more elements • usually contain a polyatomic ion:

– if polyatomic is (+) it’s bonded to something (-) charged

– either(-) non-metal ion or (-) poly ion

– if polyatomic is (–) it’s bonded to (+)metal ion

Formulas with PolyatomicsFormulas with Polyatomics

• What’s the formula for the compound formed from Al+3 and SCN-1?

• charges must add up to zero

• write symbols, positive first!

• figure out how many of each ion needed

Al(SCN)Al(SCN)33

easy ones with equal but opposite charges:

• Na+1 and OH-1

• K +1 and HCO3-1

• Mg+2 and CO3-2

• Li +1 and NO3-1

• Ca+2 and SO4-2

NaOHNaOH

KHCOKHCO33

MgCOMgCO33

LiNOLiNO33

CaSOCaSO44

these are more challenging:

• Mg+2 and (PO4)-3

• Al+3 and (NO3)-1

• Fe+2 and OH-1

• Hg2+2 and SCN-1

• Mg+2 and HCO3-1

• Al+3 and C2O4-2

MgMg33(PO(PO44))22

Al(NOAl(NO33))33

Fe(OH)Fe(OH)22

HgHg22(SCN)(SCN)22

Mg(HCOMg(HCO33))22

AlAl22(C(C22OO44))33

more challenging

• zinc + nitrate ion• magnesium + hydroxide ion• lithium + carbonate ion• potassium + sulfate ion• calcium + phosphate ion• beryllium + chlorate ion

Zn(NOZn(NO33))22

Mg(OH)Mg(OH)22

LiLi22COCO33

KK22SOSO44

CaCa33(PO(PO44))22

Be (ClOBe (ClO33))22