Ionic Compounds: Writing Formulas

Empirical Formulas formulas with smallest whole-number ratio of elements in compound ionic compounds are only written as empirical formulas

Formula Unit simplest ratio of ions in ionic compound examples: KBr is a formula unit potassium ions & bromide ions are in a 1-to-1 ratio AlCl3 is a formula unit aluminum & chloride ions in 1-to-3 ratio

Oxidation Number or State Na+1 charge of monatomic ion (right superscript) some elements form only 1 ion some elements can form more than 1 ion Ca+2 O-2 Al+3 Ag+1 Fe+2 Fe+3

Writing Formulas for Binary Ionic Compounds Write positive ion first Equal but opposite charges cancel out (no subscripts) Unlike charges use criss-cross method to determine subscripts

reminder: compounds are electrically neutral therefore the sum of all charges in the 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 Formula: write symbols, (+) 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 LiI CaO AlN KF BaS

Criss-Cross Method charges opposite but NOT equal: Mg+2 and Cl-1, CROSS and DROP! (# only, forget signs!) Mg1Cl2 (if subscript is 1, don’t write 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 CaCl2 Na+1 + O-2 Cs+1 + S-2 Na2O Al+3 + Cl-1 Al+3 + Se-2 Mg+2 + F-1 K+1 + N-3 CaCl2 Na2O Cs2S AlCl3 Al2Se3 MgF2 K3N

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) KF ZnI2 Na2O MgO Al2O3 CaBr2 CsI Ag2S

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 IONS SO42- CO32- PO43- OH1- 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 SO42- CO32- PO43- OH1-

Ternary Compounds contain 3 or more elements usually contain a polyatomic ion: if polyatomic ion is (+) it’s bonded to a (-) non-metal ion or a (-) polyatomic ion if polyatomic ion is (–) it’s bonded to a (+) metal ion or a (+) polyatomic ion

Formulas 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)3 **You must use parentheses for more than 1 polyatomic ion**

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 NaOH KHCO3 MgCO3 LiNO3 CaSO4

these are more challenging: Mg3(PO4)2 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 Al(NO3)3 Fe(OH)2 Hg2(SCN)2 Mg(HCO3)2 Al2(C2O4)3

more challenging zinc + nitrate ion magnesium + hydroxide ion Zn(NO3)2 zinc + nitrate ion magnesium + hydroxide ion lithium + carbonate ion potassium + sulfate ion calcium + phosphate ion beryllium + chlorate ion Mg(OH)2 Li2CO3 K2SO4 Ca3(PO4)2 Be (ClO3)2