Ionic Compounds: Writing Formulas

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

Identify Formulas: ? empirical vs molecular ? ionic H 2 O H 2 O 2 CaF 2 NaCl CH 4 C 2 H 6 NiO FeCl 3 empirical – NOT ionic molecular – NOT ionic empirical, ionic empirical – NOT ionic molecular – NOT 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 AlCl 3 is formula unit –aluminum & chloride ions in 1-to-3 ratio

Binary 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: Al 2 O 3 )

Oxidation 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 O -2 Fe +2 Fe +3 Al +3 Ag +1

Oxidation States A 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 FIRST 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 FIRSTrule: 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 -2LiI CaO AlN KF BaS

Criss-Cross Method charges opposite but NOT equal: CROSS and DROP!Mg +2 and Cl -1, CROSS and DROP! (# only, forget signs!) (# only, forget signs!) Mg 1 Cl 2 (if subscript is 1, forget it!) MgCl 2

Check the Math MgCl 2 means 1 Mg +2 and 2 Cl -1 (1)(+2) + (2)(-1) = = 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 CaCl 2 Na 2 O Cs 2 S AlCl 3 Al 2 Se 3 MgF 2 K3NK3NK3NK3N

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 Na 2 O MgO Al 2 O 3 CaBr 2 CsI ZnI 2 Ag 2 S

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 AuF 3 TiO Ti 2 O 3 Ti 2 O 4 Ti O 2 MnBr 4 MnBr 7 

POLYATOMIC 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 SO 4 2- CO 3 2- PO 4 3- OH 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 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

easy ones with equal but opposite charges: Na +1 and OH -1 K +1 and HCO 3 -1 Mg +2 and CO 3 -2 Li +1 and NO 3 -1 Ca +2 and SO 4 -2 NaOH KHCO 3 MgCO 3 LiNO 3 CaSO 4

these are more challenging: Mg +2 and (PO 4 ) -3 Al +3 and (NO 3 ) -1 Fe +2 and OH -1 Hg 2 +2 and SCN -1 Mg +2 and HCO 3 -1 Al +3 and C 2 O 4 -2 Mg 3 (PO 4 ) 2 Al(NO 3 ) 3 Fe(OH) 2 Hg 2 (SCN) 2 Mg(HCO 3 ) 2 Al 2 (C 2 O 4 ) 3

more challenging zinc + nitrate ion magnesium + hydroxide ion lithium + carbonate ion potassium + sulfate ion calcium + phosphate ion beryllium + chlorate ion Zn(NO 3 ) 2 Mg(OH) 2 Li 2 CO 3 K 2 SO 4 Ca 3 (PO 4 ) 2 Be (ClO 3 ) 2