1. Ionic Compounds: Writing Formulas

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

3. Identify Formulas: ? empirical vs molecular ? ionic
H2O
H2O2
CaF2
NaCl
CH4
C2H6
NiO
FeCl3
empirical – NOT ionic
molecular – NOT ionic
empirical, ionic
empirical, ionic
empirical – NOT ionic
molecular – NOT ionic
empirical, ionic
empirical, ionic

4. Formula Unit simplest ratio of ions in ionic compound examples:
KBr is formula unit
K+1 ions & Br-1 ions in 1-to-1 ratio
AlCl3 is formula unit
Al+3 & Cl-1 ions in 1-to-3 ratio

5. Binary Ionic Compounds
composed of two different elements
since ionic composed of:
(+) monatomic metal ion
(-) monatomic non-metal ion
note: binary compound may contain more than
2 ions but only 2 kinds of ions (ex: Al2O3)

6. 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 Ag+1
Fe Fe+3

7. Oxidation States A group metal elements: group 1 ions always +1
MOST group 13 ions are +3
groups 14, 15, & 16 have multiple (+) oxidation states
B group metal elements (transition metals):
can have more than 1 oxidation state
but are always (+)

8. 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

9. sum of ALL charges in compound
reminder: compounds are electrically neutral therefore:
sum of ALL charges in compound
MUST be equal to zero
(+) and (-) charges must = each other

10. Equal but Opposite Charges
ions formula
Na+1 and Cl-1: NaCl (-1) = 0
Mg+2 and O-2: MgO (-2) = 0
Al+3 and P-3: AlP (-3) = 0
rule: write symbols, (+) FIRST!

11. 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

12. Criss-Cross Method charges opposite but NOT equal:
Mg+2 and Cl-1, CROSS and DROP!
(# only, forget signs!)
Mg1Cl (if subscript is 1, forget it!)
MgCl2

13. Check the Math MgCl2 means 1 Mg+2 and 2 Cl-1
(1)(+2) + (2)(-1) = = 0
charges MUST add up to zero!

14. 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

15. of course, it gets more difficult
potassium (K) and fluorine (F)
zinc (Zn) and iodine (I)
sodium (Na) and oxygen (O)
aluminum (Al) and oxygen (O)
silver (Ag) and sulfur (S) KF
ZnI2
Na2O
Al2O3
Ag2S

16. 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

17. POLYATOMIC IONS (SO4)2- (CO3)2- (PO4)3- (OH)1-
Table E: groups of covalently bonded atoms that have a charge (+) or (–)
polyatomic ions have “names”
(-) polyatomic ions can form ionic bonds with (+) metal ion
(SO4) (CO3) (PO4) (OH)1-

18. Ternary Compounds contain 3 or more different elements
contain polyatomic ions:
(+)metal ions can bond to (–) polyatomic ions
(+) polyatomic ions bond to something (-) charged
either (-) non-metal ion or (-) poly ion

19. 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

20. ions with equal but opposite charges are easiest:
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
Na(OH)
K(HCO3)
Mg(CO3)
Li(NO3)
Ca(SO4)

21. 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
Mg3(PO4)2
Al(NO3)3
Fe(OH)2
Hg2(SCN)2
Mg(HCO3)2
Al2(C2O4)3

22. even more challenging:
zinc + nitrate ion
magnesium + hydroxide ion
lithium + carbonate ion
potassium + sulfate ion
calcium + phosphate ion
beryllium + chlorate ion
Zn(NO3)2
Mg(OH)2
Li2(CO3)
K2(SO4)
Ca3(PO4)2
Be (ClO3)2