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. 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

4. 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

5. 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

6. reminder: compounds are electrically neutral therefore the sum of all charges in the compound MUST be 0
(+) and (-) charges must = each other

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

8. 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

9. 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

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

11. 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

12. 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

13. 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

14. 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
SO CO PO OH1-

15. 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

16. 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**

17. 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

18. 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

19. 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