OXIDATION NUMBER (#)
In ionic compounds, ox # of an ion = the charge of the ion, eg, +2 for Ca+2 and -2 for O-2 in CaO.
In polar compounds, ox # of an atom = the charge it would have if it were an ion (if all electrons
in each bond are assigned to the more electronegative* atom), eg, +2 for C and -2 for O in CO.
In elements and pure covalent compounds, ox # of all atoms = zero, eg, H2, N2, O2, F2, Cl2, Br2,
I2, P4, S8, PH3
In ionic and polar compounds most atoms can have more than one ox #. Memorize the few that
have fixed ox #'s. They are:
+1 = Ag+, Group IA metals
+2 = Cd+2, Zn+2, and Group IIA metals
+3 = most Group IIIA atoms (B+3, Al+3, Ga+3 & In+3)
-1 = F-
Rules for Assigning Oxidation Numbers
1. Noble gases and atoms in elemental compounds have ox # = 0.
eg. He, Cl2, O2, S8, etc.
2. The sum of ox#'s of all atoms in a compound = 0 eg. NaCl: Na = +1, Cl = -1, [(+1) + (-1)] = 0
3. The sum of ox#'s of all atoms in a polyatomic ion = the charge on the polyatomic ion e.g. NH4+
4. The ox # of a monatomic ion = its ionic charge, eg. ox # of Na+ = +1, of Ca+2 = +2, etc.
5. In compounds containing unlike atoms:
A ‘-’ ox # is assigned to the more electronegative atom
A ‘+’ ox # is assigned to the more electropositive atom
eg. ICl
EN (I) = 2.5
EN (Cl) = 3.0
ox # (I) = +1
ox # (Cl) = -1
6. The ox # of H is either +1 or -1
H is +1 when combined with more electronegative atoms as in H2O and HCl
H is -1 when combined with less electronegative atoms as in LiH, CaH2, and AlH3
7. The ox # of O is always -2 except in peroxides where it is always –1. Peroxides have one
more oxygen than the normal oxide.
in hydrogen oxide (H2O), O = -2
but
in hydrogen peroxide (H2O2), O = -1
in sodium oxide (Na2O), O = -2
but
in sodium peroxide (Na2O2), O = -1
8. When a nonmetal is the more electronegative atom in a compound, its ox # equals the number
of electrons needed to be isoelectronic with the nearest noble gas.
‘-’ ox # for Group IVA atoms = -4 in K4C
C = -4
‘-’ ox # for Group VA atoms = -3
in K3N
N = -3
‘-’ ox # for Group VIA atoms = -2 in K2O
O = -2
‘-’ ox # for Group VIIA atoms = -1 in KF
F = -1
Note: In chemical formulas the less electronegative atom is usually written first and the more
electronegative atom is usually written last, eg, H2O, BaBr2, AlF3, SO2, etc.
Exceptions include NH3, CH4, and other organic compounds.

The more electronegative atom has the the highest ‘electronegativity’ as per Linus Pauling’s Table
of Electronegativity. For example, F (EN=4.0) is more electronegative than C (EN=2.5).
Nomenclature Review
Page 1 of 13
OXIDATION NUMBER EXERCISE
Using the rules from the previous page, assign oxidation numbers to the underlined atoms
in each compound.
1. SiF4
13. Sb2O5
2. CrI3
14. As2O3
3. CO2
15. OsO4
4. MnO4-
16. PtH4
5. CaC2
17. BaO2
6. SO3-2
18. Cu2C
7. BrO3-
19. Cr2O7-2
8. SnBr4
20. W3N5
9. Fe2O3
21. H2SiF6
10. Co2S3
22. Na2B4O7
11. P3O10-5
23. Cu3N
12. Mo3P4
24. HClO4
Answers:
1.
2.
3.
4.
5.
6.
7.
8.
+4
+3
+4
+7
–1 (an exception)
+4
+5
+4
Nomenclature Review
9.
10.
11.
12.
13.
14.
15.
16.
+3
+3
+5
+4
+5
+3
+8
+4
17.
18.
19.
20.
21.
22.
23.
24.
–1 (a peroxide)
+2
+6
+5
+4
+3
+3
+7
Page 2 of 13
RULES FOR NAMING INORGANIC COMPOUNDS (The Stock System)
Naming Binary Compounds (compounds made of two different atoms):
1. Think of compounds as having ‘+’ and ‘-’ atoms (even if they are not ionic).
The name of the more ‘+’ atom (the atom on the left side of the periodic table) is written first.
If both atoms are in the same group, the atom which is lower in the table is more ‘+’.
The name of the more ‘-’ atom is written last.
in NaCl
sodium is more ‘+’ and chlorine is more ‘-’.
2. The name of the ‘+’ ion (cation) is the same as the element.
in NaCl
Na+ is called sodium (same as the element Na).
If the cation has more than one possible ox #, then state its ox # in Roman Numerals.
FeCl2 is named iron(II) chloride
FeCl3 is named iron(III) chloride
MnCl7 is manganese(VII) chloride TiCl4 is titanium(IV) chloride
3. To name the ‘-’ ion (anion), drop the ending of the element name and add the suffix "ide".
SiC is silicon carbide
AlP is aluminum phosphide
CaO is calcium oxide
KBr is potassium bromide
Formulas of Binary Compounds:
1. Write the symbol of the ‘+’ ion (cation) first and the symbol of the ‘-’ ion (anion) last.
Recall that the anion ends in "ide".
2. Using number subscripts, make the sum of the ox #'s in the compound = 0.
It is often helpful to write the ox #'s of the ions before writing the formula of the compound.
name
iron(II) iodide
chromium(VI) oxide
zinc sulfide
formula
FeI2
CrO3
ZnS
cation
Fe+2
Cr+6
Zn+2
anion
IO-2
S-2
When combining ions with more complex ox #'s, use the inverse rule, i.e., use the ox # of the one
symbol as the subscript for the other symbol.
+3
Al
O
-2
+4
Sn
O
-2
Al 2 O 3
Sn 2 O 4
SnO 2
Note that formulas such as Sn2O4 must be simplified to the lowest whole number subscripts.
Try these.
arsenic(V) oxide
arsenic(III) oxide
aluminum sulfide
manganese(VII) sulfide
magnesium carbide
calcium nitride
Nomenclature Review
As2O5
As2O3
Al2S3
Mn2S7
Mg2C
Ca3N2
THE PREFIX SYSTEM
As+5
As+3
Al+3
Mn+7
Mg+2
Ca+2
O-2
O-2
S-2
S-2
C-4
N-3
Page 3 of 13
For binary compounds containing 2 non-metals, a Greek or Latin prefix is attached to the
name of an element to indicate the number of atoms of that element in the compound.
Number
Prefix
Formula
Name
1
mono-
CO
carbon monoxide
2
di-
SO2
sulfur dioxide
3
tri-
SO3
sulfur trioxide
4
tetra-
CCl4
carbon tetrachloride
5
penta-
PCl5
phosphorus pentachloride
6
hexa-
SF6
sulfur hexafluoride
7 = hepta, 8 = octa, 9 = ennea, 10 = deca, 11= hendeca, 12 = dodeca, 1/2 = hemi, 3/2 = sesqui
Note: A prefix is attached to the 1st element only if more than 1 is present. Although this
system is used almost exclusively for non-metal/non-metal compounds, occasionally, it is used
when a metal is present.
Name the following using the Prefix system and the Stock System system.
Prefix System Name
Stock System Name
N2O
dinitrogen monoxide
nitrogen(I) oxide
NO
nitrogen monoxide
nitrogen(II) oxide
NO2
nitrogen dioxide
nitrogen(IV) oxide
N2O3
dinitrogen trioxide
nitrogen(III) oxide
N2O4
dinitrogen tetr(a)oxide
nitrogen(IV) oxide (dimer)
N2O5
dinitrogen pentoxide
nitrogen(V) oxide
PCl3
phosphorus trichloride
phosphorus(III) chloride
P2O5
diphosphorus pentoxide
phosphorus(V) oxide
S2Cl2
disulfur dichloride
sulfur(I) choride (dimer)
As2O3
(di)arsenic trioxide
arsenic(III) oxide
As2O5
diarsenic pentoxide
arsenic(V) oxide
PbO2
lead dioxide
lead(IV) oxide
MnO2
manganese dioxide
manganese (IV) oxide
U3O8
triuranium octaoxide
uranium(5 1/3) oxide (not used)
Fe3O4
triiron tetr(a)oxide
iron(2 2/3) oxide (not used)
common names include: black iron oxide or magnetite ore or magnetic iron ore
Note: In the stock system, there is no space between the cation name and the oxidation number.
Nomenclature Review
Page 4 of 13
THE “ous-ic” SYSTEM
For binary compounds in which the cation has only 2 valences, the old “ous-ic” system is
sometimes used. Use the Latin name of the first element and add the suffix “ous” for the lower
valency and use “ic” for the higher valency.
Symbol
Name
Latin Name
Cu
copper
cuprum
Fe
iron
ferrum
Pb
lead
plumbum
Sn
tin
stannum
Au
gold
aurum
Name the following compounds using the “ous-ic” system.
ous-ic Name
Stock System Name
PbCl2
plumbous chloride
lead(II) chloride
PbCl4
plumbic chloride
lead(IV) chloride
SnBr2
stannous bromide
tin(II) bromide
SnBr4
stannic bromide
tin(IV) bromide
Cu2O
cuprous oxide
copper(I) oxide
CuO
cupric oxide
copper(II) oxide
FeO
ferrous oxide
iron(II) oxide
Fe2O3
ferric oxide
iron(III) oxide
Hg2O
mercurous oxide
mercury(I) oxide
HgO
mercuric oxide
mercury(II) oxide
PtCl2
platinous chloride
plantinum(II) chloride
PtCl4
platinic chloride
platinum(IV) chloride
CoO
cobaltous oxide
cobalt(II) oxide
Co2O3
cobaltic oxide
cobalt(III) oxide
Ce2(SO4)3
cerous sulfate
cerium(III) sulfate
Ce(SO4)2
ceric sulfate
cerium(IV) sulfate
Nomenclature Review
Page 5 of 13
OXYACIDS
Oxyacids, like H2SO4 contain, in addition to a nonmetal, such as S, both hydrogen and oxygen.
Students must memorize the names and formulas of the eight main oxyacids and their anions
since many others are derived from these.
The Eight Main Oxyacids
1
2
Acid Name
Acid Formula
Polyatomic Anion
Formula
Polyatomic
Anion Name
acetic acid
CH3COOH or HC2H3O2
CH3COO- or C2H3O2-
acetate
chloric acid
* HClO3
ClO3
-
chlorate
-
bromate
3
bromic acid
* HBrO3
BrO3
4
iodic acid
* HIO3
IO3-
iodate
5
nitric acid
HNO3
NO3-
nitrate
H2CO3
-2
carbonate
-2
sulfate
6
carbonic acid
CO3
7
sulfuric acid
H2SO4
SO4
8
phosphoric acid
H3PO4
PO4-3
phosphate
* Cl, Br and I are all in Group 7A of the periodic table and form analogous acids and polyatomic anions.
Many main oxyacids also exist in forms with more and/or less oxygens. For example, a sulfur
oxyacid with one less oxygen than the main acid is sulfurous acid, H2SO3. A chlorine oxyacid
with one more oxygen than the main acid is perchloric acid, HClO4. In the naming system of
oxyacids, ‘per…ic’ means one more oxygen than the ‘ic’ acid, ‘ous’ means one less oxygen than
the ‘ic’ acid and ‘hypo…ous’ means two less oxygens than the ‘ic’ acid.
A
Car
Never
Stays
Perfectly
per …. ic
…. ic
Clean
* HClO4
HC2H3O2
H2CO3
…. ous
HNO3
H2SO4
H3PO4
* HClO3
HNO2
H2SO3
H3PO3
* HClO2
H3PO2
* HClO
hypo …. ous
Note the patterns and learn the memory aid.
* Cl may be replaced by Br or I.
Oxyacid anions and their salts are also named systematically. The anion of the ‘ic’ acid ends
with the suffix ‘ate’. ‘per…ate’ means the anion of the ‘per…ic’ acid, ‘ite’ means the anion of
the ‘ous’ acid, and ‘hypo…ite’ means the anion of the ‘hypo…ous’ acid.
A
Car
Never
Stays
Perfectly
per …. ate
…. ate
Clean
* ClO4C2H3O2-
…. ite
CO3-2
NO3-
SO4-2
PO4-3
* ClO3-
NO2-
SO3-2
** HPO3-2
* ClO2-
** H2PO2-
* ClO-
hypo …. ite
* Cl may be replaced by Br or I.
** H3PO3 has only 2 acidic hydrogens so sodium phosphite is Na2HPO3
** H3PO2 has only 1 acidic hydrogen so sodium hypophosphite is NaH 2PO2
Nomenclature Review
Page 6 of 13
SALTS OF OXYACIDS
Salts of oxyacids are named using the Stock System rules like binary compounds.
Formula
Zn(C2H3O2)
Name
Formula
Name
zinc acetate
Sn(ClO3)2
tin(II) chlorate
iron(III) acetate
Pb(ClO2)4
lead(IV) chlorite
Cr(NO2)6
chromium(IV) nitrite
Bi(ClO)5
bisthmus(V) hypochlorite
Mn(NO3)7
manganese(VII) nitrate
HgBrO
mercury(I) hypobromite
Al2(CO3)3
aluminum carbonate
Co(BrO3)3
cobalt(III) bromate
Ga2(SO4)3
gallium sulfate
Mo(BrO4)6
molybdenum(VI) perbromate
K2SO3
potassium sulfite
Sr(IO2)2
strontium iodite
Ba3(PO4)2
barium phosphate
NaIO4
sodium periodate
CaHPO3
calcium phospite
LiIO
lithium hypoiodite
Mg(H2PO2)2 magnesium hypophosphite
Be(IO3)2
beryllium iodate
Cd(ClO4)2
Cu(BrO2)2
copper(II) bromite
2
Fe(C2H3O2)
3
cadmium perchlorate
BINARY ACIDS
Binary acids contain hydrogen and a non metal. They are all named as ‘hydro…ic’ acids when
dissolved in water. In the anhydrous state (in parenthesis) they are named using standard
notation.
HF = hydrofluoric acid
(hydrogen fluoride)
HCl = hydrochloric acid
(hydrogen chloride)
HBr = hydrobromic acid
(hydrogen bromide)
HI =
(hydrogen iodide)
hydroiodic acid
H2S = hydrosulfuric acid
(hydrogen sulfide)
HCN = hydrocyanic acid
(hydrogen cyanide)
Nomenclature Review
Page 7 of 13
NAMING ACID(IC) SALTS
Partially ionized polyprotic acids produce important polyatomic anions whose charges can be deduced from
the original oxyacid and whose names must be memorized. The names may seem confusing at first because
several conventions are used. It is important to remember that any chemical name must be unambiguous,
i.e., it must be definitive, that is, each name can correspond to only one possible formula and the number of
atoms must be clearly stated or must be readily determined from the name.
HCO3HSO4HSO3H2PO4-1
H2PO3-1
HPO4-2
HPO3-2
H2PO3-1
H2PO3-2
(mono)hydrogen carbonate
(mono)hydrogen sulfate
(mono)hydrogen sulfite
dihydrogen phosphate
dihydrogen phosphite
monohydrogen phosphate
monohydrogen phosphite
dihydrogen phosphite
dihydrogen hypophosphite
or
or
or
bicarbonate
bisulfate
bisulfite
Note that the prefix “bi” means ‘half’ is only applicable to acid salts of diprotic acids.
Since acid salts of diprotic acids can only have one hydrogen, the prefix “mono” is optional.
In the case of acid salts of triprotic acids, “bi” cannot be used and further, the number of hydrogen’s
remaining must be identified by the use of “mono” or “di”.
Keep in mind that in the foregoing, we are naming ions not compounds. These ions can and do form
neutral compounds, called acid salts, by combining with metal cations and these are named as follows.
NaHSO4
Ca(HSO4)2
Al(HSO4)3
sodium hydrogen sulfate
calcium hydrogen sulfate
aluminum hydrogen sulfate
or
or
or
sodium bisulfate
calcium bisulfate
aluminum bisulfate
NaH2PO4
or
sodium dihydrogen phosphate
or
monosodium (di)hydrogen phosphate
monosodium phosphate
Na2HPO4
or
sodium monohydrogen phosphate
or
disodium (mono)hydrogen phosphate
disodium phosphate
For compounds of monovalent cations like Na above, i.e., Li+, K+, etc., there are at least 3
acceptable names (as shown). For polyvalent cations, like Ca+2, Al+3, etc. the convention is to
state the number of H’s but not the number of metal cations, as follows.
Ca(H2PO4)2
CaHPO4
calcium dihydrogen phosphate
calcium monohydrogen phosphate
Al(H2PO4)3
Al2(HPO4)3
aluminum dihydrogen phosphate
aluminum monohydrogen phosphate
Acid salts of sulfite are named like sulfate and acid salts of phosphite are named like phosphate.
Nomenclature Review
Page 8 of 13
Calculate the oxidation # of the underlined atom or groups of atoms in each compound.
1. N2O
11. MnO2
2. N2O5
12. Zn(HCO3) 2
3. AlPO4
13. K2CO3
-2
4. SO4
14. NO2-1
5. Cd(BrO2)2
15. SnO2
6. Ga(ClO3)3
16. AgClO4
7. SiF4
17. As2O5
8. Mg2SiO4
18. P4O10
9. Na2CrO4
10. CrO3
Answers:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
+1
+5
+5
+6
+3
+5
+4
+4
+6
+6
Nomenclature Review
19. P2O7-4
20. TiBr3
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
+4
+4
+4
+3
+4
+7
+5
+5
+5
+3
Page 9 of 13
Quiz Binary Compounds and Special Groups
Write the name of formula of the following compounds. Spelling counts.
The column on the right will not be marked.
Formula
Name
Symbols & Oxid. #
Cs3P
Cu3N2
Mn2O7
PtCl2
SiH4
AlB
Hg3As2
Cd(CN)2
Zn(OH)2
Sb2O5
nickel(III) sulfide
ammonium phosphide
chromium(III) oxide
molybdenum(V) silicide
silver cyanide
gold(III) bromide
gallium carbide
iron(III) sulfide
lead(IV) hydroxide
beryllium nitride
Nomenclature Review
Page 10 of 13
Quiz Binary Compounds and Special Groups
ANSWERS:
Write the name of formula of the following compounds. Spelling counts.
The column on the right will not be marked.
Formula
Name
Symbols & Oxid. #
Cs3P
cesium phosphide
Cs+1(only) P-3(only)
Cu3N2
copper(II) nitride
(Cu?)3 (N-3)2
Mn2O7
manganese(VII) oxide
(Mn?)2 (O-2)7
PtCl2
platinum(II) chloride
(Pt?) (Cl-1)2
SiH4
silicon hydride
Si+4 (H-1)4
AlB
aluminum boride
Al+3(only) B-3
Hg3As2
mercury(II) arsenide
(Hg?)3 (As-3)2
Cd(CN)2
cadmium cyanide
Cd+2(only) [(CN)?]2
Zn(OH)2
zinc hydroxide
Zn+2(only) [(OH)?]2
Sb2O5
antimony(V) oxide
(Sb?)2 (O-2)5
Ni2S3
nickel(III) sulfide
(Ni+3)? (S-2)?
(NH4)3P
ammonium phosphide
[(NH4)+1(only)]? (P-3)?
Cr2O3
chromium(III) oxide
(Cr+3)? (O-2)?
Mo4Si5
molybdenum(V) silicide
(Mo+5)? (Si-4)?
AgCN
silver cyanide
AuBr3
gold(III) bromide
(Au+3)? (Br-1)?
Ga4C3
gallium carbide
(Ga+3)? (C-4)?
Fe2S3
iron(III) sulfide
(Fe+3)? (S-2)?
Pb(OH)4
lead(IV) hydroxide
(Pb+4)? [(OH)-1]?
Be3N2
beryllium nitride
[Be+2(only)]? [N-3]?
Nomenclature Review
Ag+(only)
CN-(only)
Page 11 of 13
Oxyacids & Oxyacid Salts
Name .........................................
Write the name of formula of the following compounds. Spelling counts.
The column on the right will not be marked.
Formula
Name
Symbols & Oxid. #
(NH4)2SO3
chromium(III) sulfate
Ca(ClO)2
magnesium phosphate
barium acetate
Cu(BrO2)2
HBrO4
calcium iodite
mercury(II) hypobromite
lead(IV) chlorate
manganese(VII) carbonate
hydriodic acid
iodic acid
lithium peroxide
H2SO3
magnesium bicarbonate
LiHSO3
KHSO4
Na2HPO412H2O
gold(II) nitrite hexahydrate
HIO
HBrO2
HClO3
nitrous acid
sulfurous acid
calcium dihydrogen phosphate
aluminum monohydrogen phosphate
Oxyacids & Oxyacid Salts
Answers
Nomenclature Review
Page 12 of 13
Write the name of formula of the following compounds. Spelling counts.
The column on the right will not be marked.
Formula
Name
Symbols & Oxid. #
(NH4)2SO3
ammonium sulfite
SO3-2 = sulfite
Cr2(SO4)3
chromium(III) sulfate
(Cr+3)? (SO4-2)?
Ca(ClO)2
calcium hypochlorite
OCl- = hypochlorite
Mg3(PO4)2
magnesium phosphate
Mg+2(only), (PO4-3)only
Ba(CH3COO)2
barium acetate
Ba+2, CH3COO- = acetate
Cu(BrO2)2
copper(II) bromite
Cu+?, BrO2- = bromite
HBrO4
perbromic acid
Ca(IO2)2
calcium iodite
Ca+2, IO2- = iodite
Hg(BrO)2
mercury(II) hypobromite
Hg+2, BrO- = hypobromite
Pb(ClO3)4
lead(IV) chlorate
Pb+4, ClO3- = chlorate
(Mn)2(CO3)7
manganese(VII) carbonate
Mn+7, CO3-2 = carbonate
HI
hydriodic acid
HIO3
iodic acid
Li2O2
lithium peroxide
H2SO3
sulfurous acid
Mg(HCO3)2
magnesium bicarbonate
Mg+2, HCO3- = bicarbonate
LiHSO3
lithium bisulfite
Li+1, HSO3- = bisulfite
KHSO4
potassium bisulfate
K+, HSO4- = bisulfate
Na2HPO412H2O
disodium phosphate dodecahydrate
Au(NO2)26H2O
gold(II) nitrite hexahydrate
HIO
hypoiodous acid
HBrO2
bromous acid
HClO3
chloric acid
HNO2
nitrous acid
H2SO3
sulfurous acid
Ca(H2PO4)2
calcium dihydrogen phosphate
Ca+2, H2PO4-1 = diH phos.
Al2(HPO4)3
aluminum monohydrogen phosphate
Al+3, HPO4-2 = monH phos.
Nomenclature Review
Li+1, O-1 = peroxide
Au+2, NO2-1 = nitrite
Page 13 of 13