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CHEM 113
The Language of Chemistry: Writing Chemical Formulas
Chemical Symbol




An abbreviation for the name of a specific element
Consists of one, two or three letters
If one letter is used, then it is capitalized
If two or three letters are used, the first letter is capitalized and the succeeding letter or
letters are small letters
Oxygen
Uranium
O
U
Chlorine
Unnihexium
Cl
Unh
Chemical Formula
 Representation used to denote one molecule of a pure substance
 Indicates the relative amount of atoms of each element in a molecule or unit particle of a pure
substance
 Consists of symbols of the elements composing the pure substance and subscripts denoting the
relative number of atoms of each element in a unit particle or molecule of the element of
compound
Oxidation State (Oxidation Number)
 Apparent charge on an atom in a compound
 In some cases, gives the actual charge, when electrons are transferred from one atom to
another to form ions
 In most cases, gives an indication of which atom in the compound has a greater attraction for
electrons and how many electrons are shifted towards or away from a particular atom
Rules in Assigning Oxidation States
 The oxidation number of an element in the free or uncombined state is always zero.
Cu0
S0
N2 0
Cl20
 The oxidation number of a monoatomic ion is the same as the charge of the ion.
K+
Br-
Ca2+
Al+3
 The algebraic sum of the oxidation numbers for all the atoms in the formula of a compound is
zero.
H3PO4
Writing Chemical Formulas
H:
P:
O:
3 (+1) = +3
1(+5) = +5
4(-2) = -8
0
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
The sum of the oxidation numbers of the atoms in a polyatomic ion must equal the charge of
the ion.
(CO3)-2
C:
O:
1(+4)
3(-2)
= +4
= -6
-2
Some common oxidation numbers
Group IA elements
Group IIA elements
Hydrogen (usually)
Hydrogen in hydrides (ionic compounds with IA and IIA elements)
Oxygen (usually)
Oxygen in peroxides (ex: H2O2)
Group VIIA when in binary compounds with less electronegative elements
Sulfur and other elements in Group VIA in binary compounds with more
electropositive elements
Nitrogen in binary compounds with more electropositive elements
Aluminum
Zinc
Silver
Some metals have more than one oxidation state
Copper (cuprous / cupric)
Mercury (mercurous / mercuric)
Gold (aurous / auric)
Iron (ferrous / ferric)
Tin (stannous / stannic)
Lead (plumbous / plumbic)
Manganese
Writing Chemical Formulas
+1
+2
+1
-1
-2
-1
-1
-2
+3
+3
+2
+1
+1
+2
+1
+2
+1
+3
+2
+3
+2
+4
+2
+4
+2
+3
+4
+7
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Rules in Writing Formulas

Metals, nonmetals, and inert gases have their formulas the same as their symbols.
Calcium
Ca
Neon gas
Ne
Sulfur S
 Active gaseous elements are written correctly by taking two atoms of the elements since these
are diatomic molecules.
Oxygen
Hydrogen
O2
H2
Iodine
Bromine
I2
Br2
 Write the symbol of the positive element then followed by the negative element.
When the positive oxidation number and the negative oxidation number are the same, the
formula is the same as it stands.
K+ Br-
Sr+2 O-2
KBr
SrO
When the positive oxidation number and the negative oxidation number are not the same,
apply criss-cross rule so that the algebraic sum of the oxidation numbers will be equal to
zero.
Al+3 S-2
Ca+2 Cl-
Al2S3
CaCl2
Whenever a radical or polyatomic ion in a formula is taken more than once, enclose the
radical inside a parenthesis and write the correct subscript after the parenthesis.
K+ (Cr2O7)-2
K2Cr2O7
Ba+2 (OH)-
Ba(OH)2
Some common polyatomic ions
Ammonium
Acetate
Perchlorate
Chlorate
(NH4)+
(C2H3O2)(ClO4)(ClO3)-
Thiocyanate
Chlorite
Carbonate
(CNS)(ClO2)(CO3)-2
Chromate
(CrO4)-2
Hypochlorite
(ClO)
Oxalate
(C2O4)-2
Cyanide
(CN)-
Sulfate
(SO4)-2
Nitrate
(NO3)-
Sulfite
(SO3)-2
Nitrite
(NO2)-
Permanganate
-
(MnO4)
-
Dichromate
(Cr2O7)-2
Thiosulfate
(S2O3)-2
Zincate
(ZnO2)-2
Hydroxide
(OH)
Bisulfate
(HSO4)-
Phosphate
(PO4)-3
Bisulfite
(HSO3)-
Phosphite
(PO3)-3
Cyanate
(CNO)-
Pyrophosphate
(P2O7)-4
Writing Chemical Formulas
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