Chapter 4 Formula Writing.notebook
Chemical Formula ­ shorthand way of representing the composition of substances (compounds) by using chemical symbols and subscripts.
Empirical Formula ­ October 06, 2014
Structural Formula ­
molecule
represents the simplest whole number ratio of atoms in a compound
e.g. C2H6 = e.g. CH4, NO2, NaCl, Al2O3
a two­dimensional representation of the way atoms are joined in a H H
H ­ C ­ C ­ H
H H
the chemical formulas for ionic compounds are given by their empirical formulas
Molecular Formulas ­ represents the total number of each kind of atoms in one molecule of a substance. A molecule is the smallest part of substance that exists free and still has the composition and properties of that substance.
e.g. methane = CH4 which is both empirical and molecular formula
e.g. ethane = C2H6 = molecular formula
what is its empirical formula?
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Writing Chemical Formulas
Sep 23­8:06 AM
The oxidation numbers for elements within the following groups is summarized:
Group 1 = +1
2 = +2
13 = +3
15 = ­3 for N and ­3 for P
16 = ­2
17 = ­1
Chemical Formulas of Binary Ionic Substances
When two elements react in such a way that an electron(s) is transferred from one element to the other, an ionic compound results. Such reactions typically involve metals (which tend to lose electrons) and nonmetals (which tend to gain electrons). The resulting electrically charged atoms, called ions, exhibit very strong electrostatic forces of attraction between the positive metal ion (cation) and the negative nonmetal ion (anion). This attraction results in an ionic bond.
e.g.
Li + F à LiF
metal nonmetal 3+9+
protons 3+ 9+
3­ 9­ 2­10­
electrons
0 0 +1 ­1 = 0
net charge
In writing the chemical formulas for any neutral ionic compound, subscripts are adjusted so the sum of the charge of the different atoms adds up to zero (charge conservation). In order to determine the charge on an atom we need to know its oxidation number. The oxidation numbers of elements reflect how atoms gain, lose, or share electrons when forming chemical bonds with other atoms. The atom with the positive oxidation number is customarily written first.
Common Simple Cations and Anions
Cation
Name
Anion
Name*
H+
Li+
Na+
K+
Cs+
Be2+
Mg2+
Ca2+
Ba2+
Al3+
Ag+
hydrogen
lithium
sodium
potassium
cesium
beryllium
magnesium
calcium
barium
aluminum
silver
H­
F­
Cl­
Br­
I­
O2­
S2­
Se2­
N3­
P3­
hydride
fluoride
chloride
bromide
iodide
oxide
sulfide
selenide
nitride
phosphide
*the root is given in bold.
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We will first consider binary ionic compounds in which the metal ion (cation) can have only one possible oxidation number. Your textbook refers to these as Type I compounds. In naming these compounds, the name of the metal is unchanged while the nonmetal takes an “ide” suffix. If you are given the name, in writing the formula we must keep in mind the idea of conservation of charge. This means the total positive charge must equal the total negative charge on a neutral compound. Adjusting the subscripts of the cation and anion does this. Use the “criss­cross” method to write these formulas. Let’s try some problems.
write the chemical formula for
The metals found in groups 3­12, called transition metals, may have multiple oxidation numbers. In this case, we must specify the oxidation number of the cation by using a roman numeral. Your textbook designates ionic compounds containing these metals as Type II compounds. Your Periodic Table lists the possible oxidation numbers for all elements (upper right hand corner). Using the roman numeral in a name is called the “stock” name. For certain cations, we will also use traditional names. The old or traditional system of designating oxidation numbers (for those metals having only two oxidation numbers) is by using the suffix “ous” for the lower oxidation number and the suffix “ic” for the higher one. You will be responsible for memorizing the following cations with their charges, stock designations, and traditional names:
magnesium bromide:
aluminum selenide:
calcium nitride:
sodium sulfide:
Give the names of the following compounds:
LiF:
AlBr3:
SrO:
Mg3N2:
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Chapter 4 Formula Writing.notebook
Traditional
Cation
Symbol Name
Stock
Write chemical formulas for the following compounds:
Co
cobalt
Co2+
Co3+
cobalt(II)
cobaltous
cobaltic cobalt(III)
Cu
copper
Cu+
Cu2+
cuprous
cupric
copper(I)
copper(II)
Fe
iron
Fe2+
Fe3+
ferrous
ferric
iron(II)
iron(III)
mercury(I)
mercury(II)
2+
October 06, 2014
Iron(II)chloride:
Iron(III)chloride:
Plumbic oxide:
Cuprous bromide:
Mercuric fluoride:
Hg
dimercury
mercury
Hg2
Hg2+
mercurous
mercuric
Pb
lead
Pb2+
Pb4+
lead(II)
plumbous
plumbic lead(IV)
Sn
tin
Sn2+
Sn4+
stannous
stannic
Mercurous fluoride:
tin(II)
tin(IV)
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Sep 23­8:08 AM
Name
Chemical Formulas for Compounds Containing Molecular (Polyatomic) Ions
Ion
Name
Ion
Here the same rules apply as for binary ionic compounds only the name of the molecular ion is unchanged. Keep in mind that you balance the charge of the whole molecular ion (treat it as one unit). If you need to balance the charge by adjusting the subscript of a molecular ion, then enclose it in parenthesis with the subscript outside the parenthesis. You will need to memorize the formulas and charges of the common polyatomic ions given in Table 4.4 of your text and reproduced below:
NH4+
ammonium
MnO4­
permanganate
C2H3O2­
acetate
NO2­
nitrite
CN­
cyanide
NO3­
nitrate
CO32­
carbonate
O22­
peroxide
HCO3­
hydrogen carbonate (bicarbonate)
OH­
hydroxide
C2O42­
oxalate
PO43­
phosphate
hydrogen phosphate
H3 O +
hydronium
HPO42­
ClO­
hypochlorite
H2PO4­
dihydrogen phosphate
ClO2­
chlorite
SCN­
thiocyanate
ClO3­
chlorate
SO32­
sulfite
ClO4­
perchlorate
SO42­
sulfate
CrO42­
chromate
HSO4­
hydrogen sulfate
Cr2O72­
dichromate
S2O32­
thiosulfate
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Sep 23­8:09 AM
Write the chemical formulas for the following compounds:
sodium chlorate:
Give the stock and traditional (where applicable) names:
potassium chromate:
CuCO3:
magnesium acetate:
Sn(NO3)4:
ferrous sulfate:
AlPO4:
Al2(SO4)3:
ammonium oxalate:
calcium hydroxide:
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Chapter 4 Formula Writing.notebook
October 06, 2014
Formulas for Binary Compounds Containing Only Nonmetals (Molecular Compounds)
Let’s try some examples.
Molecular compounds result from the sharing of electrons between two nonmetals. The sharing of electrons results in a covalent bond. Your textbook refers to these as Type III compounds. In the traditional naming of molecular compounds prefixes are used to designate the number of atoms of each element in the compound.
Name the following compounds using traditional and stock systems.
1
2
3
4
5
6
7
8
9
10
mono
di
tri
tetra
penta
hexa
hepta
octa
nona
deca
traditional
stock
compound
SnO
SnO2
PbI2
CuO
The elements in a molecular compound may not share electrons equally. The element with the stronger attraction for electrons are said to be more electronegative and acquire a partial negative charge. In naming these molecular compounds, the less electronegative element (carrying a partial positive charge) is named first, and the name of the second element takes the suffix “ide”. We need not be concerned with determining the electroegativities at this point. We use oxidation numbers in writing formulas for molecular compounds using the stock system. In this case the second (or negative) element of the compound dictates the oxidation number of the first (positive) element. Hg2F2
Sep 23­8:10 AM
Name the following compounds using traditional and stock systems.
Sep 23­8:10 AM
Write the formulas of these compounds given their names:
nitrogen trifluoride:
traditional
stock
SO3
carbon(II)oxide:
CO2
dihydrogen monoxide:
N2 O5
PCl3
sulfur(VI)chloride:
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Sep 23­8:12 AM
Ternary acids contain a molecular ion containing oxygen (an oxyanion). If the oxyanion ends in “ate”, the acid takes a suffix “ic”. If the oxyanion ends in “ite”{, the acid takes the suffix “ous”.
Naming Acids
An acid is any substance containing hydrogen as a positive ion that splits up (ionizes) to H+ and a negative ion when dissolved in water:
oxyanion
acid
oxyanion
acid
“ate”
“ic”
“ite”
“ous”
HX(s) à H+(aq) + X­(aq)
The (aq) designation for phase is “aqueous”, which means dissolved in water. In naming binary acids (acids composed of two elements only, one of which must be H), we use the prefix “hydro”, followed by the stem of the anion, and followed by the suffix “ic” and the word acid. This will be the case if the anion does not contain oxygen.
e.g.
e.g.
HNO2
HCl:
HNO3
HF:
H2S:
H3PO4
HBr:
H2SO4
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Chapter 4 Formula Writing.notebook
Give the chemical formulas for the following acids:
October 06, 2014
In some cases, typically with the chlorate ion, the prefixes “hypo” (meaning one less oxygen) and “per” (meaning one more oxygen) can be used.
Anion
phosphorous acid:
Formula
Acid Formula
Acid Name
chlorate
chloric acid:
chlorite
hypochlorite
sulfurous acid:
perchlorate
acetic acid:
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