Chapter 7: Chemical Formulas and Chemical Compounds
Section 7-1: Chemical Names and Formulas
7-1-1 Explain the significance of a chemical formula.
The subscript indicates that
The subscript indicates that
there are 8 carbon atoms in the there are 18 hydrogen atoms in
molecule of octane.
the molecule of octane.
Subscript 2
refers to 2
aluminum
atoms.
Subscript 4
refers to 4
oxygen atoms
in sulfate ion.
Subscript 3 refers to
everything inside
parentheses giving 3 sulfate
ions, with a total of 3 sulfur
atoms and 12 oxygen atoms.
Ionic Binary Compounds
7-1-2 Determine the formula of an ionic compound formed between two given ions.
1. Write the symbols for the ions side by side. Write the cation
first: Al 3+ O 2 −
2. Cross over the charges by using the absolute value of each
ion’s charge as the subscript for the other ion: Al23+ O32−
3. Multiplying the charge by the subscript shows that the charge
on two Al 3+ cations (2 × 3+ = 6+) equals the charge on three
O 2− anions (3 × 2− = 6−). The largest common factor of the
subscripts is 1. The formula is: Al2O3
4. Check the subscripts and divide them by their largest
common factor to give the smallest possible whole-number
ratio of ions. Then write the corrected formula. Al2O3
1. Write the symbols for the ions
side by side. Write the cation
first.
2. Place the Cation’s charge as a
subscript for the Anion.
3. Place the absolute value of
Anion’s charge as a subscript for
the Cation.
4. Place the Cation next to the
Anion.
5. Check the subscripts and
divide them by their largest
common factor to give the
smallest possible whole-number
ratio of ions.
1. Write the symbols for the ions
side by side. Write the cation
first.
2. Place the Cation’s charge as a
subscript for the Anion.
3. Place the absolute value of
Anion’s charge as a subscript for
the Cation.
4. Place the Cation next to the
Anion.
5. Check the subscripts and
divide them by their largest
common factor to give the
smallest possible whole-number
ratio of ions.
7-1-3 Name an ionic compound given its formula.
Nomenclature - naming system, of binary ionic compounds
Name of cation
aluminum
Page 207 – Practice:
Name of anion
oxide
The Stock System of Nomenclature: Some elements (mostly transition metals) form two
or more cations with different charges. To distinguish these ions, the Stock system of
nomenclature is used.
The Stock system - uses a Roman numeral to indicate an
ion’s charge. The numeral is enclosed in parentheses and
placed immediately after the metal name.
Stock system Examples
Fe2+
Fe3+
iron(II)
iron(III)
Names of metals that commonly form only one cation do not
include a Roman numeral.
Na+
Ba2+
Al3+
Sodium
barium
aluminum
Using the stock system:
Name of + Roman numeral
cation indicating
charge
Name of anion
(Drop the ending
add –ide to name)
chloride
copper (II)
Put it together and you get copper (II) chloride.
The way we know that the copper is a +2 is by analyzing the
formula and see that there are 2 Cl – 1’s and to be neutral copper
has to have a +2: (+2) + (2(– 1) = 0 thus a neutral compound
Do Practice Exercises: page 209 #’s 1 & 2
Oxyanions - polyatomic ions of oxygen
Two or more oxyanions are formed by the same two elements. When naming compounds
containing such oxyanions, the most common ion is given the ending -ate. The ion with
one less oxygen atom is given the ending -ite.
Examples: Nitrate,
NO3—
Nitrite;
NO2—
Sulfate,
SO42 —
Sulfite
SO32 —
When more than two different oxyanions form, the anion with one less oxygen than the ite anion is given the prefix hypo-. An anion with one more oxygen than the -ate anion is
given the prefix per-.This nomenclature is illustrated by the four oxyanions formed
between chlorine and oxygen.
Hypochlorite
ClO –
Chlorite
ClO2–
Chlorate
ClO3–
Perchlorate
ClO4–
SAMPLE PROBLEM 7- 3 Write the formula for tin(IV) sulfate.
SOLUTION: Write the symbols for the ions side by side. Write the cation first.
Sn4+ SO4 2−
Cross over the charges to give subscripts. Add parentheses around the polyatomic ion if
necessary.
Sn4+2 (SO4)4 2−
Check the subscripts and write the formula. The total positive charge is 2 × 4+ = 8+.The
total negative charge is 4 × 2− = 8−. The overall charge is neutral. Check.
The largest common factor of the subscripts is 2, so the smallest whole-number ratio of
ions in the compound is 1:2. The correct formula is therefore
Sn(SO4)2.
Do Practice Exercise page 211 #’s 1 & 2
Homework: Page 235 #s 1-6
Binary Ionic compounds: A summary
Why do metals form cations?
They have low ionization energy & low electron affinity, so
they don't particularly care to keep their outer shell electrons;
they fall back to Nobel Gas Configuration of the next lower
energy level.
Why do nonmetals form anions?
They have high ionization energy & high electron affinity; they
want to keep not only their electrons, but also what ever
electrons are around to fill their outer shell.
So for main group elements (the s and p blocks), cations &
anions have an octet of electrons.
Covalent Bonds (review): Covalent bonds consist of shared
electron pair(s) and are formed between nonmetals. Octet rule
as applied to Covalent bond formation, atoms try to complete
octet by sharing electron pairs.
The Question arises “How do we name the Covalent
Compound?” The old system of naming molecular
compounds is based on the use of prefixes.
Naming Binary Molecular Compounds:
7-1-4 Using prefixes, name a binary molecular compound from its formula.
First question is what are the prefixes?
A prefix indicates how many atoms of an element are in a
molecule.
The newer system is the Stock system for naming molecular compounds, which requires
an understanding of oxidation numbers. This system will be discussed in Section 2.
The old system of naming molecular compounds is based on
the use of prefixes; so we will start with prefixes!
First things first, we must know the
prefixes. TABLE 7-3 page 212
Number
1
2
The rules for the prefix system of
3
nomenclature of binary molecular
4
compounds are as follows.
5
6
1. The less electronegative element is
7
given first. It is given a prefix only if it
8
contributes more than one atom to a
9
molecule of the compound.
10
Prefix
monoditritetrapentahexaheptaoctanonadeca-
2. The second element is named by combining (a) a prefix
indicating the number of atoms contributed by the
element, (b) the root of the name of the second element,
and (c) the ending -ide. With few exceptions, the ending ide indicates that a compound contains only two
elements.
3. The “o” or “a” at the end of a prefix is usually dropped
when the word following the prefix begins with another
vowel, e.g., monoxide or pentoxide.
Because the less electronegative element is written first,
oxygen and the halogens are usually given second.
In general, the order of nonmetals in binary compound names
and formulas is C, P, N, H, S, I, Br, Cl, O, F.
Prefix needed
if less
Name of
electronegative
Less
element
electronegative
contributes
element
more than one
atom
tetraphosphorus
Prefix
indicating
Root name of
number
more
of atoms
electronegative
contributed by
element + ide
more
electronegative
element
decoxide
Put it all together you get tetraphosphorus decoxide
More Examples:
C3P4 – tricarbon tetraphosphide
H2S – dihydrogen monosulfide
CCl4 – carbon tetrachloride
H2O – dihydrogen monoxide
Turn to page 213: example 7-4
Homework: Practice Problems page 213
Examples: Binary Compounds of Nitrogen and Oxygen
Formula
N2O
NO
NO2
N2O3
N2O4
N2O5
Prefix-system name
dinitrogen monoxide
nitrogen monoxide
nitrogen dioxide
dinitrogen trioxide
dinitrogen tetroxide
dinitrogen pentoxide
Write the formulas using the prefixes to describe the number of
the elements in the compound for:
Dihydrogen monoxide, Water (H2O)
Carbon tetrachloride, CCl4
Carbon dioxide, CO2
Carbon monoxide CO
Acids and Salts
Acids
Most acids can be classified as binary acids or oxyacids.
Binary acids are acids that consist of two elements, usually
hydrogen and one of the halogens: fluorine, chlorine,
bromine, iodine.
Oxyacids are acids that contain hydrogen, oxygen, and a
third element (usually a nonmetal). Many polyatomic ions
are produced by the loss of hydrogen ions from oxyacids. A
few examples of the relationship between oxyacids and
oxyanions are shown next page
(in your text on page 214).
sulfuric acid
H2SO4
sulfate
SO42nitric acid
HNO3
nitrate
NO3 phosphoric acid H3PO4
phosphate
PO43Oxyacids of the “-ate” ending polyatomic ion are the “-ic”
acids. Sulfate  Sulfuric Acid
Oxyacids of the “-ite” ending polyatomic ion are the “-ous”
acids. Sulfite  Sulfurous Acid
Also from page 214 learn the Common Binary Acids and Oxyacids
Salts
Salt is an ionic compound composed of a cation and the
anion from an acid.
Table salt, NaCl, contains the anion from hydrochloric acid.
Calcium sulfate, CaSO4, is a salt containing an anion from
sulfuric acid.
Finally, some salts contain anions in which one or more
hydrogen atoms from the acid are retained. Such anions are
named by adding the word hydrogen or the prefix bi- to the
anion name. The best known such anion comes from
carbonic acid, H2CO3
HCO3 - hydrogen carbonate ion
(bicarbonate ion)
Ex of the salt: (sodium dissolved in weak carbonic acid)
Sodium Bicarbonate NaHCO3
Or Sodium Hydrogen Carbonate
Best known as Baking soda