Chemical Compounds: Part II
Naming Binary Molecular
Compounds
Molecular compounds are
composed of individual
covalently bonded units or
molecules
 Two systems of nomenclature

Old System – Prefixes
 Stock System – Uses Oxidation
Numbers

Prefix System
Number
Prefix
1
Mono-
2
Di-
3
Tri-
4
Tetra-
5
Penta-
6
Hexa-
7
Hepta-
8
Octa-
9
Nona-
10
Deca-
Prefix System
Rules
1. The less electronegative element is
given first. It is given a prefix only
if it contributes more than one
atom to a molecule of the
compound.

Prefix System
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 (c)
the ending –ide.
Prefix System
3.

Drop o or a at end of prefix when
root word of element begins with a
vowel.
Example – P4O10 tetraphosphorus
decoxide
Acids and Salts



Acids – most are binary or oxyacids
Binary acids – consist of two
elements, usually hydrogen and one
of the halogens
Oxyacids – contain hydrogen,
oxygen, and a third element
(usually a nonmetal)
Acids and Salts

In chemical nomenclature, the
term acid usually refers to a
water solution of the compound
rather than the compound itself

Example – hydrochloric acid – a
water solution of the molecular
compound hydrogen chloride, HCl
Acids and Salts

Many polyatomic ions are produced
by the loss of hydrogen ions from
oxyacids
Sulfuric
Acid
H2SO4
Sulfate
SO42-
Nitric Acid
HNO3
Nitrate
NO31-
Phosphate
PO43-
Phosphoric H3PO4
Acid
Acids and Salts

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 bito the anion name.
 Example - H2CO3 – carbonic acid
 HCO3-
– hydrogen carbonate
ion or bicarbonate ion
Oxidation Numbers

Oxidation numbers aka oxidation
states are assigned to the atoms
composing the compound or ion –
these are used in order to indicate
the general distribution of electrons
among the bonded atoms in a
molecular compound or a molecular
ion
Oxidation Numbers

Oxidation numbers do not have an
exact physical meaning. However,
they are useful in naming
compounds, writing formulas, and
balancing equations.
Assigning Oxidation Numbers


Shared electrons are assumed to
belong to the more-electronegative
atom in each bond.
Rules
1.
The atoms in a pure element have an
oxidation number of zero.
Assigning Oxidation Numbers
2.
3.
The more electronegative element in a
binary molecular compound is assigned
the number equal to the negative charge
it would have as an anion. The less
electronegative element is assigned the
number equal to the positive charge it
would have as a cation.
Fluorine has an oxidation number
of -1 in all of its compounds (most
electronegative element).
Assigning Oxidation Numbers
4.
Oxygen has an oxidation
number of -2 in almost all
compounds

Exceptions – in peroxides such as
H2O2 in which it is -1, and in
compounds with halogens such as
OF2 which it is +2
Assigning Oxidation Numbers
5.
6.
7.
Hydrogen - +1 in all compounds
containing elements that are more
electronegative than it; -1 in
compounds with metals.
The algebraic sum of the oxidation
numbers of all atoms in a neutral
compound is equal to zero
The algebraic sum of the oxidation
numbers of all atoms in a
polyatomic ion is equal to the
charge of the ion.
Assigning Oxidation Numbers
8.

Although rules 1-7 apply to
covalently bonded atoms, oxidation
numbers can also be assigned to
atoms in ionic compounds.
A monatomic ion has an oxidation
number equal to the charge of the
ion
How do we use oxidation
numbers?


Suppose you want to know the
formula of a binary comound
formed between sulfur and oxygen
Look at common oxidation states of
sulfur +4, +6 so maybe SO2 or SO3
(Remember oxidation numbers
alone cannot be used to predict the
existence of a comound)
How do we use oxidation
numbers?

Stock System Nomenclature
Formula
Prefix System
Stock System
SO2
Sulfur dioxide
Sulfur (IV)
oxide
SO3
Sulfur trioxide
Sulfur (VI)
oxide