Unit 4.4 Inorganic Binary Covalent Bonding Type III
Why do so many relatives in royalty share the same name? Some families
name a son (usually the firstborn) after his father. So it is somewhat
common to find John Smith, Jr. named after John Smith the father. A
few families may take it further and name the grandson John Smith
III. Countries with long histories of royalty take the naming even
further. One line of kings named Henry goes up to Henry the Eighth (not
the nicest guy in the world – he had six wives and two of them came to
untimely ends). The use of numbering for names adds clarity to a system –
we always know which Henry we are talking about.
Inorganic chemical compounds can be broadly classified into two groups: ionic compounds and molecular –
covalent compounds. The structure of all ionic compounds is an extended three-dimensional array of
alternating positive and negative ions. Since ionic compounds do not take the form of individual
molecules, they are represented by empirical formulas. Now we will begin to examine the formulas and
nomenclature of molecular covalent compounds.
Molecular Covalent Compounds
Molecular covalent compounds are inorganic compounds that take the form of
discrete molecules. Examples include such familiar substance as water (H 2O) and carbon dioxide
(CO2). These compounds are very different from ionic compounds like sodium chloride (NaCl). Ionic
compounds are formed when metal atoms lose one or more of their electrons to nonmetal atoms. The
resulting cations and anions are electrostatically attracted to each other.
So what holds the atoms of a molecule together? Rather
than forming ions, the atoms of a molecule share
their valence electrons (outer energy level electrons) in
such a way that a bond forms between pairs of atoms. In a
carbon dioxide molecule, there are two of these bonds, each
occurring between the carbon atom and one of the two
oxygen atoms. In the molecular diagram, carbon is shown as
the black central atom, with red oxygens on each side.
Larger molecules can have many, many bonds that serve to keep the molecule together. In a large sample
of a given molecular compound, all of the individual molecules are identical.
Naming Binary Molecular Compounds
Recall that a molecular formula shows the number of atoms of each
element that a molecule contains. A molecule of water contains two
hydrogen atoms and one oxygen atom, so its formula is H2O. A molecule
of octane, which is a component of gasoline, contains 8 atoms of carbon
and 18 atoms of hydrogen. The molecular formula of octane is C8H18.
Nitrogen dioxide (NO2) is a reddish-brown toxic gas that is a
prominent air pollutant produced by internal combustion engines. The
bottle shown to the right is filled with nitrogen dioxide.
A binary molecular covalent compound is a molecular compound that is composed of two elements. The
elements that combine to form binary molecular compounds are both nonmetal atoms. This contrasts
with ionic compounds, which were formed from a metal ion and a nonmetal ion. Therefore, binary
molecular compounds are different because ionic charges cannot be used to name them or to write their
formulas. Another difference is that two nonmetal atoms will frequently combine with one another in a
variety of ratios. Consider the elements nitrogen and oxygen. They combine to make several compounds
including NO, NO2, and N2O. They all can’t be called nitrogen oxide. How would someone know which one
you were talking about? Each of the three compounds has very different properties and reactivity. A
system to distinguish between compounds such as these is necessary.
Prefixes are used in the names of binary molecular compounds to
Numerical Prefixes
identify the number of atoms of each element. The chart shows the
prefixes up to ten. The rules for using the prefix system of
Number of
Atoms
Prefix
1
mono-
2
di-
formula and hydrogen is after nitrogen in a formula such as NH 3. The
3
tri-
order of common nonmetals in binary compound formulas is C, P, N, H, S,
4
tetra-
I, Br, Cl, O, F.
5
penta-
6
hexa-
7
hepta-
8
octa-
9
nona-
10
deca-
nomenclature of binary molecular compounds can be summarized as
follows.
1. Generally, the less-electronegative element is written first in the
formula, though there are a few exceptions. Carbon is always first in a
2. When naming, the appropriate prefix is used for the first element
only if there are more than one atom of that element in the formula.
3. The second element is named after the first, but with the ending of
the element’s name changed to –ide. The appropriate prefix is always
used for the second element.
4. The a or o at the end of a prefix is usually dropped from the name
when the name of the element begins with a vowel. As an example, four
oxygen atoms is tetroxide instead of tetraoxide.
Formula
Name
Some examples of naming are shown in the chart.
NO
nitrogen monoxide
Notice that the mono- prefix is not used with the
N2O
dinitrogen monoxide
S2Cl2
disulfur dichloride
Cl2O7
dichlorine heptoxide
nitrogen in the first compound, but is used with the
oxygen in both of the first two examples. The
S2Cl2 emphasizes that the formulas for molecular
compounds are not reduced to their lowest
ratios. The o of mono- and the a of hepta- are dropped
from the name when paired with oxide.
Summary



Molecular covalent compounds are inorganic compounds that take the form of discrete molecules.
The atoms of these compounds are held together by covalent bonds, the sharing of electrons.
Prefixes are used to indicate the number of atoms of an element that are in the compound.
Review
1.
What are molecular covalent compounds?
2.
How do they differ from ionic compounds?
3.
List the prefixes for the number of atoms of an element when there are four atoms of that
element in the compound.
4.
List the prefixes for the number of atoms of an element when there are seven atoms of that
element in the compound.
5.
Name the following compounds:
a.
ClF3
b.
As2O5
c.
B4H10
Review
1.
Molecular covalent compounds are inorganic compounds that take the form of discrete molecules.
2.
They are not reduced to the simplest numbers, they share electrons, they are made of
nonmetals.
3.
Four atoms of the same element uses the prefix tetra.-.
4.
Seven atoms of the same element uses the prefix hepta-.
5.
Name the following compounds:
a.
Chlorine trifluoride
ClF3
b.
Diarsenic pentoxide
As2O5
c.
Tetraboron decahydride
B4H10