Molecular Compounds:
Formulas & Naming
Ch 6.3b - Covalent Bonds
What type of bond?
Covalent
– Electrons are shared between atoms
– Forms between non-metals
– Covalent Bonds  Molecular
Compounds
Which
elements are non-metals?
Describing Molecular
Compounds

What information do the name and formula
of a molecular compound provide?
 Ionic Bonds (Ionic Compounds) – The name distinguishes the compound from
other ionic compounds containing the same
elements.
– The formula of an ionic compound describes
the ratio of the ions in the compound.
Describing Molecular
Compounds

Covalent Bonds (Molecular
Compounds)
– The name and formula of a
molecular compound describe the
type and number of atoms in a
molecule of the compound.
Naming Molecular Compounds
Naming Order:
 Step 1: Element that is farthest to the left side of the
periodic table is typically named 1st. (left to right)
– The most metallic element appears first in the name.
These elements are farther to the left in the periodic
table.

If both elements are in the same group, the more metallic
element is closer to the bottom of the group.
– Formulas are written same way

Example:
–
Sulfur is named before chlorine  SCl2
Naming Molecular Compounds

Exception: Oxygen & Halogens
–
–

Oxygen comes 2nd when combined with lowerelectronegativity halogens (Group 17)
Example: ClO2 (electronegativity: Cl = 3.0; O = 3.5)
Step 2: Second element name is changed to end in
“ide” (like the anion in an ionic compound)
Examples:

–
–
Chlorine becomes chloride
Oxygen becomes oxide
Naming Molecular Compounds

There may be more than one molecular compound
that can exist with the same two elements.
–

Greek prefixes are used to name molecular compounds.
Step 3: Add prefixes to elements names to specify
the number of atoms of each element in the
compound.
 mono - one  hexa - six
 di - two
 hepta - seven
 tri - three
 octa - eight
 tetra - four  nona - nine
 penta - five  deca - ten
Naming Molecular Compounds
Prefixes:
• mono - one
• di - two
• tri - three
• tetra - four
• penta - five
Example:
Both Compounds contain
Nitrogen and Oxygen
 N2O 
 Dinitrogen monoxide
N4O5 
 Tetranitrogen pentoxide

Naming Molecular Compounds
 Exception
to Step 3– No prefix is used
if there is only one atom of the 1st
element in the compound.
 Carbon monoxide
 CO2  Carbon dioxide
 SO10  Sulfur decaoxide
 CO
Naming Molecular Compounds
More Examples:

– Dinitrogen
pentachloride
 N2Cl5

– Dinitrogen
pentoxide
 N2O5

– Pentaphosphorous
hexachloride
 P5Cl6

– Sulfur decaoxide
 SO10
Writing Molecular Formulas

Step 1: Write the symbols for the elements in
the order the elements appear in the name.
 Step 2: Write the numerical value of the prefixes
as subscripts for the corresponding element in
the formulas.
– The prefixes indicate the number of atoms of each
element in the molecule.
– If there is no prefix for an element in the name, there
is only one atom of that element in the molecule.
Writing Molecular Formulas
 Examples:
– Diphosphorus Pentoxide  P2O5
– Silcon Dioxide  SiO2
– Carbon Tetrachloride  CCl4
– Trineptunium Octoxide  Np3O8
You Try
 Give
the formula for:
– Oxygen Difluoride 
– Phosphorus Tribromide 
– Trisilicon Tetranitride 
 Give the Name
– NO2 
– N2O3 
– SO3 
for:
You Try - Answers
 Give
the formula for:
– Oxygen Difluoride  OF2
– Phosphorus Tribromide  PBr3
– Trisilicon Tetranitride  Si3N4
 Give
the Name for:
– NO2  Nitrogen Dioxide
– N2O3  Dinitrogen Trioxide
– SO3  Sulfur Trioxide