Binary Molecular Compounds:

 

Molecular compounds are composed of combinations of nonmetals and / or metalloids.  Because all nonmetals need to increase the number of valence electrons, electrons are shared instead of being transferred as in ionic bonding.

When electrons are shared, they are being attracted to the nuclei of both atoms at the same time.  This creates a force that holds the atoms together known as a covalent bond.

An example of a molecule would be hydrogen gas (H2).  Each atom has one proton in the nucleus and one electron in the 1s orbital.  Hydrogen needs two electrons to become stable like helium.  The two electrons are ‘shared’ or attracted to both nuclei at the same time.

Nonmetals other than hydrogen need eight valence electrons to become stable.  This is known as the octet rule.

Because each group of elements has a common number of valence electrons they form bonds in the same way.  For example group 14 elements all have 4 valence electrons.  They need four additional electrons to obtain an octet.  Each covalent bond provides 1 additional electrons. Therefore, the group 14 elements form four bonds in molecules.  For example, carbon and hydrogen combine to make methane, CH4.  Each hydrogen atom provides 1 electron for the carbon.

image: Bonding in methane. Four hydrogen atoms each share one electron and a carbon atom shares four electrons.

XO represents a shared pair of electrons.  X is the electron from the hydrogen atoms, O is the electron from the carbon atoms.  The overlapping circle demonstrate the sharing, each electron is a part of both atoms.  Carbon has 8 and each hydrogen has 2.

 

Group 15 elements have 5 valence electrons and need 3 to have an octet.  Therefore, they need to form 3 bonds.  Nitrogen and hydrogen form the compound ammonia, NH3:

 

Group 16 elements have 6 valence electrons and need 2 to have an octet.  Therefore, they need to form 2 bonds.  Oxygen and hydrogen form the compound water, H2O:

image: Bonding in water. Two hydrogen atoms each share one electron and an oxygen atom shares two electrons.

Group 17 elements have 7 valence electrons and need 1 to have an octet.  Therefore, they need to form 1 bond.  Chlorine and hydrogen form the compound hydrochloric acid, HCl:

image: Bonding in hydrogen chloride. A hydrogen atom and a chlorine atom each share one electron.

To summarize:

Group

Number of valence electrons

Number of bonds required to achieve octet

14

4

4

15

5

3

16

6

2

17

7

1

 

Naming Molecular Compounds:

In ionic compounds, common charges were used to predict the ratio in which atoms would combine.  Because molecular compounds are not made of ions another method is needed to predict the ratio of elements.

A prefix system is used in the name to indicate the number of atoms of that element.

 

Prefix

Meaning

Mono

1

Di

2

tri

3

Tetra

4

Penta

5

Hexa

6

Hepta

7

Octa

8

Nona

9

deca

10

 

 

 

Example:  Dinitrogen pentoxide –N2O5         Carbon monoxide – CO

                 Carbon Dioxide – CO2                  Phosphorus trichloride – PCl3

 

Notice the second name always ends with ide suffix like it did with ionic compounds.

Oxygen is always written last.  If the compound does not contain oxygen, the leftmost element on the periodic table is written first.  When adding a prefix to oxygen, always drop the a/o off the prefix.  Mono is never added to the first name.