Chemistry Reference
In-Depth Information
hydrogen atoms were to bond with another one of the hydrogen atoms, they
wouldn't have any bonding electrons left to attach to a carbon atom. There-
fore, each hydrogen atom must use its one bonding electron to connect to a
carbon atom, where there is an available bonding site. The following Ex-
ample 1 summary shows the completed structural formula for propene:
A.
Molecular formula: “prop-” = 3 carbon atoms, or C
3
“-ene” means the # of hydrogen atoms = 2n = 2 × 3 = 6 or H
6
so, the molecular formula for propene is C
3
H
6
B.
Structural formula for propene:
H
HCCCH
HHH
This may seem confusing at first, so try as many examples as you can.
Always pay attention to the valence configurations of the elements in or-
der to figure out how many bonds each can make. Let's try another ex-
ample. I show the work for the molecular formula already completed, but
I will go over the structural formula more slowly.
Example 2
Write the molecular formula and structural formula for a molecule
of ethyne.
A.
Molecular formula: “eth-” = 2 carbon atoms, or C
2
“-yne” means # of hydrogen atoms = 2n - 2 = (2 × 2) - 2
= 2 or H
2
so, the molecular formula for ethyne is C
2
H
2
I hope that you find writing the molecular formula easy. It is as easy as
looking up what the prefix means, then calculating the number of carbon
atoms and then calculating the number of hydrogen atoms from the suffix.
For the structural formula, we will start by laying down the two carbon
atoms and connecting them with a triple bond. How do we know that the
carbon atoms are joined by a triple bond? Remember: The suffix “-yne” in-
dicates the alkyne series, which contain a triple bond. The triple bond can't
be between a carbon and hydrogen atom, because hydrogen atoms only have
