Molecules | AP Chemistry
Bonds
• Fluorine is one electron deficient
from filling its valence
(outermost) electron shell
Electron Distribution of Neon
•
Fluorine
Each fluorine atom is one electron short of filling
its valence shell.
•
If each fluorine shares an
electron with the other, each will
achieve a filled valence shell
o This type of bond is a
covalent bond
F2 Covalent Bond
Each fluorine shares an electron with the other,
giving rise to a covalent bond, and allowing each
fluorine to achieve stability.
•
•
From our previous illustration of
F2, notice that the covalent bond
allows both fluorine atoms to
achieve the same electron
configuration as neon
o 2 electrons in the 1st shell
o 8 electrons in the 2nd shell
So, the covalent bond allows
fluorine to achieve the stability
exhibited by neon
Bond Length
• The length of a covalent bond is
determined by two factors:
o The repulsion between he
positive nuclei of the two
atoms
o The attraction between
each nucleus and the
shared electrons
This explains why fluorine
naturally exists diatomically
Octets
• We’ve seen that atoms (like
fluorine) are stable with a
valence electron count of 8
• To understand why, consider the
fact that fluorine sits besides
neon on the periodic table
• Neon is a noble gas, meaning it’s
extremely stable
o It has 10 electrons,
arranged in shells like
this:
Determining Bond Length
The length of the bond is determined by (1) the
repulsion between the 2 fluorine nuclei (red
arrows), and (2) the attraction between each
nucleus and the shared electrons (blue arrows).
•
Together, these attractions and
repulsions compromise at a
lowest-energy bond length
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Molecules | AP Chemistry
Possible Bond Lengths of H2
At very short and very long bond lengths, the H2
bond is relatively high in energy. The dip in the
graph represents the point where attractive and
repulsive forces yield the lowest-possible energy.
That will be the observed bond length.
•
When bonds form, the energy of
the participating atoms is reduced
o So breaking the bond will
require energy
That sum of
energy is called
the bond energy
Molecules
• Molecules form when two or
more atoms bond
o If the molecule contains
at least 2 different
elements, it’s a compound
• A molecule may be denoted
according to its molecular
formula or empirical formula
• Glucose has 6 carbons, 12
hydrogens, and 6 oxygens
o Its molecular formula is
C6H12O6
o Its empirical formula
consists of the ratio in
which those elements
appear- CH2O
Finding Percent Composition
• You may be asked to find the
percent composition of an
element, like carbon, in glucose
•
•
o And you may be tempted
to conclude that there are
6 carbons out of 24 total
atoms in glucose, and so
carbon occupies 25% of
the molecule
This is incorrect!
C, H, and O have
very different
masses, and that
must be
considered
Let’s assume we have 1 mole of
glucose
o Each mole of glucose has:
6 mol carbon
12 mol hydrogen
6 mol oxygen
Multiplying those mol counts by
each elements molar mass, we
can ascertain the grams of each
element in a mole of glucose
Mass Composition of Glucose
Multiplying the moles of each element by their
molar masses, we ascertain the mass of each
element in a mole of glucose.
•
•
•
•
Summing these masses, we get
the total mass of a mole of
glucose: 180g
The percent composition of
carbon can now be found:
o 72g / 180g = 40%
Hydrogen’s mass composition
would equal 6.6%
Oxygen’s mass composition
would equal 53.3%
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© 2017 J Co Review, Inc., Accessed by Guest on 06-16-2017
Molecules | AP Chemistry
Finding the Empirical Formula
• Given percent compositions, you
may be asked to find the
empirical formula
• Suppose you are given the
percent compositions:
o 40% carbon
o 6.6% hydrogen
o 53.3% oxygen
• First, assume you have 100g of
our mystery molecule
o Of those 100g, there will
be:
40g carbon
6.6g hydrogen
53.3g oxygen
• Next, convert those masses to
moles by multiplying by the
inverse of each element’s molar
mass
Conversion to Moles
Multiplying the masses of each element by the
inverse of their molar masses, we ascertain the
moles of each element in 100g of glucose.
•
•
Carbon, hydrogen, and oxygen
appear in the ratio of 1:2:1
o This tells us the empirical
formula: CH2O
This information is insufficient,
however, to produce a molecular
formula
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© 2017 J Co Review, Inc., Accessed by Guest on 06-16-2017