Group Members:
In Class Exercise #7
How Many Bonds to an Atom?
adapted from Chemistry. A Guided Inquiry, Preliminary Edition, Richard S. Moog and John. J.
Farrell, Wiley and Sons: New York, 1996
The valence shell depictions of some atoms are given below:
A general property of the Inert or Noble gases (He, Ne, Ar, Kr, Xe) is that they are
not very reactive with themselves or other elements.
Lewis structures are representations of the sharing of electrons to form covalent
bonds. In general, atoms try form bonds such that each atom achieves a "noble gas
configuration", i. e., the same valence shell as the noble gas from the same row of
the periodic table. For example, to complete its valence shell Oxygen must form
bonds with two H atoms.
O
+ 2 H
O
H
H
A chemist would say that water has two bonds and that the oxygen atom has two
lone pairs.
1) Why do you think that Lewis proposed a maximum of two electrons for
hydrogen and a maximum of eight for carbon, nitrogen, oxygen, and fluorine?
2) Consider the Nitrogen atom.
a)How many valence electrons does N have?
b) How many of the N electrons are not in the valence shell?
c) What is the Lewis representation of N?
d) How many bonds to H will N make? Draw the Lewis Dot structure of the
most likely compound that can be made from H and N. What is the number
of lone pairs?
2
e) How many bonds are made between the N atoms of N2? Draw the Lewis Dot
structure below. What is the number of lone pairs?
3) Now consider Arsenic (As).
a) How many valence electrons does As have?
b) What is the Lewis representation for As?
c) How many of the As electrons are not in the valence shell?
d) What is the likely formula for a molecule composed of H atoms and one As
atom? Draw the Lewis structure below. What is the number of lone pairs?
4)Write Lewis structures for the following. Note whether the molecule can achieve
an octet or not.
3
a) F2
b) O2
c) Al2
d) LiH
Bond Order and Bond Length
A single bond is defined as the sharing of two electrons and has bond order equal
to 1.0. The sharing of four electrons defines a double bond which has a bond
order of 2.0. In a triple bond, six electrons are shared and the bond order is 3.0.
The bond length is defined as the distance between two nuclei of bonded atoms.
1Å = 10-10m. The lengths of some C-C bonds are shown below:
Bond Length
H
H C
H
C
H
H
H
1.52Å
H
H
C
1.34Å
C
H
H
4
H
C
C
H
1.20Å
1) On the next page make a graph of bond length vs. bond order for C-C bonds.
Make sure that you label the axes.
a) Do the points on your graph describe a line or a curve?
b) In graphite and buckyballs the observed bond length is around 1.4Å whereas the
distance is 1.54Å for diamond. Estimate the bond orders for buckyball,
diamond, and graphite.
5
c) Which of the following C-N bonds is longer? HCN or H3CN. Draw the Lewis
structures and explain.
d) Draw two plausible Lewis structures for the greenhouse gas CO2. Each
structure should have carbon bonded to each oxygen atom and should not have
an O-O bond.
e) How would the measured C-O bond lengths differ for the two descriptions? Use
complete sentences.
f) Draw a valid Lewis structure for the poisonous gas CO.
6
g) The molecule C6H6 (benzene) forms a six-membered ring of C atoms with one
H attached to each C. Draw a Lewis structure for this compound. Do you expect
all C-C bonds to be the same length or not? Explain.
7