Prelims Inorganic Chemistry
St Anne’s and Oriel Colleges
HT 2017
Bonding in molecules
Suggested reading:
• Structure and Bonding, Barrett
• Introduction to Molecular Symmetry, Ogden (Primer)
• Chemical Bonding, Winter (Primer)
• Chemical Structure and Bonding, DeKock & Gray
Questions:
(i) Give an overview of the VSEPR approach for understanding molecular geometries, using the
following compounds (and any others you might wish to consider) to illustrate your answer:
BH3 , NH3 , SF4 , IF7 , XeF4 . Are there any known exceptions to the VSEPR rules?
(ii) Define and discuss the different bonding descriptions for N2 given by (a) Lewis structures, (b)
valence bond theory, and (c) molecular orbital theory.
(iii) Consider the general case of a molecule AX formed between a group 1 element A and a halogen
element X. Draw a simplified MO diagram to describe bonding in the AX molecule, using an
appropriate sp hybrid as the frontier orbital for the X atom. Explain the effect of increasing
electronegativity difference between A and X on the form of your MO diagram and on the
nature of the bonding and antibonding MOs. With reference to your answer, explain how the
MO picture for bonding is consistent with conventional views of covalency and ionicity.
(iv) The valence shell electronic configuration of oxygen, together with its high electronegativity,
mean that O atoms often behave as excellent electron pair acceptors. In this context, an
‘electron pair’ may correspond either to a lone pair or to a bonding pair of electrons. Show
that this interpretation of oxygen bonding is consistent with the molecular structures of the
series of compounds P4 , P4 O6 , P4 O10 . Use this way of thinking to rationalise the structures
−
2−
of the following species: O3 , SO2−
4 , S2 O3 (thiosulfate), N2 O4 , ClO4 XeO4 .
(v) Draw a molecular orbital diagram for C2 , labelling and sketching each MO. The order of the
2σg and 1πu MOs is different in C2 and O2 : can you think of how one might tell that this is
the case from physical measurements of the two species? Determine the expected trends in
2−
bond order and bond length for the series C2 , C−
2 , C2 .
(vi) How many independent linear combinations are possible for four 1s orbitals? Draw pictures of
the linear combinations of H1s orbitals for a hypothetical linear H4 molecule, arranging these
in order of increasing energy. How do you think this picture would change if the four atoms
were arranged on the vertices of a square? Or a tetrahedron?
(vii) Using MO diagrams as appropriate, rationalise the following trend in H–E–H angles for EH3
compounds (E ∈ group 15): NH3 , 106◦ ; PH3 , 94◦ ; AsH3 , 92◦ .
Prelims Inorganic Chemistry
St Anne’s and Oriel Colleges
HT 2017
(viii) For each of the following compounds, assign the corresponding point group and illustrate each
of the symmetry elements present.
SO3
BrF5
H2 O
S8
P4
(ix) Compare the MO diagrams for N2 and CO. On the basis of your answer, predict the direction
of the dipole moment in CO and rationalise (to whatever level you feel confident) the preference
for M–CO or M–OC isomers in its complexes with transition metals.
(x) Compare and contrast the bonding in XeF2 with the bonding in the bridging B–H–B groups
in B2 H6 .
quantum number n and the effective nuclear charge Zeff.
(b)
[3]
Comment on the following ionisation energies (eV) for some group I and
group II elements. The ionization energy of hydrogen is 13.6 eV.
Li
5.39
Prelims Inorganic Chemistry
St Anne’s and Oriel Na
Colleges5.14
K
4.34
Be
9.32
Mg
7.64
Ca
6.12
HT 2017
[7]
Past paper questions (2014 Prelims Resit Examination):
2.
Answer all parts (a) to (c).
(a)
Predict the structures of the following four species using the valence shell
electron-pair repulsion (VSEPR) approach:
SF5−
(b)
(c)
SO32−
For two of the species you have drawn above identify on your sketch: (i)
the highest order proper rotation axis, and (ii) one additional symmetry
element.
[2]
Which of the following species has (or have) D2h symmetry ?
cis-[PtCl2Br2]
9.
SF3+
SF4
[4]
[Fe(CN)6]4−
trans-[PtCl2Br2]
B2H6
[4]
Answer all parts (a) to (c).
(a)
(b)
Sketch molecular orbital diagrams for N2 and F2. Depict the form of
the molecular orbitals in terms of linear combination of atomic
orbitals and provide appropriate symmetry labels for them.
Comment on the following bond energies and bond lengths.
Bond energy / kJ mol−1
CCHE 4271 Bond length / pm
N2
942
2 110
O2
494
121
F2
159
142
Bond energy / kJ mol−1
Bond length / pm
P2
485
189
S2
422
189
Cl2
241
199
Bond energy / kJ mol−1
Bond length / pm
As2
382
210
Se2
367
217
Br2
190
228
(c)
How would you expect the bond energies in N2+, P2+, F2+ and
Cl2+ to compare with those in the neutral molecules.
END OF PAPER
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