Cambridge International AS Level Chemistry
■
■
■
■
68
Electronegativity differences can be used to predict the type of weak intermolecular forces between molecules.
■
Hydrogen bonding occurs between molecules that have a hydrogen atom covalently bonded to an atom of a very electronegative element (fluorine, oxygen or nitrogen).
■
■
The reactivities of covalent bonds can be explained in terms of bond energy, bond length and bond polarity.
■
Metallic bonding can be explained in terms of a lattice of positive ions surrounded by mobile electrons. The physical properties of substances may be predicted from the type of bonding present. Substances with ionic bonding have high melting and boiling points, whereas simple molecules with covalent bonding have low melting points. The presence of hydrogen bonding in a molecule influences its melting point and boiling point.
Intermolecular forces are based on either permanent dipoles, as in CHCl3(l), or temporary induced dipoles (van der Waals’ forces), as in Br2(l).
End-of-chapter questions 1 The table shows the atomic number and boiling points of some noble gases. Gas
helium
neon
argon
krypton
xenon
Atomic number
–1 2
–110
–118
–136
–154
Boiling point / °C
–253
–246
–186
–152
–107
a Use ideas about forces between atoms to explain this trend in boiling points. b Xenon forms a number of covalently bonded compounds with fluorine. i What do you understand by the term covalent bond? ii Draw a dot-and-cross diagram for xenon tetrafluoride, XeF4. iii Suggest a shape for XeF4. Explain why you chose this shape. c The structure of xenon trioxide is shown below.
[2] [1] [1] [3]
Xe O
O
O
i By referring to electron pairs, explain why xenon trioxide has this shape. ii Draw the structure of xenon trioxide to show the partial charges on the atoms and the direction of the dipole in the molecule.
[2] [2] Total = 11