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Chemical Bonding Remedial 2_ Factors affecting the strength of

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					Chemical Bonding Remedial 2: Factors affecting the strength of different types of bonds

   Structure                   Bonding                                       Factors                                How they affect strength of bonding
                                                          charge density of metallic cation
                                                                                                        higher charge density (i.e. smaller cationic size and higher
                 strong metallic bonds between             charge density of ion  charge of ion        charge on cation), stronger the metallic bond
Giant Metallic   metallic cations and freely mobile                                size of ion
                 valence electrons                                                                      greater the number of freely mobile valence electrons,
                                                          no. of delocalised valence electrons
                                                                                                        stronger the metallic bond

                                                          charge on positive and negative ions          higher charge on ions, stronger ionic bonds
                 strong ionic bonds between positive
Giant Ionic
                 and negative ions                        size (ionic radii) of positive and negative
                                                                                                        smaller size (ionic radii) of ions, stronger ionic bonds
                                                          ions
                 strong covalent bonds between atoms      size of atoms involved in the covalent        smaller the size of atoms, more effective the overlap of the
Giant Covalent
                                                          bonding                                       orbitals and stronger the covalent bonds
                 strong covalent bonds within
                 molecules (NOT broken in melting or
                 boiling)
                                                                                                        higher average no. of H-bonds per molecule (i.e. greater no.
                                                          no. of hydrogen atoms per molecule
                                                                                                        of H atoms and greater no. of lone pairs available), stronger
                                                          no. of available lone pairs on X
                                   hydrogen bonding                                                     the H-bonding

                                                          electronegativity of X                        more electronegative the X, the stronger the H-bonding
Simple
                 weak                                     magnitude of net dipole moment (difference    greater the magnitude of net dipole moment (greater
Molecular                          permanent-dipole
                 intermolecular                           in electronegativity between atoms in the     difference in electronegativity between atoms), stronger pd-
                 forces between    (pd-pd) interactions
                                                          molecule)                                     pd interactions – for similar size molecules
                 molecules
                                                                                                        greater size of molecule, higher no. of electrons, greater the
                                   induced-dipole (id-    size of molecule (i.e. Mr)                    electron cloud which is more easily distorted, hence larger
                                   id) or van der                                                       induced dipole and stronger vdw forces
                                   Waals (vdw)
                                   interactions                                                         spherical shape / more branching, lower the surface area of
                                                          shape of molecule (assume same size)
                                                                                                        contact and hence weaker vdw forces
Exercise:

Use the summary in the table to identify the substance with the higher melting point:

Si vs C
HCl vs HI
F2 vs Cl2
K vs Ca
MgO vs NaF
CH3OH vs CH3CH2OH
(CH3)2CHCH3 vs CH3CH2CH2CH3
NaCl vs NaF
H2O vs NH3
NaCl vs KI
MgCl2 vs NaCl

				
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posted:1/10/2011
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