116
Sizes and Shapes of Molecules
ELECTRONEGATIVITY
Unfortunately, as in so many sharing processes, the pair of electrons m a
covalent bond often is not shared equally by the two atoms The atom with the
greater electron affinity will hold the pair closer to its nucleus, with the result
that its end of the bond (and its end of the molecule) will be somewhat more
negatively charged than the other end When this happens, we say that the bond
is partially ionic and, because opposite charges attract each other, this partially
ionic bond will be stronger than it would have been with equal sharing
Linus Pauling made a careful study of a tremendous number of partially ionic
covalent bonds and came to the conclusion that, as a measure of its electron
affinity, each element could be assigned anelectmnegativity value (e) that would
make it possible to estimate the energy of these bonds To calculate the approximate bond energy (A// A -e) between the atoms A and B, his formula requires
knowledge of the A-A bond energy (A// A J, the B-B bond energy (A//B-B),
and the electronegativities of A and B (e A and e B ) Table 9-1 lists some electronegativity values along with covalent bond energies Pauling's finding, which
we might call generalization #3, is the following
3 The energy of a partially ionic covalent bond between two atoms (A
and B) is equal to the energy expected from a strictly covalent bond
between the two atoms plus an amount of energy related to the square
of the difference in their electronegativities In equation form, Pauling's formula (with units in kcal) is
A//, B = i[A// A _, + A//B-B] + 23 06(e, - e B )
2
(9-2)
You can see that, if A and B have equal electronegativities, then A// A B is
simply the average of the two covalent bond energies The greater the difference in electronegativites, the greater the percentage of ionic character and the
stronger the bond If the difference in electronegativity becomes too great, the
bond is essentially ionic, and the atoms are held together by electrostatic forces
as in an ionic crystal, the concept of a molecule disappears
From this brief discussion you can see that more often than not most covalent
bonds will be partially ionic, ' and most ionic bonds will be partially covalent " We shall describe a bond as being ionic or covalent according to its
predominant characteristic Some chemists like to say that a bond possesses a
certain percent ionic character One way of calculating an apptoximate value
for this quantity is with the expression
logF< = -(^-=-^)'
(9-3)
where F c is the fraction ofco\aknt character The fraction of ionic character is
F = I - F c
Sizes and Shapes of Molecules
ELECTRONEGATIVITY
Unfortunately, as in so many sharing processes, the pair of electrons m a
covalent bond often is not shared equally by the two atoms The atom with the
greater electron affinity will hold the pair closer to its nucleus, with the result
that its end of the bond (and its end of the molecule) will be somewhat more
negatively charged than the other end When this happens, we say that the bond
is partially ionic and, because opposite charges attract each other, this partially
ionic bond will be stronger than it would have been with equal sharing
Linus Pauling made a careful study of a tremendous number of partially ionic
covalent bonds and came to the conclusion that, as a measure of its electron
affinity, each element could be assigned anelectmnegativity value (e) that would
make it possible to estimate the energy of these bonds To calculate the approximate bond energy (A// A -e) between the atoms A and B, his formula requires
knowledge of the A-A bond energy (A// A J, the B-B bond energy (A//B-B),
and the electronegativities of A and B (e A and e B ) Table 9-1 lists some electronegativity values along with covalent bond energies Pauling's finding, which
we might call generalization #3, is the following
3 The energy of a partially ionic covalent bond between two atoms (A
and B) is equal to the energy expected from a strictly covalent bond
between the two atoms plus an amount of energy related to the square
of the difference in their electronegativities In equation form, Pauling's formula (with units in kcal) is
A//, B = i[A// A _, + A//B-B] + 23 06(e, - e B )
2
(9-2)
You can see that, if A and B have equal electronegativities, then A// A B is
simply the average of the two covalent bond energies The greater the difference in electronegativites, the greater the percentage of ionic character and the
stronger the bond If the difference in electronegativity becomes too great, the
bond is essentially ionic, and the atoms are held together by electrostatic forces
as in an ionic crystal, the concept of a molecule disappears
From this brief discussion you can see that more often than not most covalent
bonds will be partially ionic, ' and most ionic bonds will be partially covalent " We shall describe a bond as being ionic or covalent according to its
predominant characteristic Some chemists like to say that a bond possesses a
certain percent ionic character One way of calculating an apptoximate value
for this quantity is with the expression
logF< = -(^-=-^)'
(9-3)
where F c is the fraction ofco\aknt character The fraction of ionic character is
F = I - F c