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CHAPTER EIGHT
BONDING: GENERAL CONCEPTS
For Review
- Electronegativity is the ability of an atom in a molecule to attract electrons to itself. Electronegativity is a bonding term. Electron affinity is the energy change when an electron is added to a substance. Electron affinity deals with isolated atoms in the gas phase.
A covalent bond is a sharing of electron pair(s) in a bond between two atoms. An ionic bond is a complete transfer of electrons from one atom to another to form ions. The electrostatic attraction of the oppositely charged ions is the ionic bond.
A pure covalent bond is an equal sharing of shared electron pair(s) in a bond. A polar covalent bond is an unequal sharing.
Ionic bonds form when there is a large difference in electronegativity between the two atoms bonding together. This usually occurs when a metal with a small electronegativity is bonded to a nonmetal having a large electronegativity. A pure covalent bond forms between atoms having identical or nearly identical eletronegativities. A polar covalent bond forms when there is an intermediate electronegativity difference. In general, nonmetals bond together by forming covalent bonds, either pure covalent or polar covalent.
Ionic bonds form due to the strong electrostatic attraction between two oppositely charged ions. Covalent bonds form because the shared electrons in the bond are attracted to two different nuclei, unlike the isolated atoms where electrons are only attracted to one nuclei. The attraction to another nuclei overrides the added electron-electron repulsions.
- Anions are larger than the neutral atom and cations are smaller than the neutral atom. For anions, the added electrons increase the electron-electron repulsions. To counteract this, the size of the electron cloud increases, placing the electrons further apart from one another. For cations, as electrons are removed, there are fewer electron-electron repulsions and the electron cloud can be pulled closer to the nucleus.
Isoelectronic: same number of electrons. Two variables, the number of protons and the number of electrons, determine the size of an ion. Keeping the number of electrons constant, we only have to consider the number of protons to predict trends in size. The ion with the most protons attracts the same number of electrons most strongly resulting in a smaller size.
- Lattice energy: the change in energy that takes place when separated gaseous ions are packed together to form an ionic solid. The reason ionic compounds form is the extremely favorable lattice energy value (large and negative). Looking at Figure 8.11, there are many processes that occur when forming an ionic compound from the elements in their standard state. Most of these processes (if not all) are unfavorable (endothermic). However, the large, exothermic lattice energy value dominates and the ionic compound forms.
The lattice energy follows Coulomb’s law (E%Q 1 Q 2 /r). Because MgO has ions with +2 and !2 charges, it will have a more favorable lattice energy than NaF where the charge on the ions are !1 and +1. The reason MgO has +2 and !2 charged ions and not +1 and !1 charged ions is that lattice energy is more favorable as the charges increase. However, there is a limit to the magnitude of the charges. To form Mg3+O^3 −, the ionization energy would be extremely unfavorable for Mg2+^ since an inner core ( n = 2) electron is being removed. The same is true for the electron affinity of O^2 −; it would be very unfavorable as the added electron goes into the n = 3 level. The lattice energy would certainly be more favorable for Mg3+O^3 −, but the unfavorable ionization energy and electron affinity would dominate making Mg3+O^3 − energetically unfavorable overall. In general, ionic compounds want large charges, but only up to the point where valence electrons are removed or added. When we go beyond the valence shell, the energies become very unfavorable.
- When reactants are converted into products, reactant bonds are broken and product bonds are formed. Thus, ∆H for a reaction should be the energy it takes to break the reactant bonds minus the energy released when bonds are formed. Bond energies give good estimates for gas phase reactions, but give poor estimates when solids or liquids are present. This is because bond energy calculations ignore the attractive forces holding solids and liquids together. Gases have the molecules very far apart and they have minimal (assumed zero) attractive forces. This is not true for solids and liquids where the molecules are very close together. Attractive forces in substances are discussed in Chapter 10.
For an exothermic reaction, stronger bonds are formed in the products as compared to the strength of the bonds broken in the reactants so energy is released. For endothermic reactions, the product bonds are weaker overall and energy must be absorbed.
As the number of bonds increase, bond strength increases and bond length decreases.
- Nonmetals, which form covalent bonds, have valence electrons in the s and p orbitals. Since there are 4 total s and p orbitals, there is room for only 8 valence electrons (the octet rule). The valence shell for hydrogen is just the 1s orbital. This orbital can hold 2 electrons, so hydrogen follows the duet rule.
Drawing Lewis structures is mostly trial and error. The first step is to sum the valence electrons available. Next, attach the bonded atoms with a single bond. This is called the skeletal structure. In general, the atom listed first in a compound is called the central atom; all other atoms listed after the first atom are attached (bonded) to this central atom. If the skeletal structure is something different, we will generally give you hints to determine how the atoms are attached. The final step in drawing Lewis structures is to arrange the remaining electrons around the various atoms to satisfy the octet rule for all atoms (duet role for H).
Be and B are the usual examples for molecules that have fewer than 8 electrons. BeH 2 and BH 3 only have 4 and 6 total valence electrons, respectively; it is impossible to satisfy the octet rule for BeH 2 and BH 3 because fewer than 8 electrons are present.
All row three and heavier nonmetals can have more than 8 electrons around them, but only if they have to. Always satisfy the octet rule when you can; exceptions to the octet rule occur when there are no other options. Of the molecules listed in review question 10, KrF 2 , IF 3 , SF 4 ,
Number of bonded atoms plus lone pairs about a central atom Geometry Bond Angle(s)
2 linear 180 Ε 3 trigonal planar 120 Ε 4 tetrahedral 109.5Ε 5 trigonal bipyramid 90 Ε, 120Ε 6 octahedral 90 Ε
To discuss deviations from the predicted VSEPR bond angles, let us examine CH 4 , NH 3 , and H 2 O. CH 4 has the true 109.5Ε bond angles, but NH 3 (107.3Ε) and H 2 O (104.5Ε) do not. CH 4 does not have any lone pairs of electrons about the central atom, while H 2 O and NH 3 do. These lone pair electrons require more room than bonding electrons, which tends to compress the angles between the bonding pairs. The bond angle for H 2 O is the smallest because oxygen has two lone pairs on the central atom; the bond angle is compressed more than in NH 3 where N has only one lone pair. So, in general, lone pairs compress the bond angles to a value slightly smaller than predicted by VSEPR.
- The two general requirements for a polar molecule are:
- polar bonds
- a structure such that the bond dipoles of the polar bonds do not cancel.
CF 4 , 4 + 4(7) = 32 valence electrons XeF 4 , 8 + 4(7) = 36 e−
C
F
F F F
F
Xe
F F
F
tetrahedral, 109.5Ε square planar, 90Ε
SF 4 , 6 + 4(7) = 34 e−
≈ 90 Ε
S
F
F F
F
see-saw, ≈ 90 Ε, ≈ 120 Ε
The arrows indicate the individual bond dipoles in the three molecules (the arrows point to the more electronegative atom in the bond which will be the partial negative end of the bond
dipole). All three of these molecules have polar bonds. To determine the polarity of the overall molecule, we sum the effect of all of the individual bond dipoles. In CF 4 , the fluorines are symmetrically arranged about the central carbon atom. The net result is for all of the individual C−F bond dipoles to cancel each other out giving a nonpolar molecule. In XeF 4 , the 4 Xe−F bond dipoles are also symmetrically arranged and XeF 4 is also nonpolar. The individual bond dipoles cancel out when summed together. In SF 4 , we also have 4 polar bonds. But in SF 4 , the bond dipoles are not symmetrically arranged and they do not cancel each other out. SF 4 is polar. It is the positioning of the lone pair that disrupts the symmetry in SF 4.
CO 2 , 4 + 2(6) = 16 e−^ COS, 4 + 6 + 6 = 16 e−
O C O^ S^ C^ O
CO 2 is nonpolar because the individual bond dipoles cancel each other out, but COS is polar. By replacing an O with a less electronegative S atom, the molecule is not symmetric any more. The individual bond dipoles do not cancel since the C−S bond dipole is smaller than the C−O bond dipole resulting in a polar molecule.
- To predict polarity, draw in the individual bond dipoles, then sum up the net effect of the bond dipoles on each other. If the net effect is to have the bond dipoles cancel each other out, then the molecule is nonpolar. If the net effect of the bond dipoles is to not cancel each other out, then the molecule will have a partial positive end and a partial negative end (the molecule is polar). This is called a dipole moment or a polar molecule.
CO 2 , 4 + 2(6) = 16 valence electrons SO 2 , 6 + 2(6) = 18 e−
S O O
S
O C O O O
linear, 180Ε, nonpolar V-shaped, ≈ 120 Ε, polar
KrF 2 , 8 + 2(7) = 22 e−^ SO 3 , 6 + 3(6) = 24 e−
- 2 other resonance structures
linear, 180Ε, nonpolar trigonal planar, 120Ε, nonpolar
S
O O
O F Kr F
