The Foundations of Chemistry

COVALENT BONDING

Ionic bonding cannot result from a reaction between two nonmetals, because their elec-

tronegativity difference is not great enough for electron transfer to take place. Instead,

reactions between two nonmetals result in covalent bonding.

A covalent bondis formed when two atoms share one or more pairs of electrons.

Covalent bonding occurs when the electronegativity difference, (EN), between

elements (atoms) is zero or relatively small.

Figure 7-2 A schematic
representation of the energy changes
that accompany the process
Na(g)Cl(g) nNaCl(s).
The red arrow represents the positive
energy change (unfavorable) for the
process of ion formation,
Na(g)Cl(g) nNa(g)Cl(g).
The blue arrow represents the
negativeenergy change (favorable)
for the overall process, including the
formation of the ionic solid.

278 CHAPTER 7: Chemical Bonding

(more stable) than the original mixture of atoms (the blue arrow in Figure 7-2). Thus, we

see that a major driving force for the formation of ionic compounds is the large electro-

static stabilization due to the attraction of the ionic charges (step 3).

In this discussion we have not taken into account the fact that sodium is a solid metal

or that chlorine actually exists as diatomic molecules. The additional energy changes

involved when these are changed to gaseous Na and Cl atoms, respectively, are small enough

that the overall energy change starting from Na(s) and Cl 2 (g) is still negative.

Na(g)  e  Cl(g)

Na(g)  Cl(g)

Increasing potential energy (less stable)

Na(g)  Cl(g)

EA of Cl  349 kJ/mol

Net energy change for

Na(g)  Cl(g) Na(g)  Cl(g)

 147 kJ/mol

Net energy change for

 642 kJ/mol

Na(g)  Cl(g) NaCl(s)

1st IE of Na

 496 kJ/mol

Crystal lattice

energy of NaCl

789 kJ/mol

NaCl(s)

1

2

3

(Enrichment, continued)