Basic Concepts of Chemical Bonding

Ionic Bonding


Ionic Bonding

Sodium metal reacts with chlorine gas in a violently exothermic reaction to produce NaCl (composed of Na+ and Cl- ions):

2Na(s) + Cl2(g) -> 2NaCl(s)

These ions are arranged in solid NaCl in a regular three-dimensional arrangement (or lattice):

The chlorine has a high affinity for electrons, and the sodium has a low ionization potential. Thus the chlorine gains an electron from the sodium atom. This can be represented using electron-dot symbols (here we will consider one chlorine atom, rather than Cl2):

The arrow indicates the transfer of the electron from sodium to chlorine to form the Na+ metal ion and the Cl- chloride ion. Each ion now has an octet of electrons in its valence shell:

Na+ 2s22p6

Cl- 3s23p6

Energetics of Ionic Bond Formation

The formation of ionic compounds (like the addition of sodium metal and chlorine gas to form NaCl) are usually extremely exothermic.

The loss of an electron from an element:

The gain of an electron by a nonmetal:

The formation of NaCl from Na and Cl would thus require the input of 147 kJ/mol. However, it appears to be a highly exothermic reaction.

Ionic compounds are stable due to the attraction between unlike charges:

Lattice energy:

the energy required to separate completely a mole of a solid ionic compound into its gaseous ions

It is a measure of just how much stabilization results from the arranging of oppositely charged ions in an ionic solid.

To completely break up a salt crystal:

NaCl(s) -> Na+(g) + Cl-(g) DHlattice = +788 kJ/mol

Thus, -788 kJ/mol is given off as heat energy when 1 mol of NaCl is incorporated into the salt lattice.

So, forming the ions from Na(g) and Cl(g) requires the input of +147 kJ/mol, these ions incorporate into the salt lattice liberating -788 kJ/mol, for an overall highly exothermic release of -641 kJ/mol.

The magnitude of the lattice energy depends upon the charges of the ions, their size and the particular lattice arrangement.

The potential energy of two interacting charged particles is:

Q1 = charge on first particle

Q2 = charge on second particle

d = distance between centers of particles

k = 8.99 x 109 J m/C2

Thus, the interaction increases:

The minimum distance between oppositely charged ions is the sum of the atomic (ionic) radii. Although atomic radii do vary, it is not over a considerable range, thus, the attraction between two ions is determined primarily by the charge of the ions.

Electron configuration of ions

How does the energy released in lattice formation compare to the energy required to strip away another electron from the Na+ ion?

Since the Na+ ion has a noble gas electron configuration, stripping away the next electron from this stable arrangement would take far more energy than what is released during lattice formation (Sodium I2 = 4,560 kJ/mol). Thus, sodium is present in ionic compounds as Na+ and not Na2+.

Likewise, adding an electron to fill a valence shell (and achieve noble gas electron configuration) is exothermic or only slightly endothermic. To add an additional electron into a new subshell requires tremendous energy - more than the lattice energy. Thus, we find Cl- in ionic compounds, but not Cl2-.

Lattice energies range from around 700 kJ/mol to 4000 kJ/mol:

Compound

Lattice Energy

(kJ/mol)

LiF

1024

LiI

744

NaF

911

NaCl

788

NaI

693

KF

815

KBr

682

KI

641

MgF2

2910

SrCl2

2130

MgO

3938

This amount of energy can compensate for values as large as I3 for valence electrons (i.e. can strip away up to 3 electrons).

Because most transition metals would require the removal of more than 3 electrons to attain a noble gas core, they are not found in ionic compounds with a noble gas core (thus they may have color). Some examples which can form a noble gas core (and be colorless):

Ag: [Kr]5s14d10 Ag+ [Kr]4d10 Compound: AgCl

Cd: [Kr]5s24d10 Cd2+ [Kr]4d10 Compound: CdS

The valence electrons do not adhere to the "octet rule" in this case (a limitation of the usefulness of this rule)

Note: The silver and cadmium atoms lost the 5s electrons in achieving the ionic state

When a positive ion is formed from an atom, electrons are always lost first from the subshell with the largest principle quantum number

A transition metal always loses electrons first from the higher 's' subshell, before losing from the underlying 'd' subshell.

Iron will not have a noble gas core (iron salts will have color)

Fe: [Ar]4s23d6 Fe2+ [Ar] 3d6

Fe: [Ar]4s23d6 Fe3+ [Ar] 3d5

Polyatomic ions

In polyatomic ions, two or more atoms are bound together by covalent (chemical) bonds. They form a stable grouping which carries a charge (positive or negative). The group of atoms as a whole acts as a charged species in forming an ionic compound with an oppositely charged ion.


1996 Michael Blaber