Resonance and resonance hybrid structures of O3, SO3, SO2, CO3-2, SO4-2, PO4-2,NO3-

he phenomenon of resonance was put forward by Heisenberg to explain the properties of certain covalent molecules. Most covalent molecules have a unique Lewis formula (dot structure), which explains the bonding in that molecule. Some molecules lend themselves two or more dot structures. None of these structures can exactly describe all the properties of the molecule, but each structure contributes to the true structure of the molecule. Such different structures are called 'resonance structures' and the actual molecule is termed as the 'resonance hybrid' of all these structures. In every case the actual molecule is found to be energetically more stable than would be expected from any of its resonance structures.The energy difference between the most stable resonating structure and the resonance hybrid structure is termed as resonance energy.

Characteristics of resonance

• The contributing structures do not have real existence. These are only imaginary structures, proposed to explain the properties of the molecule. None of these 'resonance structures' can be prepared in the laboratory. Only the resonance hybrid is the real molecule structure.

• Because of resonance, the bond lengths in resonating structures become equal. For example, both the O-O bond lengths in O₃ are equal. All the C-C bonds in benzene are equal.

Resonance structure of ozone

Structure1

Structure2

Resonance hybrid structure of ozone

Both the O-O bond lengths are equal and intermediate of single and double bond.

Benzene

All the C-C bonds in the resonance hybrid structure are intermediates of single and double bonds and the bond lengths are equal.

• The resonance hybrid has lower energy and thus greater stability than any of the contributing structures.

• Greater the resonance energy, greater is the stability of the molecule.

• Concept of resonance is theoretical.

Conditions for writing resonance structures

The contributing structures should:

• Have the same atomic positions.

• Posses the same number of unpaired electrons.

• Have nearly the same energy.

• Written in a way that negative charge is present on an electronegative atom and positive charge is present on an electropositive atom.

• Not place the like charges on adjacent atoms.

The resonance structures of a few more molecules and ions are given below:

NO^-₃ ion: The three possible resonance structures for the nitrate ion (NO^-₃) are

Benzene

Resonance energy is the difference between the actual bond energy of the molecule and that of the most stable of the resonating structures (having least energy). For example, the resonance energy of carbon dioxide is 138 kJ mol^-1. This means that the actual molecule of CO₂ is about 138 kJ more stable than the most stable structure among the contributing structures.

Problems

7. Identify the atoms which do not obey the octet rule in the following compounds and draw their Lewis structures? SO₂ or SF₆

Solution

SO₂

Here all atoms obey the octet rule. Oxygen and sulphur atoms have six electrons in their outermost shells. The Lewis structure of SO₂ is,

S is attached to one O atom through a double bond while to the other O atom through a coordinate bond. All atoms have eight electrons in their outer most shells. The octet rule is obeyed.

SF₆

S does not obey the octet rule. The Lewis Structure of SF₆ gives 12 electrons (6 pairs of electrons) around S. Thus, S does not obey the octet rule.

(Only the sharing electron of F is shown)

8. Write and explain the Lewis Structure for (I) H₂SO₄ (II) H₃PO₄ (III) BCl₃ compounds.

Solution

(I) Sulphur atom has six valence electrons. The two hydrogens in this compound are present as OH groups. The possible Lewis structure of H₂SO₄ is,

(II) Phosphorous has 5 valence electrons. The three hydrogens are present as three OH groups. The possible Lewis Structure of H₃PO₄ is

(III) Boron atom (B) has 3 electrons in its outermost shell, and Cl has 7. B does not obey the octet rule. The possible Lewis structure for BCl₃ is

Thus, there are only six electrons around B. The octet rule, thus is not obeyed by B.