Characteristics of Bond

Bond Length

The distance between the nuclei of two atoms bonded together is termed as bond length or bond distance. It is expressed in angstrom (Å)  units or picometer (pm).

[1Å = 10^–8 cm; 1 pm = 10^–12 m]

Bond length in ionic compound = r_c⁺ + r_a^– 

Similarly, in a covalent compound, bond length is obtained by adding up the covalent (atomic) radii of two bonded atoms.

Bond length in covalent compound (AB) = r_A + r_B

Important Features of Bond Length

• The bond length of the homonuclear diatomic molecules are twice the covalent radii.

• The lengths of double bonds are less than the lengths of single bonds between the same two  atoms, and triple bonds are even shorter than double bonds.Single bond > Double bond > Triple bond (decreasing bond length)

• Bond length decreases with increase in s-character since s-orbital is smaller than a p – orbital. 

• sp³ C – H = 1.112Å:                    sp² C – H = 1.103Å;                   sp C – H = 1.08Å;                                                 

• (25% s-character as in alkanes) (33.3% s-character as in alkenes) (50% s-character as in alkynes)

• Bond length of polar bond is smaller than the theoretical non-polar bond length.
   

Bond Energy or Bond Strength

 Bond energy or bond strength is defined as the amount of energy required to break a bond in molecule.  

Important features of bond energy:

• The magnitude of the bond energy depends on the type of bonding. Most of the covalent bonds have energy between 50 to 100 kcal mol^–1 (200-400 kJ mol^–1). Strength of sigma bond is more than that of a π-bond.

• A double bond in a diatomic molecules has a higher bond energy than a single bond and a triple bond has a higher bond energy than a double bond between the same atoms. C ≡ C > C = C > C – C (decreasing bond length)

• The magnitude of the bond energy depends on the size of the atoms forming the bond, i.e. bond length. Shorter the bond length, higher is the bond energy.

• Resonance in the molecule affects the bond energy.

• The bond energy decreases with increase in number of lone pairs on the bonded atom. This is due to electrostatic repulsion of lone pairs of electrons of the two bonded atoms.

• Homolytic and heterolytic fission involve different amounts of energies. Generally the values are low for homolytic fission of the bond in comparison to heterolytic fission.

• Bond energy decreases down the group in case of similar molecules.

• Bond energy increase in the following order:

s < p < sp < sp² < sp³
C – C          >         N – N         >          O – O
^{(No lone pair)        (One lone pair)              (Two lone pair)}

Refer to the following video for bond length and bond strength of covalent bonds

Bond Angles

Angle between two adjacent bonds at an atom in a molecule made up of three or more atoms is known as the bond angle.

Bond angles mainly depend on the following three factors:

(i) Hybridization: Bond angle depends on the state of hybridization of the central atom

(ii) Lone pair repulsion: Bond angle is affected by the presence of lone pair of electrons at the central atom. A lone pair of electrons at the central atom always tries to repel the shared pair (bonded pair) of electrons. Due to this, the bonds are displaced slightly inside resulting in a decrease of bond angle.

(iii) Electronegativity: If the electronegativity of the central atom decreases, bond angle decreases.
 

Resonance

There may be many molecules and ions for which it is not possible to draw a single Lewis structure. For example we can write two electronic structures of O₃.

Ionic Bonding - occurs when two or more ions combine to form an electrically-neutral compound

The positive cation "loses" an electron (or 2 or 3)
The negative anion "gains" the electron (or 2 or 3)
The anion steals the electrons from the cation. 

The electrons are shared between the two atoms. Both atoms don't have charge in the beginning and the compound remains with zero charge.

The chemical activity of an atom is determined by the number of electrons in its valence shell. With the help of concept of chemical bonding one can define the structure of a compound and is used in many industries for manufacturing products in which the true structure cannot be written at all.

Some other examples

(i) CO₃^2– ion

      Example
(ii) Carbon-oxygen bond lengths in carboxylate ion are equal due to resonance.

(iii) Benzene

Difference in the energies of the canonical forms and resonance hybrid is called resonance stabilization energy and provides stability to species.
 

Rules for writing Resonating Structures

Only electrons (not atoms) may be shifted and they may be shifted only to adjacent atoms or bond positions.
The number of unpaired electrons should be same in all the canonical form.

The positive charge should reside as far as possible on less electronegative atom and negative charge on more electronegative atom.

Like charge should not reside on adjacent atom

The larger the number of the resonating structures greater the stability of species. Greater number of covalency adds to the stability of the molecule.

Example: Out of the following resonating structures for CO₂ molecule, which are important for describing the bonding in the molecule and why?

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