What are Bond Parameters?

Many factors decide the covalent bonding between atoms. Some of the bond parameters are bond angle, bond order, enthalpy, bond length, etc. These parameters decide what kind of bond will form in atoms. Hence it is crucial to understand these parameters in detail and understand how changing these parameters affects the kind of bonding or various characteristics.

There are four types of bond parameters which we will know about in detail.

Chemical Bond

A chemical bond is a force that binds two or more atoms, molecules, or ions to form a molecule or compound. Different atoms or molecules from a chemical bonding to gain stability. These bonds may have strong intra-molecular interactions, like the covalent and ionic bonds, or weaker intermolecular forces, such as the London dispersion forces, the dipole-dipole interactions, and hydrogen bonding.

Types of Chemical Bonds

Chemical bonds are of three types, i.e., ionic bonds, covalent bonds, and coordinate bonds. In addition to these, there are also hydrogen bonds (intra-molecular & intermolecular H-bonds).

An ionic bond is a chemical bond in which there is a shift of electrons from one atom or molecule to another takes place. In such bonds, an atom loses an electron while the other gains. When such an electron transfer takes place, one of the atoms develops a negative charge (anion) and the other atom develops a positive charge (cation) held together by electrostatic forces.

A co-ordinate bond is a dative covalent bond formed by two atoms sharing a pair of electrons in which both the electrons come from the same atom.

A covalent bond is a chemical bond in which electrons (mostly valence electrons) are shared between two atoms. These bonds include sigma-bond (σ) and pi-bond (π) interactions.

Bond Parameters

Bond parameters are defined as the characterization of covalent bonds. It includes certain factors or measurable properties affecting a covalent bond. These parameters are used to strengthen and stabilize a chemical compound. Covalent bonding parameters are of the following types:

• bond length
• bond angle
• bond energy or bond enthalpy
• bond order

Bond Length

Bond length is the distance from the center of one nucleus to the center of the other of the another when two atoms are bonded together in a molecule in an equilibrium position. Each atom of the bonded pair is accountable to the bond length as bond length is dependent on the radii of both the atoms. Covalent radius is the contribution from each atom at the time of covalent bond formation. Van Der Waals radius is half the distance between the total size of the atom including its valence shell in a non-bonded situation of two similar atoms. The Van Der Waals radii are always larger than covalent radii. Bond length is expressed in Angstrom units (Α⁰ ) or picometers (pm).

Covalent radius = Bond length/2

The representation of the bond length of a covalent bond is shown in Fig 1.

The total bond length of a covalent molecule AB is given by

R= r_A + r_B
where R is the total bond length and r_A and r_B are the covalent radii of atoms A and B respectively.

Experimentally, we can measure this bond parameter with different techniques, such as X-ray diffraction, spectroscopic techniques like rotational spectroscopy, Electron diffraction techniques. Bonded atoms easily absorb heat energy from their surroundings and tend to vibrate continuously. Thereby, this vibration leads to the variation in the bond length. Hence, we should note that the bond length of a covalent bond forms the average distance between the nuclei of the bonded atoms.

Factors on which the bond length depends

• Bond multiplicity is indirectly proportional to its bond length, i.e, it decreases with a rise in bond multiplicity. Thus, the force of attraction between the bonding atom is stronger if their bond is shorter.
• Therefore we conclude the order as, single bond >double bond >triple bond.

For instance,

C-C bond > C=C bond > C ≡ C

N-N> N=N> N ≡ N

O-O> O=O

• The size of the atom is directly proportional to its bond length, i.e., it increases with the increase in the size of the atoms. The bond is longer because of the bigger size of the atom as with the addition of new shells the distance between valence electrons becomes larger and thereby the size of the atom becomes bigger.

F–F < Cl–Cl < Br–Br < I–I

C-C< Si-Si< Ge-Ge

H–F < H–Cl < H–Br < H–I

• Hybridization: Bond length decreases with the rise in ‘s’ character or lesser the ‘p’ character,

C(sp³) –H is 110 pm (25%, s character) < C(sp²)–H is 109 pm (33% ) <C(sp)–H is 106 pm (50%)

• The smaller the bond length, the greater the bond energy and stability of the molecule increases.
• Due to resonance, bond length decreases.

Since bond lengths are directly dependent on the atomic radii (size) of the bonded atoms. Therefore, it decreases as we move across the period and increases as we go down the group.

Few examples of bond Length of different elements and molecules are given below:

• Carbon-Carbon in C-C is 154 pm, C=C bond is 134 pm and C≡C bond is 120 pm.
• Carbon-oxygen, C-O is 360 pm
• Carbon-chlorine, C-Cl is 177 pm
• H-Cl molecule is 127 pm
• H-H is 74 pm
• O-H is 96 pm.

Bond Angle

Bond angle is the angle between any two covalently bonded atoms which has bonded pair around the central atom in a molecule/ion. It is measured in or its unit is degrees/minute/second. It can be calculated through spectroscopic methods.

It gives us a clear view as to how the bonded electrons are distributed around the central atom in a molecule. It also gives us a clear understanding of the molecular geometry of a compound by helping us determine the shape of the molecules. In some cases we may observe a decrease of bond angle, this may be because of repulsion between the lone electron pairs at the central atom and the bonded-pair electrons which successively slightly displaces the bonds inside which will decrease the angle of the bond. Greater the angle more will be the stability of the molecule.

Factors on which the bond angle depends

• The variance in electronegativity between two bonded atoms. Larger is the electronegativity difference between the central atom and shared atoms, its bond angle decreases.
• Size of the central atom and surrounding atoms. The angle decreases with the rise in the size of the central atom and increases with a rise in the size of surrounding atoms.

A few examples of bond angles are given below:

The bond angle of

                                   H-C-H in methane is 109.5°

                                   H-N-H in ammonia is 107 °

                                   H-O-H in H₂O is 104.5°

Bond Energy or Bond Enthalpy

Bond enthalpy or energy is another bond parameter. It is the mean energy or total amount of energy needed to split one mole of a chemical bond of a particular type between two atoms in a molecule. The bond-breaking process is endothermic, and the bond enthalpy involved is represented with a positive sign. The bond energy measurement is represented by kJ mol^-1and measured at 298 K.

Factors on which the bond energy depends

Bond length

• It is oppositely dependent on the bond length, i.e. larger the bond, the lower is the bond energies.

Bond order

• It is directly dependent on the bond order, i.e., energy increases with an increase in bond order.
  Example: Order of bond enthalpy in carbon bonds: C  ≡ C > C = C > C – C
  The bond dissociation enthalpy of diatomic molecules is the same as enthalpy. For polyatomic molecules, different bonds of the same type can have different bond energy and because of these differences in the energy, the average bond energy is taken into consideration.

Size

• In this case, the smaller the size of the bonded atoms, the tougher is the bond which means the greater the value of bond enthalpy. For instance, the enthalpy of H-H bond (435.8 kJ) > Cl-Cl bond (243.5 kJ)

Electronegativity

• Another factor that impacts the bond energy is the variance in the electronegativities of the atoms in the chemical covalent bond. In simpler terms, greater is the electro-negativity between the atoms, larger is the bond dissociation energy.

Average bond enthalpy

• It is the mean bond dissociation energy present in polyatomic molecules.

Bond Order

Another Bond Parameter is the bond order. Bond order is defined as the total number of bonds (chemical bonds) present between two atoms in a molecule. The larger the order, the greater is the stability of molecules.

According to the Lewis interpretation, in a molecule, the bond order of a bond (covalent) can also be defined as the total of covalently bonded electron pairs between two atoms.

We can calculate the bond order of diatomic molecules or ions between two covalently bonded atoms by the following steps:

1. Draw out the Lewis structure of the molecule.
2. Calculate the number of bonds present between the two atoms.

We can determine the bond order of polyatomic molecules or ions with the following steps:

1. Draw out the Lewis structure of the molecule.
2. Calculate the total number of bonds present in the molecule.
3. Calculate the total number of bond groups that is present between single atoms.
4. Divide the number of bonds which is present between atoms by the number of bond groups (we calculated in point 3) in the molecule.

• The bond order of single bonds is 1. eg, H-H           
• The bond order of double bonds is 2. eg, O=O
• The bond order of triple bonds is 3. eg, N≡N, C≡O 
• A covalent bond with 0 bond order means that the bond does not exist.

It is to be noted that isoelectronic species have the same bond order. For example, bond order 1 is seen in F₂, O₂^2-  having 18 electrons. Bond order of 3 is seen in N₂, CO, and NO⁺ which have14 electrons.

Context and Application

• Masters in Chemistry
• Masters in Physics
• Bachelors in Chemistry
• Masters in Physics

Practice Problems

Q.1 What is the relationship between bond order and stability?

1. Directly proportional
2. Inversely proportional
3. No relation
4. First directly proportional then constant

Answer : (a)

Explanation: Bond order and stability are directly proportional to each other. As the bond order increases, the stability of molecules also increases.

2 How is a pi- bond formed?

1. By overlap of s-p orbitals
2. By overlap of p-p orbitals
3. By overlap of p-p orbitals in sidewise manner
4. By overlap of s-s orbitals

Answer : (c)

Explanation: Pi- bond is formed by overlapping of p-p orbital in sidewise manner.

Q.3 The unit for measuring a bond angle is?

1. M
2. Kg
3. Mole
4. Degrees

Answer : (d)

Explanation: The unit for measuring a bond angle is degrees.

4 What is the value of bond angle for hydrogen atoms?

1. 105 degrees
2. 104 degrees
3. 100 degrees
4. 5 degrees

Answer : (d)

Explanation: The value of bond angle for hydrogen atoms is 104.5 degrees.

5 What will be the bond order of Carbon monoxide (CO)?

1. 1
2. 2
3. 3
4. 4

Answer : (c)

Explanation: The bond order of carbon monoxide, CO will be 3.

Related concepts

• Isotopes
• Enthalpy
• Allotropes
• Isomers
• Resonance
• Chemical bonding