Bond Parameters - Definition, Order, Angle, Length

Several bond parameters, such as bond length, bond angle, bond order, and bond energy, can be used to characterize covalent bonds (also known as bond enthalpy). These bond parameters provide information about the stability of a chemical compound as well as the strength of the chemical bonds that hold its atoms together.

What is a Bond?

Covalent bonds are classified into two types: sigma bonds and pi bonds. The number of shared bonds varies depending on the number of electrons. When two, four, or six electrons are shared, the number of bonds formed is one, two, or three, depending on the number of electrons shared. Covalent bonds are thus classified as sigma bonds or pi bonds based on the type of overlapping.

Types of Bonds:

• Sigma Bond: End-to-end (head-on) overlap of bonding orbitals along the internuclear axis creates this type of covalent bond. This is known as head-on or axial overlap. This can be formed by any of the atomic orbital combinations listed below.
  • s-s Overlapping: In this scenario, two half-filled s-orbitals are overlapping along the internuclear axis. This type of overlapping can be seen in the creation of the H₂ molecule.
  • s-p Overlapping: Along the internuclear axis, there is an overlap of one atom's half-filled s orbital and another atom's half-filled p orbital. The formation of methane, ammonia, and water exhibits this type of overlapping.
  • p-p Overlapping: This type of overlapping occurs along the internuclear axis between one half-filled p orbital and another half-filled p orbital. This type of overlapping can be seen when fluorine atoms combine to form F₂ molecules.
• Pi (π) Bond: During the creation of Pi bonds, atomic orbitals overlap in such a way that their axes remain parallel to each other and perpendicular to the internuclear axis. Atomic orbitals overlap sideways during bond formation, forming a saucer-shaped charged cloud above and below the internuclear axis.

Strength of Sigma and Pi Bond

1. The extent to which atomic orbitals overlap determines the strength of a bond.
2. The sigma bond, which overlaps along the internuclear axis, is more powerful than the pi bond, which overlaps sideways.
3. Pi bonds have a smaller area of overlap than sigma bonds. This is why the pi-bond breaks first, followed by the sigma bond.
4. During multiple bond formation, a pi bond is formed in addition to a sigma bond.

Bond Parameters

In order to become stable, various atoms must join together. This combination occurs as a result of the formation of bonds. Ionic or electrovalent bonds, covalent bonds, and coordinate bonds are the three types of bonds. As a result, every bond has a characteristic linked with it

Bond parameters are a set of different features or characteristics that may be observed in bonds. 

The parameters that define a covalent bond are as follows:

1. Bond Order
2. Bond Angle
3. Bond Length
4. Bond Enthalpy or Energy

Bond Order

A covalent bond's bond order is the total number of covalently bonded electron pairs between two atoms in a molecule. It can be calculated by drawing the molecule's Lewis structure and counting the total number of electron pairs between the atoms in question. For example, the carbon-hydrogen bond in C₂H₂ (ethyne/acetylene) has a bond order of 1 and the carbon-carbon bond has a bond order of 3. The oxygen-oxygen bond in an O2 molecule has a bond order of 2. The carbon-oxygen bond in a carbon monoxide molecule has a bond order of 3.

• Single bonds have a bond order of 1.
• Double bonds have a bond order of 2.
• Triple bonds have a bond order of 3.

Bond Order according to the Molecular Orbital Theory: The bond order of a covalent bond, according to molecular orbital theory, is equal to half of the difference between the number of bonding and antibonding electrons, as represented by the following formula:

Bond Order = (½) × (Total number of bonding electrons – Total number of antibonding electrons)

Bond Angle

Bond angle is defined as the angle formed between two covalent bonds formed by the same atom. A bond angle is a geometric angle formed by any two adjacent covalent bonds. This bond parameter provides information about a compound's molecular geometry.

Bond Length

The bond length of a molecule is the distance between the nuclei of two chemically bonded atoms. It is close to the sum of the covalent radii of the two bonded atoms. For covalent bonds, the bond length is inversely related to bond order; greater bond orders result in stronger bonds, which are accompanied by stronger attraction forces that hold the atoms together. These strong forces of attraction result in short bonds. Rotational spectroscopy, X-ray diffraction, and neutron diffraction can all be used to determine this bond parameter experimentally. Bonded atoms absorb thermal energy from their surroundings and vibrate incessantly. The bond length varies as a result of this vibration. As a result, it is critical to understand that the bond length of a covalent bond represents the average distance between the nuclei of the atoms involved.

Periodic Trends in Bond Length: Bond lengths are proportional to the atomic radii of the atoms involved. The periodic trends that can be observed in element bond lengths are similar to the periodic trends that can be observed in element atomic radii (decreases across the period, increases down the group).

Bond Energy or Bond Enthalpy

Bond energy is a measurement of the strength of a chemical bond. It is defined as the amount of energy required to break all covalent bonds of a particular type in one mole of a chemical compound (which is in its gaseous state).

Factors Affecting Bond Energy

The amount of energy required to break a chemical bond is directly proportional to its strength. As a result, bond energy is:

1. Bond energies are inversely proportional to bond length, so longer bonds have lower bond energies.
2. Bond energies are directly proportional to bond order, implying that multiple bonds have high bond energies.
3. The atomic radii of the atoms involved in the bond are inversely proportional (since the atomic radius is directly proportional to bond length).

Sample Questions

Question 1: What is the bond angle?

Answer:

The angle created by three atoms spanning at least two bonds is known as a bond angle. The angle of torsion for four atoms bonded together in a chain is the angle formed by the first three atoms and the plane formed by the last three atoms.

Question 2: What is a stronger double bond or triple bond stronger?

Answer:

Triple bonds are stronger than double bonds because they contain two pi bonds rather than one. Each carbon atom has two hybrid sp-orbitals, one of which overlaps to form a sp-sp sigma bond with the corresponding one from the other carbon atom.

Question 3: What is the enthalpy of bond dissociation?

Answer:

Bond enthalpy, also known as bond-dissociation enthalpy, average bond energy, or bond strength, is the amount of energy contained in a bond between atoms in a molecule. In fact, it is the energy that must be applied to the homolytic or symmetrical cleavage of a bond in the gas phase.

Question 4: How is a pi bond formed?

Answer: 

A pi bond is formed when a bond between two atoms is broken when one of the atoms rotates around the bond axis. A pi bond is not the same as an axial bond. Pi bonds are formed when parallel p orbitals on adjacent atoms overlap sideways. They are not made up of hybrid orbitals.

Question 5: Why is there no rotation around a double bond?

Answer:

Only alkanes allow for free rotation. Because it will break during rotation, it is restricted in both alkenes and alkynes.