[Chemistry Class Notes] on Bond Parameters Pdf for Exam

Covalent bonds are characterized based on several bond parameters such as bond angle, bond length, bond energy (which is also called bond enthalpy), and bond order. All these bond parameters offer insight into the stability of a chemical compound and the strength of the chemical bonds that hold its corresponding atoms together. Let us discuss more on its important terms like bond order and bond length; bond length and bond energy.

Note: Also, the electronegativity differences of the atoms participating in the chemical bond contributes to the bond energy.

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Bond Order

Bond Order of a covalent bond can be given by the total number of covalently bonded electron pairs between the two atoms in a molecule where it can be found by forming the Lewis structure of the molecule and counting all the electron pair count between the atoms in question.

• Single bonds contain a bond order of 1

• Double bonds contain a bond order of 2

• Triple bonds contain a bond order of 3

It is to note that if the bond order of a covalent bond is given as 0, then the two atoms in question are not bonded covalently (it means no bond exists).

Examples

• The bond order of an oxygen-oxygen bond in the O2 molecule is 2.

• Bond order of a carbon-hydrogen bond of C2H2 (ethyne/acetylene) can be given as 1 to that of the carbon-carbon bond is 3.

• In a carbon monoxide molecule, the bond order of carbon-oxygen is given as 3, where the illustration in the Lewis structure is given below.

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• As the nitrate ion is stabilized by the resonance, the bond order of the nitrogen-oxygen bond is given as either 1.33 or 4/3. It is calculated by dividing the total nitrogen-oxygen bond count (4) by the total covalently bonded nitrogen-oxygen group count (3).

Bond Order, Based on the Molecular Orbital Theory

According to the molecular orbital theory, the covalent bond’s bond order is equal to half of the difference between the bonding electrons and antibonding electrons count, which can be represented using the formula given below.

Bond Order = (½) * (total number of bonding electrons – total number of antibonding electrons)

Bond Angle

We can define the bond angle as the angle formed between the two covalent bonds that originate from a similar atom. A detailed illustration of the bond angle in a water molecule (104.5 °C) is depicted below.

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The Bond Angle can be referred to as the geometric angle between any two adjacent covalent bonds. At the same time, this bond parameter provides insights into the compound’s molecular geometry.

Define Bond Length?

Bond length can be given as a measure of the distance between the nuclei of two chemically bonded atoms present in a molecule. Approximately, it is equal to the sum of the two bonded atom’s covalent radii. For the covalent bonds, the bond length is inversely proportional to the bond order, whereas the higher bond orders result in stronger bonds, accompanied by the stronger forces of attraction that hold the atoms together. In contrast, short bonds are the consequence of these strong attraction forces. The bond length formula can be given as r1+r2.

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The description of the bond length illustration of a covalent bond concerning the sum of the covalent radii individual of the participating atoms is represented above. Experimentally, this bond parameter can be determined with the techniques given below:

• X-ray diffraction

• Rotational spectroscopy

• Neutron diffraction

The bonded atoms tend to absorb the thermal energy from their surroundings and vibrate constantly. This vibration further causes the bond length to differ. Thus, it is very important to make a note that the bond length of a covalent bond describes the average distance between the nuclei of the participating atoms.

Periodic Trends in Bond Length

The bond lengths are always directly proportional to the atomic radii of the participating atoms. The periodic trends that are observed in the bond lengths of elements are the same as the periodic trends present in the atomic radii of the elements (increases down the group and decreases across the period).

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A detailing illustration of the periodic trends in bond length is given above. It can also make a note that the H-H bond is the shortest bond length having 74 picometers.

Bond Enthalpy or Bond Energy

Bond Enthalpy is given as a measure of the strength of a chemical bond. It can also be defined as the energy needed to break all the covalent bonds of a specific type in one mole of a chemical compound (which exists in its gaseous state).

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It is also important to make a note that the bond energy is not similar to the bond dissociation energy. The latter is the enthalpy change associated with the homolytic cleavage of a bond, whereas the former is the average of the bond dissociation enthalpies of total bonds (of a particular type) present in a molecule.

Factors Affecting the Bond Energy

The chemical bond’s strength is directly proportional to the amount of energy needed to break it. Thus, bond energy is given as follows:

• Directly proportional to the bond order; that is, multiple bonds contain high bond energies.

• Inversely proportional to the bond length, that is, longer bonds contain lower bond energies, and

• Inversely proportional to the atoms’ atomic radii, participating in the bond (because the atomic radius is directly proportional to bond length).