Determining certain parameters such as size, length and angle on an atomic scale is not easy. Because of the importance of these parameters, scientists have developed methods to derive or calculate the parameters at the atomic level. Bond order and bond length are two important parameters that indicate the type and strength of the bonds between a pair of atoms.
This article explains
1. What are bond order and bond length?
2. How to calculate the bond order and length - How to calculate the bond order - How to calculate the bond length
What are bond order and bond length?
Bond length and bond order are two parameters associated with covalent bonds . The bond order is the number of chemical bonds between two atoms and the bond length is the distance between two atomic nuclei that are covalently linked to each other. This article explains how to calculate bond order and bond length at the atomic level.
How to calculate the bond order
The bond order is the number of chemical bonds between two atoms. It indicates the stability of a bond. In the case of covalent bonds, the bond order is the number of shared electrons. For example, the bond order of a pair of atoms linked by a single bond is one, while the bond order of a pair of atoms linked by a double bond is two. The zero bond order indicates that there is no bond between atoms. The stability of a molecule increases as the bond order increases. The molecules with a resonance bond need not be an integer. In covalent bonds with two atoms, the bond order between a pair of atoms is determined by first drawing the Lewis structure and then determining the type of bond between the atoms - zero bond, single, double or triple bond. For example, the bond order of hydrogen gas is determined as follows.
- Draw the Lewis structure
- Determine the number of bonds / valence electron pair
One pair of electrons, therefore the bond order is 1.
If there are more than two atoms, the bond order is determined as follows. See the example: nitration.
- Draw the Lewis structure
- Count the total number of bonds (after nitration, its 4)
- The number of bonding groups between individual atoms (according to ammonia, its 3)
- Therefore the bond order = total number of bonds / number of bonding groups
Therefore the bond order of the nitrate ion is 1.33
How to calculate the bond length
The bond length is the distance between two atomic nuclei that are covalently linked to one another. The bond length is usually in the range 0.1-0.2 nm. When two similar atoms are bonded together, half the bond length is called the covalent radius . The bond length depends on the number of bound electrons in two atoms or the bond order. The higher the bond order, the shorter the bond length due to the strong tensile forces of positively charged nuclei. The unit of the bond length is picometer . For single, double and triple bonds, the bond length increases in the order of magnitude. to
Triple bond <double bond <single bond
Electronegativity can be used to calculate the bond length between two atoms with different electronegativity. The following empirical formula was proposed by Shoemaker and Stevenson to calculate bond length.
d AB = r A + r B - 0.09 (x A - x B )
d AB is the bond distance between two atoms A and B, r A and r B are covalent radii of A and B, and
(x A - x B ) is the electronegativity difference between A and B.
Another method is used to calculate the approximate bond length. In this method, a Lewis structure is first drawn to determine the type of covalent bond. Then, using a graph * based on the studies by Cordero et al . and Pyykkö and Atsumi determined the corresponding radii of the bonds made by each atom. Then the bond length is determined by taking the sum of the two radii. For example, when determining the bond length of carbon dioxide, a Lewis structure is first drawn.
According to the diagram, the covalent radius of the carbon double bond is 67 picometers and that of the oxygen double bond is 57 picometers. Therefore, the bond length of carbon dioxide is about 124 picometers (57 pm + 67 pm).
* You can find the covalent table here
Cordero, Beatriz, et al. "Covalent Radii Reinterpreted." Dalton Transactions 21 (2008): 2832-2838.
Free texts. "Loan Order and Lengths." Chemistry LibreTexte. Libretexts, December 5th, 2016. Web. January 10, 2017.
Lister, Ted and Janet Renshaw. Understand chemistry for advanced learners. Np: Nelson Thornes, 2000. Print.
Pratiyogita-Darpan. “Properties of elements in relation to their electronic structure.” Science Vision competition Aug. 1998: n. Pag. Print.
Pyykkö, Pekka and Michiko Atsumi. “Molecular covalent double bond radii for the elements Li – E112.” Chemistry – A European Journal 15.46 (2009): 12770-12779.