Ø  Covalent Bond Parameters:
1.     Bond Length: It is defined as the distance between the nuclei of two bonded atoms in a molecule. It is measured by spectroscopic, X-ray diffraction and electron diffraction techniques. It is expressed in pico-metre (pm) or in angstrom unit (A0). Bond lengths depend upon the relative sizes of the atoms. For example, the bond lengths of hydrogen halides follow the sequence H – F < H – Cl < H – Br < H – I because the size of the halogens increases down the periodic group as F < Cl < Br < I. In case of atoms joined by multiple bonds, greater is the bond order, smaller will be the bond length.






2.  Bond Angle: It is defined as the angle between the orbitals containing bonding electron pairs around the central atom in a molecule. It can be experimentally determined by spectroscopic methods. It is expressed in degree.

3.     Bond Enthalpy (Bond Energy): Energy is released during the process of covalent bond formation between atoms which leads to the lowering of energy and energy is required to break a bond. Bond enthalpy (or bond energy) is defined as the amount of energy required to break one mole of a particular bond between 2 atoms in gaseous state. Its unit is kJ/mol.


4.  Bond Order: The bond order is the number of bonds present between two atoms in a molecule or ion. The bond order is said to be 1 when there is only one covalent bond formed between two atoms. For example: Bond order in H2 and Cl2 is 1, 
                  

    

Bond order for O2 is 2


Generally, with increase in bond order, bond enthalpy increases and bond length decreases.

·       Dative bond or Co-ordinate bond: When in the formation of a bond between two atoms, only one atom contributes the pair of electrons and the other atom simply participates in sharing, the bond formed is called dative bond or Co-ordinate bond. This may be explained as under:
In this case, one atom has its octet complete and has at least one lone pair (unshared) of electrons. The lone pair of electrons belonging to the atom with the complete octet is shared with the other atom containing an empty orbital and thus a link is established. The atom contributing the shared pair of electrons is called the donor while the atom which accepts these shared electrons is called the acceptor. This linkage set up between two atoms is represented by an arrow starting from the donor to acceptor.