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Define Bond Length, Size of the atom and Bond Angle

What is Bond Length?
Bond length is the average distance between the centers of the nuclei of two bonded atoms in a molecule. Shorter the bond length, larger is the value of bond energy. It is expressed in Angstrom units (Å) or picometers (pm).

1Å = 10-10m and 1pm = 10 -12-1) is smaller than that of C=C bond (619 kJ mol-1).

What is Size of the atom?
The bond length increases with increase in the size of the atom. For example,
C-C < Si-Si < Ge-Ge This is due to the increase in the distance of the electrons from the nucleus with the addition of a new shell. Therefore the average distance between the bonding nuclei (bond length) increases. Smaller the bond length, smaller will be the size of the atom as atoms were bounded more tightly. What is Bond Angle?
The average angle between the orbitals of the central atom containing the bonding electron pairs in the molecule is known as bond angle between the atoms. The unit of bong angle is either degree or minute or second. This gives an idea about the distribution of orbital’s around the central atom in a molecule. Therefore bond angle determines the shape of a molecule.

For example, the H-O-H bond angle in H2O is 104.5° and H-N-H bond angle in NH3 is 107°.