|
Unlimited Tutoring & Homework Help
|
|
Bond length
Bond length is the average distance between the centres of the nuclei of two bonded atoms in a molecule. It is expressed in Angstrom units (Å) or picometers(pm).
1Å = 10-10m and 1pm = 10-12 m. It is determined with the help of X-rays diffraction and other spectroscopic methods.Bond length depends upon the following factors:
Bond multiplicity
Bond length decreases with increase in bond multiplicity.
C
C bond length is shorter than C=C bond which in turn is shorter than
C-C.
Similarly, NN < N=N < NN and O=O < O-O
The bond lengths of some simple bonds (in pm)are:
Size of the atom
The bond length increases with increase in the size of the atom. From the above values it is clear that the bond lengths for a given family increase with increase in atomic number. For example,
C-C < Si-Si < Ge-GeThis is because with the increase in size of the atom, the distance of the electrons from the nucleus increases successively with the addition of a new shell. Therefore the average distance between the bonding nuclei (bond length) increases.
Bond angle
It is defined as the average angle between the orbitals of the central atom containing the bonding electron pairs in the molecule. It is expressed in degree/minute/second. This gives an idea about the distribution of orbitals 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 is NH3 107°.
Bond Energy (Bond Dissociation Energy)
It is defined as the amount of energy required to break one mole of bonds of a particular type between the atoms of a molecule in the gaseous state. It is expressed in terms of kJ mol-1. When a bond is formed between the atoms, energy is released and the bonded atoms have lesser energy than the separated individual atoms. Then, same amount of energy will be needed to form the bond. This energy is called the bond dissociation energy and is a measure of bond strength. Larger the bond dissociation energy, stronger will be the bond in the molecule.
Bond dissociation energy depends upon:Size of the bonded atoms
The smaller the size of the bonded atoms, the stronger is the bond and larger is the value of bond dissociation energy.
For example, the bond dissociation energy of H-H bond in hydrogen molecules is 433 kJ mol-1. This is larger than the bond dissociation energy of Cl-Cl in Cl2, which is 242.5 kJ mol-1.Bond length
Shorter the bond length, larger is the value of bond energy.
For example, bond length (154 pm) is larger than C=C bond length (134 pm). Consequently, the dissociation energy of C-C bond(348 kJ mol-1) is smaller than that of C=C bond(619 kJ mol-1).
Bond polarity
When the atoms forming bonds are different, the electron attracting powers of the two atoms in a bond may differ. Then, the shared pair will be displaced towards the atom having more electronegativity.
For example, in a moleclule of HCl, the electronegativities of Cl and H are 3.0 and 2.1 respectively. Due to the larger electronegativity of chlorine, the bonding pair will be attracted more towards chlorine atom. The chlorine atom will experience more negative charge around it and the other atom will experience less as if it has lost some of its negative charge. Due to this, the chlorine end of the molecule will acquire slightly negative charge and the hydrogen end will have slightly positive charge. These are represented as -d and +d (delta meaning small) charges. Such molecules having oppositely charged poles are called polar molecules and the bond is said to be polar covalent bond. The magnitude of electronegativity difference reflects the degree of polarity. Greater the difference in the electronegativities of the atoms forming the bond, greater will be the charge separation and hence greater will be the polarity of the molecule
Distorted electron cloud
Similarly, molecules such as BrCl, H2O, HF, etc. are also polar molecules.