covalent radius is half of the distance between two similar atoms joined by a covalent bond in the same molecule. The van der Waals radius represents the overall size of the atom which includes its valence shell in a nonbonded situation. Further, the van der Waals radius is half of the distance between two similar atoms in separate molecules in a solid. Covalent and van der Waals radii of chlorine are depicted in Fig.
Covalent and van der Waals radii in a chlorine molecule. The inner circles correspond to the size of the chlorine atom (r vdw and r c are van der Waals and covalent radii respectively). r = pm c pm r = pm vdw pm Fig. .
The bond length in a covalent molecule AB. R = r A + r B (R is the bond length and r A and r B are the covalent radii of atoms A and B respectively) Some typical average bond lengths for single, double and triple bonds are shown in Table . . Bond lengths for some common molecules are given in Table .
. The covalent radii of some common elements are listed in Table . . .
. Bond Angle It is defined as the angle between the orbitals containing bonding electron pairs around the central atom in a molecule/complex ion. Bond angle is expressed in degree which can be experimentally determined by spectroscopic methods. It gives some idea regarding the distribution of orbitals around the central atom in a molecule/complex ion and hence it helps us in determining its shape.
For example H–O–H bond angle in water can be represented as under : . . Bond Enthalpy It is defined as the amount of energy required to break one mole of bonds of a particular type between two atoms in a gaseous state. The unit of bond enthalpy is kJ mol – .
For example, the H – H bond enthalpy in hydrogen molecule is . kJ mol – . H (g) → H(g) + H(g); ∆ a H = . kJ