Valence bond theory class 11

valence bond theory, its postulates, formation of H2 molecule ,concept of overlapping and its types, sigma and pi bond

1. VALENCE BOND THEORY Presented by : NEELU SHARMA CLASS XI

2. Postulates of vbt Covalent bonds are formed when two valence orbitals (half-filled) belonging to two different atoms overlap on each other. The electron density in the area between the two bonding atoms increases as a result of this overlapping, thereby increasing the stability of the resulting molecule. The presence of many unpaired electrons in the valence shell of an atom enables it to form multiple bonds with other atoms. The paired electrons present in the valence shell do not take participate in the formation of chemical bonds as per the valence bond theory. Covalent chemical bonds are directional. T he atomic orbitals overlap to form sigma and pi bonds .

3. Applications of Valence Bond Theory The maximum overlap condition which is described by the valence bond theory can explain the formation of covalent bonds in several molecules. This is one of its most important applications. For example, the difference in the length and strength of the chemical bonds in H 2  and F 2  molecules can be explained by the difference in the overlapping orbitals in these molecules. The covalent bond in an HF molecule is formed from the overlap of the 1s orbital of the hydrogen atom and a 2p orbital belonging to the fluorine atom, which is explained by the valence bond theory.

4. Formation of h2 molecule H(g) + H(g) → H2(g) + 433 kJ/mol When atoms combine to form molecule, there is always decrease in energy, which leads to the stability of the molecule. When atoms approaches, two different forces operate: The nucleus of one atom is attracted towards the electrons of the other and vice versa. Energy is release during attraction . The nuclei of the atoms as well as there electrons repel each other . Energy is needed to over come the force of repulson

5. Formation of hydrogen molecule If the magnitude of attractive force is more than repulsive force , a stable molecule will be form . When to hydrogen atoms approach each other , the overall potential energy of the system decreases and stable hydrogen molecule will formed. When attraction and repulsion forces balanced each other, both hydrogen atoms get bonded to form H2 molecule.

6. H2 molecule In H2 molecule, H ─ H bond length is 74 pm. The total decrease in the potential energy when one mole bond is formed between atoms in gaseous state is called bond energy. The bond energy in H2 molecule is 433 kJ/mol. (Bond dissociation energy)

7. Helium molecule New attractive force = 4 New repulsive force = 5 The overall repulsive force is more than attractive force, therefore He2 molecule don’t exist.

8. overlapping

9. Types of overlapping

10. S-p overlapping

11. Co-axial & sidewise (p-p)overlapping

12. overlapPing

13. THANK YOU