Question

Draw an MO diagram for HF and for HCl on the same drawing. Try to make clear which is which using different colors. Be sure that your relative atomic energies and relative molecular energies are correct. Base on the diagram which do you expect to be greater, CCl or CF in the bonding orbital for HCl and HF. explain your reasoning.

Answer 1

Solution:

The MO diagram are expressed as below,

MO Diagram of HCl In case of MO diagram of HCL, combination between the hydrogen 1s AO and chlorine 1s, 2s,2p and 3s orbital can be ruled out because their energies are too low. If the overlap occur between the chlorine 3py and 3pz orbitals it would be non bonding because the positive lobe of hydrogen will overlap equally with positive and negative lobes of chlorine orbitals Thus the only effective overlap is with chlorine 3px orbital. The combination of H 1s1 and CL 3px1 gives both bonding and the two electrons occupythe bonding MO, leaving te antibonding MO empty orb itals are non-bonding as they have the wrong symmetry to 43 :interact with the H 1s orbital ーーープーーー3p Cl is more electronegative than hydrogen.3p is lower energy than the H1S Energy The 33 orbital is non-bonding orbitalsare too far apart in energy to interact l ơ Ci (1s') Bond order 1

MO Diagram of HF: Similar is the case with HF. Due to low energy o 1s AO of H and 1s and 2s AO of F, combination these orbitals are ruled out from the MO diagram . Moreover the 2py and 2pzIn case of orbital behave as non bonding orbitals due to the above mentioned reason. In HF molecule the 1s1 AO of H combine with 2px orbital of F to form bonding and antibonding molecular orbitals. The two electrons occupy the bonding MO and the antibonding MO is left vacant. t orbitals are non-bonding as they have the wrong symmetry to interact with the H 1s orbital Fluorine is more electronegative than hydrogen, 2p is lower energy than the H 1s Energy The 2σ orbital is non-bonding the H 1s and F 2s... orbitals are too far apart in energy to interact 2s HF (1s') (2s2 2p5) Bond order 1

STRUCTURE ON THE BASIS OF HYBRIDISATION Ground state electronic configuration of H-1s Ground state electronic configuration of CI-3s2 3p/ 3py2 3px1 The 3px orbital of Cl combine with 1s orbital of H and form sp hybrid orbitals. Therefore, the hybridisation is sp. sp hybridisation is a linear hybridisation and therefore, the structure is linear. Similarly, in HF Ground state electronic configuration of H-1s Ground state electronic configuration of F- 2s2 2p2 2py 2px Here the 2px orbital of F combined with 1s orbital of H and form sp hybrid orbitals Hybridisation of HF is same as HCl i.e. sp and both of them have Linear structure. STRUCTURE ON THE BASIS OF VSEPR THEORY According to the VSEPR theory, type of spatial arrangement depends upon the sum of bonding electron pairs and lone pairs surrounding the central metal atom. In case of HCl as well as HF, since there are only two atoms inolved i.e. they are diatomic molecules amd we know that the geometry of such molecules is clearly linear. Here no. of bonding electrons1 No. of lone pairs 0 Therefore, the structure of HCl and HF is linear with a bond angle of 180°

: Cl