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CHEMICAL BONDING AND MOLECULAR STRUCTURE antibonding molecular orbital, the electron as the mole ular a is It is im ortant to note wa es an el ea h other due to destru ti e that atomi orbitals ha ing same or nearly interference.Asaresult,theelectrondensityin the same energy will not combine if they do not ha e the same symmetry or e am le a bonding molecular orbital is located between the nuclei of the bonded atoms because of 2pzorbital of one atom can combine with 2p z whi h the re ulsion between the nu lei is ery orbital of the other atom but not with the less while in case of an antibonding molecular 2p or 2p orbitals because of their different x y orbital, most of the electron density is locatedsymmetries. away from the space between the nuclei. 3.The combining atomic orbitals must Infa t there is a nodal lane on whi h the overlap to the maximum extent. Greater ele tron density is ero between the nu lei the e tent of o erla the greater will be the and hence the repulsion between the nuclei is ele tron density between the nu lei of a high le trons la ed in a bonding mole ular molecular orbital. orbital tend to hold the nuclei together and 4.7.3 Types of Molecular Orbitals stabilise a mole ule herefore a bonding molecular orbital always possesses lower Molecular orbitals of diatomic molecules are energy than either of the atomic orbitals that designated as σ sigma π i δ delta et ha e ombined to form it In ontrast the In this nomenclature, the sigma ( ) electrons placed in the antibonding molecular molecular orbitals are symmetrical around orbital destabilise the mole ule his is the bond-axis while pi () molecular orbitals because the mutual repulsion of the electrons are not symmetrical or e am le the linear in this orbital is more than the attraction combination of 1s orbitals centered on two between the electrons and the nuclei, which nuclei produces two molecular orbitals which causes a net increase in energy. are symmetri al around the bond a is u h It may be noted that the energy of the molecular orbitals are of the σ type and are antibonding orbital is raised abo e the designated as σ1s and σ*1s ig a age energy of the parent atomic orbitals that If internuclear axis is taken to be in ha e ombined and the energy of the bonding the z-direction, it can be seen that a linear orbital has been lowered than the parent combination of 2p z- orbitals of two atoms orbitals he total energy of two mole ular also produces two sigma molecular orbitals orbitals howe er remains the same as that designated as 2p and *2p . ig b z z of two original atomic orbitals. Molecular orbitals obtained from 2p and 4.7.2 Conditions for the Combination of 2p orbitals are not symmetrical around the y Atomic Orbitals bond a is be ause of the resen e of ositi e he linear ombination of atomi orbitals to lobes abo e and negati e lobes below the form molecular orbitals takes place only if the mole ular lane u h mole ular orbitals are labelled as π and π ig following onditions are satisfied 1.The combining atomic orbitals must π bonding MO has larger electron density have the same or nearly the same energy. abo e and below the inter nu lear a is he π* antibonding MO has a node between the his means that s orbital can combine with nuclei. another 1s orbital but not with 2s orbital because the energy of 2s orbital is appreciably 4.7.4 Energy Level Diagram for Molecular higher than that of 1s orbital his is not true Orbitals if the atoms are ery different e ha e seen that s atomic orbitals on two 2.The combining atomic orbitals must atoms form two molecular orbitals designated have the same symmetry about the as σ1s and σ*1s. In the same manner, the 2s molecular axis y on ention a is is ta en and 2p atomi orbitals eight atomi orbitals Reprint 2026-27