1.

In which of the following molecules, sigma 2p_(z) molecular orbital is filled after pi 2p_(x) and pi 2p_(y) molecular orbitals ?

Answer»

`O_(2)`
`Ne_(2)`
`N_(2)`
`F_(9)`

Solution :`N_(2)`
Total number of electrons present in `N_(2)` molecule is 14. The electronic configuration of `N_(2)`molecule will be ,
`sigma 1s^(2), sigma^(**) , sigma 2s^(2) ,sigma^(**) 2s^(2) , pi 2p_(x)^(2) = pi 2p_(y)^(2) , pi 2p_(z)^(2)`
The increasing order of energies of various molecular orbitals for `O_(2) and F_(2)` is given below `sigma 1s lt sigma^(**) lt sigma 2s lt sigma^(**) 2s lt sigma 2p_(z) lt (pi 2p_(x) = pi 2p_(y)) lt (pi^(**) 2p_(x) = pi^(**) 2p_(y) ) lt sigma^(**) 2p_(z)`
HOWEVER, this sequence of energy levels of MO is not CORRECT for the remaining MOLECULES such as `Li_(2) , Be_(2) , B_(2) , C_(2) and N_(2).` for these molecules, the increasing order of energies of various MO is
`sigma 1s lt sigma^(**) 1s lt sigma 2s lt sigma^(**) 2s lt (pi 2p_(x) = pi 2p_(y)) lt sigma 2p_(z) lt (pi^(**) 2p_(x) = pi^(**) 2p_(y)) sigma 2p_(z)`


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