(NCERT Examplar Problem) Molecular orbitals are formed by the overlap of atomic orbitals . Two atomic orbitals combine to form two molecular orbitals called bonding molecular orbital (BMO) and antibonding molecular orbital (ABMO) . Energy of antibonding orbitals is raised above the parent atomic orbitals that have combined and the energy of the bonding orbitals is lowered than the parent atomic orbitals . Energies of verious molecular orbitals for elements hybrogen to nitrogen increase in the order : sigma 1s lt sigma ^(**)1s lt sigma 2s lt sigma^(**)2s lt (pi_(2p_(x)) = pi_(2p_(y))) lt sigma2p_(z) lt (pi^(**)2p_(x) = pi^(**) 2p_(y)) lt sigma^(**) 2p_(z) and for oxygen and fluorine , order of energy of molecular orbitals is given below : sigma 1s lt sigma ^(**)1s lt sigma 2s lt sigma^(**)2s lt sigma 2p_(x) lt (pi_2p_(x) = pi_2p_(y))lt (pi^(**)2p_(x) = pi^(**) 2p_(y)) lt sigma^(**) 2p_(z) Different atomic orbitals of one atom combine with those atomic orbitals of the second atom which have comparable energies and proper orientation . Further , if the overlapping is headon, the molecular orbital is called 'sigma', (sigma) and if the overlap is lateral, the molecular orbitals is called 'pi' , (pi) . The molecular orbitalsare filled with electrons according to the same rules as followed for filling ofatomic orbitals . However, the order for filling is not In whihc of the following moelcular,sigma 2p_(z), molecular orbitals is filled afterpi 2p_(x) and pi 2p_(y) molecular orbitals ?