The sp - py,z Sequence
The standard sequence of sp and py,z	molecular orbitals.
The change in the sequence of bonding sp and py,z orbitals for oxygen and fluorine.
Electron configurations across the first complete row of the periodic table (lithium and up).
The 2s   atomic orbitals are present in all of these configurations.
The geometry of the non-bonding (1s and 2s) orbitals in Be2
The change in the geometry of the non-bonding (1s and 2s) orbitals as we move across the row of the periodic table towards oxygen and fluorine. The orbitals get smaller because the nuclear charge is increasing.
The geometry of a pi molecular orbital in, for example, B2. The shaded region shows where electron-electron repulsions are particularly unfavorable.
The geometry of a sigma molecular orbital in, for example, B2 showing that the electron-electron repulsions, most effective in the shaded regions, are even worse than in the case of the pi orbital's electrons. The sigma orbital is consequently less stable than the pi orbitals.
The geometry of a sigma molecular orbital in O2 vs B2.
The geometry of a pi molecular orbital in, for example, O2.
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