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In molecular orbitals diagram, sigma ($\sigma$) and pi ($\pi$) symbols are used. What do these symbols really mean?

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These symbols are for bonding orbitals. A covalent bond is formed when two orbitals in different atoms hybridize. $\sigma$ and $\pi$ refer to two different configurations in which this can happen.

In a $\sigma$ bond, two $s$ orbitals or two $p$ orbitals oriented towards each other join into one orbital, with the electron density concentrated between the two atoms.

In a $\pi$ bond, two $p$ orbitals oriented perpendicular to the bond axis merge, and, in the resulting orbital, electron density is concentrated above and below the bond axis.

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  • $\begingroup$ so, pi and sigma symbols are actually referring to pi bond and sigma bond, right? $\endgroup$ – Rafique Jul 12 '13 at 1:27
  • $\begingroup$ Technically they're for ABMOs as well, when used with an asterisk $\endgroup$ – ManishEarth Jul 12 '13 at 8:27
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    $\begingroup$ @MuhammadRafique Yep. Though in the case of sigma-star and pi-star, it's not exactly a bond. It's better to look at them as sigma-bond-like and pi-bond-like orbitals, as bonds don't exactly correspond to orbitals like in VSEPR here. It's similar, but not the same. $\endgroup$ – ManishEarth Jul 12 '13 at 8:30

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