difference between sigma and pi bonds

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From Wikipedia

Sigma bond

In chemistry, sigma bonds (σ bonds) are the strongest type of covalentchemical bond. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is symmetrical with respect to rotation about the bond axis. By this definition, common forms of sigma bonds are s+s, pz+pz, s+pz and dz2+dz2 (where z is defined as the axis of the bond). Quantum theory also indicates that molecular orbitals (MO) of identical symmetry actually mix. As a practical consequence of this mixing of diatomic molecules, the wavefunctions s+s and pz+pzmolecular orbitals become blended. The extent of this mixing (or blending) depends on the relative energies of the like-symmetry MO's.

For homodiatomics, bonding σ orbitals have no nodal planes between the bonded atoms. The corresponding antibonding, or σ* orbital, is defined by the presence of a nodal plane between these two bonded atoms.

Sigma bonds are the strongest type of covalent bonds, and the electrons in these bonds are sometimes referred to as sigma electrons.

The symbol σ is the Greek letter for s. When viewed down the bond axis, a σ MO resembles an s atomic orbital.

Sigma bonds in polyatomic compounds

They are obtained by head on overlap of atomic orbitals. The concept of sigma bonding is extended, albeit loosely, to describe bonding interactions involving overlap of a single lobe of one orbital with a single lobe of another. For example, propane is described as consisting of ten sigma bonds, one each for the two C-C bonds and one each for the eight C-H bonds. The σ bonding in such a polyatomic molecule is highly delocalized, which conflicts with the two-orbital, one-bond concept. Despite this complication, the concept of σ bonding is extremely powerful and therefore pervasive.

Sigma bonds in multiple-bonded species

Compounds that feature multiple bonds, such as ethylene and chromium(II) acetate, have sigma bonds between the multiple bonded atoms. These sigma bonds are supplemented by π-bonds, e.g. in the case of ethylene, and even δ-bonds, e.g. in the case of chromium(II) acetate.

Sigma bonds in organic molecules

Organic molecules are often made up of one cyclic compound or more, such as benzene, and are often made up of many sigma bonds along with pi bonds. According to the sigma bond rule, the number of sigma bonds in a molecule is equivalent to the number of atoms plus the number of rings minus one.

Nb σ = Nb atoms + Nb rings - 1

This can easily be concluded by realizing that the creation of bonds between atoms that are not connected in a ring requires the same number of atoms minus one (such as in hydrogen gas, H2, where there is only one sigma bond, or ammonia, NH3, where there are only 3 sigma bonds), and that rings do not obey this rule (such as benzene rings, which have 6 sigma bonds within the ring for 6 carbon atoms).



From Yahoo Answers

Question:

Answers:A sigma bond is formed from the head-to-head overlap of two atomic orbitals. Therefore, the shared molecular orbital lies directly between the nuclei of the bonding atoms. A pi bond is formed by the side-to-side overlap of two atomic orbitals. The shared molecular orbital therefore exists above and below the imaginary line between the atoms, but not directly between the atoms. A single covalent bond is always a sigma bond. A double bond is one sigma bond and one pi bond, while a triple bond is one sigma bond and two pi bonds.

Question:I've seen them but in my review book, but I don't know what it's talking about. I'm studying for SAT II chemistry.

Answers:No. A pi bond is 1/2 of a double bond. But let's start at the beginning. A sigma bond is the end-to-end overlap of the bonding orbitals, usually hybrid orbitals. The sigma bond is a single bond. A pi bond is the side-to-side overlap of unhybridized p-orbitals. A pi bond, along with a sigma bond form a double bond. A triple bond is two pi bonds and a sigma bond. Take a look at this diagram showing a sigma bond and a pi bond, forming a single bond and a double bond. http://metafysica.nl/ethylene01.jpg

Question:

Answers:A pi bond is the sideways over lap of orbitals. A sigma bond is the lobe to lobe (head on) overlap of orbitals. Though it depends on hydridization of orbitals, generally the sideways overlap of p-orbitals will result in the formation of a pi bond, while the sigma bond will have head-on overlap of the s-orbital (or the hydridized orbitals). Thus, it can be generalized that molecules with single bonds will be those with sigma bonds, molecules with double bonds will have a sigma AND pi bonds, and molecules with triple bonds have a sigma, and TWO pi bonds. Hope this helps.

Question:

Answers:Sigma bond is formed due to overlapping of orbitals along the internuclear axis. It is cylindrically symmetrical. Wheras, the pi bond is formed due to lateral overlapping If there is a single bond between two atoms, that is sigma bond. If u have a double bond, then one is sigma bond and the other is pi bond. Like wise, in a triple bond, one is sigma remaining two are pi bonds. Always, there is only one sigma bond between two atoms

From Youtube

Sigma and Pi Bonds :Basic information about Sigma and Pi bonds ie the different types of orbital overlappings.

sigma and pi bonds :Chemistry help! My response to, "What's a sigma and a pi bond? How can I identify them in a chemical structure?"