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Because sigma bonds overlap head on and pi bonds overlap side by side. It has to do with orbital configuration. In today's lecture, the C2H4 molecule had a pi bond where the double bond was because the left over unhybridized orbital at the 2p level was oriented at 90 degrees outward, resulting in a side by side pi bond. Single bonds are always sigma because the unpaired electrons always overlap head-on. The addition of a double or triple bond is a result of unpaired electrons in these unhybridized orbitals, resulting in side by side overlaps.
When two atoms bond head on they create a sigma bond. When there's a double bond, the first bond is a sigma bond, but the second bond of the double bond can't face the other atom head on. Instead, it overlaps side by side as a result of repulsion from the first bond. This is the reason that a single sigma bond is much stronger than a singe pi bond.
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