The third type of hybridisation commonly encountered in organic chemistry is sp. By extrapolation, it is obvious that this involves the use of one s orbital, and one p orbital. Let us have a look at an sp hybridised carbon atom:
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In this diagram, there are two unhybridised p orbitals, and two sp orbitals. The sp orbitals are separated by and angle of 180°, and the two p orbitals are perpendicular to each other, and also to the line through the two sp orbitals. Note also, that the sp orbitals do not extend as far out as the sp3 or sp2 orbitals. This we should expect, as there is more s type character in the sp orbital than there is in an sp3 for example (a half in sp compared to a quarter in sp3 if you like). This means that the orbital is more spherical in nature, and less of a dumbell shape like the p orbitals.
If we imagine two sp hybridised CH fragments, they would look something like this:
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From this diagram, we can see that one strong sp-sp sigma bond will form in the line of the molecule (ethyne in this case), and that two pi bonds will form around it (above and below, and in front and behind):
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This model fits well with observations such as ethyne is linear, and again, there is no rotation about its axis. This combination of two pi bonds and one sigma bond is called a triple bond.