Hybridised orbitals

What determines which type of hybridisation (sp3/sp2/sp) a molecule will take? Methane/ethylene/acetylene all have the same electron configuration but undergo different different types of hybridisation- why?




It's a pretty long topic and you would like to have a look at [this article](http://en.wikipedia.org/wiki/Orbital_hybridisation). In short I can present this from the same link, also some explanation at the last:


![enter image description here](https://i.stack.imgur.com/cGqBG.png)


Methane $\ce{CH4}$, Ethylene $\ce{H2C=CH2}$ and acetylene $\ce{HC#CH}$ all have same electronic configuration but the number of unhybridised orbitals( and of course the hybridised orbitals) are different in each case due to formation of $\pi$ bonds.


 4