I am really confused on hybridization and how to do the orbital drawings. For example for BeF2: The electron configuration of F is (1s^2)(2s^2)(sp^5). In my book it says that there are two electrons in the ground state of Be in 1s and 2s orbital shells. For B3, 1 electron from 2s goes to the 2p shell. How do I know that and how do I know how many electrons go in each orbital? Also how do I distinguish a sp2 and an sp3 orbital shell. I know you have to promote electrons to different orbitals but how do you promote? Is there a certain system or procedure? Also how do I distinguish localized and delocalized bonding? How do I know when one exhibits a certain type. I'm really confused. Please help asap!! Thank you; thanks!!!

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This is tough to do on a computer but here goes. First of all, I THINK you intended to write F as 1s2, 2s2, 2p5 and not sp5. Do I assume you have no trouble with BeF2. I think of it as Be= 1s2, 2s2 with 1 of the 2s electrons going to make ONE F 1s2, 2s2, 2p6 and the OTHER electron going to make ANOTHER F atom 1s2, 2s2, 2p6. That way we complete the octet for two F atoms and use the two available electrons from the one Be atom we have.
Now to B. B is 1s2, 2s2, 2p1. What happens is the promotion of one of the 2s2 to a p orbital so the final "hybridized" state is 1s2, 2s1, 2p2. First we call it hybridized because there is no difference now between the 2s and the 2p electrons; they have been rearranged, so to speak, to identical hybrid orbitals (somewhere in between a pure s and a pure p). Second, we call it sp2 by counting; we have 1 s electron and 2 p electrons so we call it simply sp2. Why didn't we promote more than 1? We have only 2 electrons in the 2s2 orbital. We promote one of them and NOT both of them. To my knowledge two are never promoted but I guess stranger things have happened. Furthermore, we know sp2 hybrids are trigonal planar so we can predict to an extent what many of the B compounds will be. Third, when we have sp3 hydbridization, that comes about because we have promoted a s electron from an element with TWO existing p electrons; for example, carbon.
C is 1s2, 2s2, 2p2. If we hybridize that, we promote one of the 2s electron to a 2p so it becomes 1s2, 2s1, 2p3 and we call it guess what? Counting we get 1 + 3 = 4 or sp3. Isn't that simple?
The energy for the promotion comes from the energy of formation of the final product. How do you know where to promote? If you have s electrons it is only logical to promote to the NEXT higher level which is a p ALTHOUGH there are some instances where it is promoted to much higher levels (Co(II) does that). Finally, you need to realize that much of this comes from experimentally determined behavior, I hope this helps but please post a follow up if anything is not a little clearer. As a parting note I should state here that BeF2 is NOT an ionic compound, as my answer above might suggest, and sp hybrids are formed abd those are linear compounds as sp would suggest.