Question

Can 1s orbital combine with 2s orbital to form sigma bond...This is asked in ncert exercise question no. 29 (d)....
the given answer is yes...But a/c to me it shouldn't combine due to difference in energy..
Plz solve my doubt

Answer 1

I understood your doubt. The answer given in your textbook is true. 1s orbital and 2s orbital can overlap. Even 1s orbital and a 4s orbital can overlap. But we see such overlappings very rarely (for instance in lithium hydride the 1s orbital of hydrogen overalps with the 2s orbital of lithium). As you said this occurs rarely because of the higher differences in energies.
Your textbook portions are limited to some concepts. There are some things called nodes and lobes for every orbital
A 1s and 1s can combine really well as they don't have any lobes or nodes. 2p and 2p also go hand in hand because their lobes and nodes don't affect that much. But if you go into 3pand 3p, there are lobes of different polarity on the same side so beside overlapping of like polar lobes some overlapping of unlike lobes also occurs effectively cancelling out the overlapping.
No if you run into 1s and 2s, 2s is a larger and has a one spherical lobes. Even if 1s gets in pretty well that's still not much for 2s. Added that quite a good part of overlapping would be cancelled out and further the overlapping effect will be decreased due to presence of nodes.
Dont panic with the answers, you will learn more about these in higher classes. Hope this helped