Question
Fri August 10, 2012 By:
 

what are orbitals and what are shells and difference between them?

Expert Reply
Sat August 11, 2012
The "shell" is the principal quantum number and has the same number as the period in the periodic table in which that shell is being filled. So the first period (H and He) is filling shell no 1, the second period (Li, Be etc) is filling shell no 2 and so on.

Within each shell you have orbitals s, p, d, etc There is one s-orbital per shell, three p-orbitals per shell, five d-orbitals per shell and so on. Each orbital can hold up to a pair of electrons.

What the orbital is (at least for A level purposes) is really just a region of space of a certain shape where you are most likely to find the electron(s) it contains. In the case of the s-orbital, that's basically a sphere centred on the nucleus. In the case of the p-orbitals, they look a bit like dumbells aligned along the x-, y- and z-axis with the nucleus at the origin. The shapes of the d and higher orbitals are a bit more involved, but you don't need to know that for A level
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