[Mark S 2014]

Q. Explain Why the Emission (or Absorption) Spectra of He is more complicated than that of H.

I know that different coloured lines will be produced because He has a greater nuclear charge and thus the energies will be different than those in H. Helium obviously has an extra electron which probably contributes in some way to it being different but I don't really know how.

Any ideas on what type of answer they might be looking for here ?

[Arkcon]

Helium obviously has an extra electron which probably contributes in some way to it being different but I don't really know how.

Really?  What causes spectral lines?

[Snowfleece]

A link that might help: http://www.ifa.hawaii.edu/~barnes/ASTR110L_F05/spectralab.html

[AdiDex]

In my view..
If He would present in a molecular form then i would blame to the formation of energy band due to overlapping of molecular orbits.
But this is not the case as He present in elemental form.
So I think that
This happens due to sheilding effect of electron..
As sheilding effect of electron varies with the orbital in which it present.
Suppose a situation of an excited He atom,
first elecron is present in n=1, l=0 (1s) and second one in the n=2,l=1(2p) , so the energy level of all shells will be slightly different from the energy levels of He in the ground state.
This is only one of the excited state ...there are many other possible state.....so now you can think about it.

This change is main reason that the no. Of spectral lines are greater in case if He.

Or see it in this way suppose there is a H atom ...suppose you can observe 'n'  energy levels.
Now in He atom there are two electrons so there will be ⁿC₂ (= n(n-1)/2 )observable energy levels.

It seems logically.. I didn't read it anywhere ..so if anyone knows that my  point of view is correct or wrong.....so please tell me....

[Irlanur]

I find the question strange. I don't think you can point out one "main reason". In general, the reason why you see different lines are that it's a different system. if you want to go into some math you could say that helium has a different Hamiltonian (which is the same as saying it's a different system), with different energy eigenvalues. of course you could argue with orbitals, but these are inherently an approximation and in this case, they don't "explain" anything.

[Irlanur]

Or see it in this way suppose there is a H atom ...suppose you can observe 'n'  energy levels.

I wish you great fun if you go to Rydberg states ...

[AdiDex]

Seems good....
Aaaaaahhh not again....another incomplete theory..
I thought that in high school ,some good concepts are taught....

So what are these Hamiltonians .

Nd i know that a atom consist of infinite no. Of shells....
My point was suppose there are n states in H atoms so in He there will nC2 states in which a He atom can exist ...therefore no. Of ways of transitions between them will be greater  than that of in case of H atom.

[Corribus]

The only reason the He emission spectrum would be "more complicated" than the H spectrum would be the presence of additional spin states in the former, and all that entails. E.g., in helium the excited-states can be divided into what are sometimes known as ortho- (S=0) and para-helium (S=1). Having one electron, hydrogen has no such higher-order (non-singlet) spin states, so there are no lines between triplet states to account for in the emission spectra.

Google the Grotrian diagrams of hydrogen and helium and you can see these differences.

[Irlanur]

So what are these Hamiltonians

no worries. it's a part of the mathematical description of Quantum Physics. Nothing for High School Chemistry.