Consider the neutral helium atom (He) and the singly-ionised helium ion (He+). Both species contain the same nucleus but differ in the number of bound electrons.
a. (i) Give one reason why the Bohr model successfully predicts the spectral lines of He+ but fails for neutral He.
b. (ii) Describe qualitatively how electron–electron interaction alters the potential energy experienced by an electron in a multi-electron atom and thereby invalidates the single-electron orbit picture.
c. (iii) State any two features introduced by the quantum-mechanical model that overcome the difficulty mentioned in part (ii).