The Bohr model successfully explains the spectrum of hydrogen but fails for multi-electron atoms such as helium. Which of the following is the primary reason for this limitation?
a) The uncertainty principle prevents electrons in heavier atoms from having well-defined orbit radii, so the Bohr quantisation rule cannot apply.
b) Changes in effective nuclear charge due to shielding make the allowed energies form a continuum instead of discrete levels.
c) Electron–electron repulsion introduces additional forces that invalidate the assumption of a single Coulomb attraction used in Bohr’s derivation.
d) The nucleus of helium is so massive that relativistic corrections dominate and alter the discrete energy levels predicted by the Bohr model.