The wave-mechanical model of the atom is required to explain the

(1) mass number and atomic number of an atom

(2) organization of atoms in a crystal

(3) radioactive nature of some atoms

(4) spectra of elements with multielectron atoms

Answer: option (4) spectra of elements with multielectron atoms.

Explanation:

1) The spectrum is the set of electromagnetic wave frequencies emitted by the atoms.

2) Such emssions are explained by the wave-mechanical model of the atom as the release of energy (photons, which is the same that light) when an excited electron jumps into a lower energy level.

3) As per the wave-mechanical model of the atom, the electrons are not able to occupy any position around tha atom, but some specific energy levels, and it is the change of energy level (jump of the electrons from one level to other) what produces the spectrum.