Which statement describes a concept included in the wave-mechanical model of the atom?
(1) Protons, neutrons, and electrons are located in the nucleus.
(2) Electrons orbit the nucleus in shells at fixed distances.
(3) Atoms are hard, indivisible spheres.
(4) Electrons are located in regions called orbitals.
Explanation: Wave mechanical model states that electrons are located outside of nucleus in regions called orbitals. Nucleus contains protons and neutrons only, and atoms are made of mostly empty space.