(a) Cite two important quantum-mechanical concepts associated with the Bohr model of the atom. (b) Cite two important additional refinements that resulted from the wave- mechanical atomic model.

Two important quantum-mechanical concepts associated with the Bohr model of the atom are: 1. Quantization of energy levels: In the Bohr model, electrons can only exist in specific energy levels, and they cannot occupy levels between these quantized states. Each energy level corresponds to a stable electron orbit around the nucleus. The quantization of energy levels arises due to the standing-wave nature of the electron orbitals, which are described by specific wavefunctions. 2. Photon emission and absorption: When an electron transitions between allowed energy levels within an atom, it either absorbs or emits a photon. The energy of the emitted or absorbed photon corresponds to the energy difference between the initial and final energy levels. This concept is represented mathematically by the equation: ΔE = hf, where ΔE is the energy difference between the levels, h is Planck's constant, and f is the frequency of the absorbed or emitted photon.

Two important additional refinements that resulted from the wave-mechanical atomic model are: 1. Electron Cloud Distribution: In the wave-mechanical atomic model, electrons cannot be precisely located at any particular point around the nucleus. Instead, their positions are described by a probability distribution called an electron cloud. This distribution shows regions where the electron is most likely to be found, based on its wavefunction. This concept replaces the fixed electron orbits of the Bohr model and takes into account the particle-wave duality of electrons. 2. Quantum numbers and atomic orbitals: The wave-mechanical atomic model introduced the concept of quantum numbers, which are used to describe the various properties of atomic orbitals. There are four quantum numbers: principal quantum number (n), azimuthal quantum number (l), magnetic quantum number (m_l), and electron spin quantum number (m_s). These quantum numbers are used to describe the energy, shape, orientation, and spin of atomic orbitals and associated electron clouds. This improvement allows for a more detailed understanding of the electron behavior and chemical bonding properties of atoms.