What does the term 'quantum number' describe?

The term 'quantum number' refers specifically to the properties of atomic orbitals and electrons. Quantum numbers are derived from the solutions to the Schrödinger equation and help in defining the various properties that characterize electrons in an atom, including their energy levels, angular momentum, and orientation in space.

There are four principal quantum numbers: the principal quantum number (n), which indicates the energy level of an electron; the azimuthal quantum number (l), which defines the shape of the orbital; the magnetic quantum number (m_l), which specifies the orientation of the orbital; and the spin quantum number (m_s), which indicates the intrinsic spin of the electron. Together, these quantum numbers provide a comprehensive description of the distribution and behavior of electrons around the nucleus in an atom.

In contrast, the other options do not relate to the concept of quantum numbers. The number of atoms in a molecule is a chemical characteristic, the mass of an atom pertains to its physical properties, and the pH level of a solution is a measure of its acidity or basicity, none of which involve quantum mechanical properties. Thus, only the properties of atomic orbitals and electrons accurately align with the definition of quantum numbers.