What term describes a region of definite energy within an atom that electrons can occupy?

The term that describes a region of definite energy within an atom that electrons can occupy is known as an energy level. Energy levels are quantized structures, meaning that electrons can only exist in specific energy states rather than a continuous range of energies. These levels are essential for understanding how electrons interact with energy and matter, which in turn dictates an atom’s chemical properties and behavior.

As electrons occupy these energy levels, they can move between them by absorbing or emitting energy, typically in the form of photons. The arrangement of electrons across these energy levels also plays a critical role in the periodic trends observed in the elements, such as electronegativity and ionization energy.

The other terms provided refer to different concepts: an electron shell is often used interchangeably with energy level but may refer more specifically to the orbitals within that level; the nucleus is the dense center of the atom containing protons and neutrons; and the atomic mass unit is a unit for measuring atomic and molecular mass. Each of these concepts is important in its own right, but they do not specifically define the regions of definite energy that electrons occupy in the manner that energy levels do.