The theory of atomic structure, as we currently understand it, is based on quantum mechanics and provides a framework for understanding the behavior of individual atoms and their constituents (electrons, protons, and neutrons). This theory has been incredibly successful in explaining a wide range of phenomena and has stood up to rigorous experimental testing.
However, as the number of atoms increases and we move towards larger and more complex systems, the theoretical and computational methods used to describe atomic structure become increasingly challenging. Analyzing the behavior of large ensembles of atoms, such as molecules or extended materials, requires more sophisticated approaches, such as quantum chemistry or solid-state physics.
There is no specific threshold or particular point in mass accumulation where a distinct theory for multiple atomic structures is required. Instead, the transition to a different theory happens gradually as the complexity and size of the system increase. For example, when studying molecules, we often employ methods like molecular orbital theory or density functional theory, which build upon the principles of quantum mechanics but incorporate additional approximations and techniques to handle the complexity.
Similarly, when dealing with macroscopic materials, such as crystals or solids, concepts from solid-state physics are used to describe their properties, which involve considering interactions between many atoms and their collective behavior.
In summary, the theory of atomic structure remains valid for individual atoms, but as systems grow in size and complexity, additional theoretical frameworks and computational methods are necessary to accurately describe and understand their properties. The transition to these new theories occurs gradually and depends on the specific characteristics of the system under study.