Which phenomenon explains how electrons are arranged in an atom?

The arrangement of electrons in an atom is best explained by quantum mechanics. Quantum mechanics provides a framework for understanding the behavior of particles at very small scales, such as electrons and the structure of atoms. It introduces concepts such as wave-particle duality and quantization of energy levels, which are essential for describing how electrons occupy specific energy levels or orbitals around the nucleus.

In an atom, electrons do not travel in fixed paths as one might expect from classical mechanics; instead, they exist in probabilistic distributions characterized by orbitals. These orbitals are defined by quantum numbers that describe their energy and spatial orientation. This probabilistic nature and the need to incorporate wave-like behavior of electrons are fundamental aspects of quantum mechanics.

Other options, like Newtonian physics and classical mechanics, primarily apply to macroscopic objects and do not adequately describe subatomic particles. Additionally, thermodynamics focuses on the principles governing heat and energy transfer, which is not directly related to the electronic structure of atoms. Thus, quantum mechanics is the most appropriate explanation for the arrangement of electrons in an atom.