In physical chemistry, the basic principles are those that govern the behavior of matter and energy on a atomic and molecular scale. These principles include the laws of thermodynamics, quantum mechanics, and statistical mechanics. They form the foundation upon which all of physical chemistry is built.
The first principle is the law of conservation of energy. This law states that energy can neither be created nor destroyed, but only converted from one form to another. The second principle is the law of conservation of momentum. This law states that momentum must be conserved in any closed system (one where there are no external forces acting on it). The third principle is the law of conservation of angular momentum. This law states that angular momentum must be conserved in any closed system as well.
The fourth principle is the uncertainty principle. This principle states that certain properties of particles (such as position and momentum) cannot both be known with absolute certainty at the same time. The fifth principle is the Pauli exclusion principle. This principle states that two identical particles (such as electrons) cannot occupy the same quantum state simultaneously.
These five principles form the foundation upon which all of physical chemistry is built. Without them, our understanding of how matter and energy behave would be greatly limited.