5. Why was quantum mechanics invented? a) To make the periodic table b) To explain why protons are positively charged. c) To explain the behavior of the tiny world of the atom. d) To explain why noble gases are unreactive.

The correct answer is: **c) To explain the behavior of the tiny world of the atom.** **Explanation:** Quantum mechanics was developed in the early 20th century to understand and describe the behavior of particles at atomic and subatomic levels, where classical physics fails to provide accurate predictions. It explains phenomena such as electron orbitals, energy levels, and the dual wave-particle nature of matter, which are essential for understanding the structure and behavior of atoms. - **a) To make the periodic table**: The periodic table was developed by Dmitri Mendeleev based on the properties of elements, not directly through quantum mechanics. - **b) To explain why protons are positively charged**: The positive charge of protons is related to the fundamental properties of subatomic particles, but quantum mechanics addresses a broader range of atomic and molecular behaviors. - **d) To explain why noble gases are unreactive**: While quantum mechanics helps explain the electron configurations that make noble gases stable and unreactive, this is a specific application rather than the primary reason for the invention of quantum mechanics. Quantum mechanics provides the foundational framework for modern chemistry and physics, enabling the understanding and development of various technologies such as semiconductors, lasers, and quantum computing.