What is the nucleus of an atom?

• A. The center of an atom containing 99.9% of its mass and comprised of neutrons and protons held together by nuclear force
• B. The region where electrons are most densely located
• C. The outermost shell of electrons responsible for bonding
• D. A cloud of quarks surrounding the atom

Answer: (A)

The nucleus is the tiny, dense center of an atom where almost all of its mass resides. It is made of protons, which carry positive charge, and neutrons, which have no charge, bound together by the strong nuclear force that acts at very short ranges. Because electrons are far lighter, the atom’s mass is concentrated in the nucleus even though the electron cloud takes up most of the atom’s volume. The strong force between nucleons overcomes the electrostatic repulsion between protons, keeping the nucleus intact. The surrounding region where electrons are found—the electron cloud or shells—determines bonding and chemical behavior, not the nucleus. A cloud of quarks would imply a separate surrounding structure, but quarks are confined inside protons and neutrons, which compose the nucleus itself.