What is the central part of an atom that contains protons and neutrons called?

The central part of an atom, which contains protons and neutrons, is known as the nucleus. This structure is vital because it holds the majority of an atom's mass due to the presence of protons, which carry a positive charge, and neutrons, which are neutral particles. The nucleus is surrounded by a cloud of electrons, which are negatively charged and exist in energy levels or shells around the nucleus.

Understanding the role of the nucleus is fundamental in atomic theory. It not only defines the identity of an element based on the number of protons it contains (the atomic number) but also determines the stability and behavior of atoms through interactions within the nucleus and between the nucleus and the electron cloud.

Other options such as the electron cloud, core, or shell do not accurately describe the central part of an atom. The electron cloud refers to the region around the nucleus where electrons are likely to be found, while the term core is more commonly used in different contexts and does not specifically refer to the atomic structure as it pertains to protons and neutrons. Shells refer to the different energy levels where electrons reside, which is entirely separate from the nucleus itself.