What is an electron shell?

An electron shell refers to a region around the nucleus of an atom where electrons are likely to be found. These shells are associated with specific energy levels, and they help organize electrons around the nucleus according to their energies. Each shell can hold a certain maximum number of electrons based on its energy level, defined by principles of quantum mechanics.

This concept is fundamental in understanding atomic structure and behavior. The arrangement of electrons within these shells influences an atom's chemical properties and bonding behavior, making it a critical aspect of chemistry. The other options do not accurately define an electron shell, as they either mischaracterize the components of an atom or refer to unrelated concepts in nuclear physics.