Which particle is NOT found in the nucleus of an atom?

In an atom, the nucleus is the central core that contains protons and neutrons. Protons are positively charged particles, while neutrons have no charge and are neutral. Both of these particles are essential for determining the mass of the atom and play a crucial role in the atom's stability and identity, as the number of protons in the nucleus defines the element.

Electrons, on the other hand, are negatively charged subatomic particles that orbit the nucleus in various energy levels or shells. They are not located within the nucleus itself, and their primary role is in chemical bonding and interactions, rather than contributing to the mass of the nucleus.

This distinction highlights why the presence of electrons outside the nucleus makes them the particle that is not found in the nucleus, confirming the accuracy of the provided answer.