Which particles make up the nucleus of an atom?

The nucleus of an atom is composed of protons and neutrons. Protons are positively charged particles, while neutrons have no charge at all. Together, these particles account for the vast majority of an atom's mass and define its identity; the number of protons determines the element's atomic number, while the number of neutrons influences the atomic mass and stability of the nucleus.

Electrons, which are negatively charged, orbit the nucleus but are not considered part of it. This distinction is crucial because the nucleus itself does not contain electrons; instead, it is surrounded by an electron cloud. Therefore, only protons and neutrons make up the nucleus, making the option that specifies these particles the correct one.