What does the nucleus of an atom contain?

The nucleus of an atom is composed of protons and neutrons. Protons are positively charged particles, while neutrons have no charge. This composition is fundamental to the identity of an element and contributes to its atomic mass. The presence of protons determines the atomic number, which defines the element itself. Neutrons, although electrically neutral, play a crucial role in stabilizing the nucleus, as they help to reduce the repulsive forces between the positively charged protons.

In contrast to the nucleus, electrons are found in the electron cloud surrounding the nucleus, and they are not part of the nucleus itself. Therefore, options mentioning only protons, only neutrons, or including electrons in the nucleus do not accurately reflect the correct structure of an atom's nucleus.