American Chemical Society (ACS) Chemistry Test 2025 – 400 Free Practice Questions to Pass the Exam

In the nucleus of an atom, protons and neutrons are the two types of particles present. Protons carry a positive charge, while neutrons are neutral and have no charge. Together, these particles account for the majority of an atom's mass and are key to determining the atomic number and overall identity of an element. The presence of protons in the nucleus defines the element, and neutrons contribute to the stability of that nucleus.

Other subatomic particles, like electrons, are found in the electron cloud surrounding the nucleus, not within it. Electrons are negatively charged and are involved in chemical bonding and reactions, but they do not reside in the nucleus itself. Positrons, which are the antiparticles of electrons, are also not normally found in the nucleus; they are typically generated in specific particle interactions or decay processes. Thus, the most accurate characterization of the nucleus is that it contains protons and neutrons.