Which of the following pairs of subatomic particles have an attractive interaction that does not involve the strong nuclear force? (A) Electrons and protons, (B) quarks and quarks, (C) protons and neutrons, (D) protons and protons, or (E) neutrons and neutrons.
An atom is made up of protons, neutrons, and electrons. An electron has a negative charge, a neutron has no charge, and a proton has a positive charge. Protons and neutrons are composed of quarks. Quarks are subatomic particles that are fundamental constituents of matter.
Let’s consider the forces that exist between particles in the nucleus. As protons have a positive charge, they repel each other as like charges repel and should force the atom apart. But nuclei can be stable upwards of a billion years. So there is an attractive force that holds the nucleus together. This attractive force is called the strong nuclear force or the residual strong force. The strong nuclear force is defined as the attractive force that holds protons and neutrons together in an atomic nucleus.
The question asks, which particles have an attractive interaction that does not involve the strong nuclear force. Since the strong nuclear force holds protons and neutrons together, we can rule out option (C) as the answer. But the strong nuclear force doesn’t just affect pairs of protons and neutrons; it affects pairs of protons and protons and neutrons and neutrons. So we can also rule out options (D) and (E). Furthermore, the strong nuclear force binds quarks together. Thus, we can rule out option (B), quarks and quarks.
The only option left is electrons and protons. These species have opposite charges, so the attraction between them is electrostatic; it does not involve the strong nuclear force. So the answer to the question “Which of the following pairs of subatomic particles have an attractive interaction that does not involve the strong nuclear force?” is (A) electrons and protons.