Final answer:
The subatomic particle used to identify an atom is the proton (option B), as it determines the atomic number and defines the element. Isotopes differ in the number of neutrons in the nucleus. Quarks were discovered in protons and neutrons, using electrons in experiments.
Step-by-step explanation:
The subatomic particle commonly used to identify an atom, whether it is considered stable or not, is the proton. Protons, found in the nucleus of atom, have a positive charge and are used to determine the atomic number, which defines the element.
Atoms that vary in the number of neutrons found in their nuclei are called isotopes. This variation does not affect the atomic number but does affect the atomic mass.
The particles launched into the proton during the original discovery of quarks were electrons. Quarks were originally discovered in the proton and the neutron, not in the electron.
Protons and neutrons, which make up the nucleus, are referred to collectively as nucleons. The neutron is a neutral particle similar in mass to the proton but slightly heavier and, along with protons, binds together in the nucleus by the strong nuclear force. In contrast, electrons are leptons, which have a negative charge and are much lighter than nucleons.