A deuterium is (a) an electron with a positive charge (b) a nucleus having two protons (c) a nucleus containing a neutron and two protons (d) a nucleus containing a neutron and a proton.

Isotopes are variants of a particular chemical element which have the same number of protons but differ in the number of neutrons within their atomic nucleus. This variation results in isotopes having the same atomic number (representing the element) but different mass numbers. Take hydrogen, for example, which has three commonly known isotopes: protium, deuterium, and tritium. Protium, the most abundant isotope, has just one proton and no neutrons in its nucleus. Deuterium, the next isotope, has one proton and one neutron, giving it a heavier mass. Lastly, tritium contains one proton and two neutrons, making it even heavier than deuterium.

Understanding isotopes is crucial when discussing elements and their respective variations. Each isotope of an element exhibits nearly identical chemical behavior but can have vastly different physical properties, such as melting point and density. Notably, some isotopes are stable, while others are radioactive and can decay over time, which is an integral concept in fields such as nuclear physics and radiometric dating.

The atomic nucleus is the small, dense region at the center of an atom where protons and neutrons are found. It is the core of an atom and holds the majority of its mass. Even though the nucleus is tiny compared to the entire atom (which is mostly empty space), it plays a critical role in the identity and stability of the atom.

The number of protons in the nucleus, known as the atomic number, determines the type of element. It is this quantity that essentially defines what the element is—oxygen, hydrogen, carbon, and so on. Neutrons, the approximately neutrally charged particles, add to the mass of the nucleus without changing the fundamental character of the element. However, variations in neutron number lead to the formation of isotopes, as previously mentioned. The strong nuclear force is responsible for holding protons and neutrons together in the nucleus, counteracting the repulsive force between the positively charged protons.

Neutrons are subatomic particles that can be found in the atomic nucleus alongside protons. They have no net electric charge, hence their name, and a slightly larger mass than protons. Despite being neutral, neutrons play a vital role in the stability of an atom as they help to hold the atomic nucleus together through the strong nuclear force.

Neutrons contribute to the mass of an atom and, as part of isotopes, influence the atomic mass but not the chemical properties of an element. The presence of neutrons stabilizes the nucleus by separating the positively charged protons, which would otherwise repel each other due to their like charges. In nuclear reactions, such as fission and fusion, neutrons can be instrumental, often initiating and sustaining the chain reactions. Their neutral nature allows them to penetrate the nucleus without being deflected by the electric forces that act on charged particles such as protons and electrons.