Resources | Subject Notes | Physics
This section explores the structure of the nucleus and the relationship between the number of nucleons and the mass of an atom.
The nucleus is the dense, central core of an atom. It contains protons and neutrons, collectively known as nucleons.
Protons are positively charged particles. The number of protons in an atom determines the element. This number is called the atomic number (Z).
Neutrons are electrically neutral particles. They contribute to the mass of the nucleus and help to stabilize it.
The total number of nucleons in the nucleus is the nucleon number (A), which is the sum of the number of protons and neutrons: A = Z + N, where N is the number of neutrons.
The mass of a nucleus is not simply the sum of the masses of its individual protons and neutrons. This is because of mass defect and binding energy.
Mass defect is the difference between the actual mass of a nucleus and the sum of the masses of its constituent nucleons when they are free. This mass defect is due to the fact that some mass is converted into binding energy that holds the nucleus together.
The relative mass of a nucleus is often expressed in terms of atomic mass units (amu) or Daltons (Da). One amu is defined as 1/12th of the mass of a carbon-12 atom.
The relative mass of a nucleus is approximately proportional to the nucleon number (A). This means that as the number of nucleons increases, the mass of the nucleus increases.
The mass defect is small but significant. It represents the energy that is released when nucleons combine to form a nucleus. This energy is known as the binding energy.
| Concept | Definition |
|---|---|
| Nucleon | A proton or a neutron in the nucleus. |
| Atomic Number (Z) | The number of protons in an atom's nucleus. |
| Neutron Number (N) | The number of neutrons in an atom's nucleus. |
| Nucleon Number (A) | The total number of nucleons in the nucleus (A = Z + N). |
| Mass Defect | The difference between the actual mass of a nucleus and the sum of the masses of its constituent nucleons. |
| Binding Energy | The energy released when nucleons combine to form a nucleus. |
The relationship between the nucleon number (A) and the relative mass is generally linear, indicating that the mass of the nucleus increases with the number of nucleons. The mass defect quantifies the energy associated with this binding.
For a deeper understanding, explore concepts like isotopes, nuclear stability, and the strong nuclear force.