Subatomic Particles and Atomic Components

Subatomic Particles

Subatomic particles refer to particles smaller than atoms. Subatomic particles include composite particles made up of smaller constituent particles, as well as elementary particles that are believed to be indivisible. In nuclear engineering, the following particles are particularly important:

• Hadrons
  • Baryons
    • Nucleons
      • Protons
      • Neutrons
• Leptons
  • Electrons
  • Positrons
  • Neutrinos

The name ‘lepton’ is derived from the Greek word ‘λεπτός’, meaning small and thin. At the time of naming, they were called this because they were thought to have smaller masses compared to other types of particles. However, the tauon, discovered in the 1970s, is a lepton with a mass close to 1.9 times that of protons and neutrons, so not all leptons are necessarily light.

Electrons & Positrons

• Rest mass: $m_e = 9.10939 \times 10^{-31} \text{kg}$
• Charge: $e = 1.60219 \times 10^{-19} \text{C}$

There are two types of electrons: $e^-$ (negatron) with a negative charge and $e^+$ (positron) with a positive charge. These two are identical in all properties except for the sign of their charge. Usually, when referring to an electron without further specification, it means a negatron.

Under certain conditions, when a positron and a negatron collide, these two electrons annihilate, emitting two photons. This process is called electron annihilation, and the photons produced are called annihilation radiation.

Image source

• Authors: Dirk Hünniger, Joel Holdsworth
• License: GFDLv1.2

Protons

• Rest mass: $m_p = 1.6726 \times 10^{-27} \text{kg}$
• Charge: + $e = 1.60219 \times 10^{-19} \text{C}$

They have a positive charge of the same magnitude as an electron.

Neutrons

• Rest mass: $m_n = 1.674929 \times 10^{-27} \text{kg}$
• Charge: $0$

They have a slightly larger mass than protons and are electrically neutral. Outside the atomic nucleus, they are unstable and decay into a proton by emitting an electron and an electron antineutrino, a process that takes an average of about 12 minutes.

Neutrinos

• Rest mass: Very small (exact value unknown)
• Charge: $0$

Originally thought to have zero rest mass, it was discovered in 1998 by the Super-Kamiokande research team in Japan that they have a very small but non-zero mass. There are several types, but in nuclear reactions, the electron neutrino and electron antineutrino are considered most important, and are usually treated as a single type without distinction.

Structure of Atoms and Atomic Nuclei

\[^A_Z X \ (\text{A: Mass number, Z: Atomic number, X: Element symbol})\]

• Atoms consist of an electron cloud and a centrally located atomic nucleus
• A neutral, non-ionized atom has the same number of electrons orbiting the nucleus as protons
• Electrons determine the chemical properties and type of element
• The atomic nucleus is composed of nucleons (protons and neutrons), which are bound together by the strong nuclear force, overcoming electrical repulsion
• Atomic number: The number of protons in the atomic nucleus, denoted by $Z$
• Total charge of the atomic nucleus: +$Ze$
• Neutron number: The number of neutrons in the atomic nucleus, denoted by $N$
• Mass number or Nucleon number: The sum of the number of protons and neutrons in the atomic nucleus. $A=Z+N.$
• Nuclide: An atomic nucleus with a specific number of protons and neutrons

Isotopes, Isobars, Isotones, and Nuclear Isomers

Classification	Definition
Isotopes	Nuclides with the same atomic number but different neutron numbers
Isobars	Nuclides with the same mass number but different numbers of protons and neutrons
Isotones	Nuclides with the same neutron number but different atomic numbers
Nuclear Isomers	Nuclides of the same type but in a metastable state due to the excitation of one or more nucleons