It is worth noting that the ancient Greeks also had the concept of "molecule", which was used to represent small groups composed of several atoms, and different groups of atoms corresponded to different physical properties. This idea of the ancient Greeks was almost identical to that of modern physicists and chemists.
With the concept of atom, there will be many "children" below, such as photons representing the smallest unit of light, electrons representing the smallest unit of electricity and so on. It is true that after atoms, photons and electrons are widely studied and discussed in physics.
Here is the concept of "quantum". Quantum was first proposed by Planck to represent the smallest unit of light energy, hν. Now we know that hν is the energy of photons.
Atoms are electrically neutral as a whole, and electrons are components of atoms, which are negatively charged, so there must be positively charged components in atoms, that is, protons, which are positively charged, with exactly the same electricity as electrons. But the mass of protons is nearly 2000 times greater than that of electrons.
Protons are located in a very small area at the center of atoms, which we call nuclei. Besides protons, there are neutrons. Neutrons and protons have almost the same mass, but neutrons are uncharged. Neutrons and protons form nuclei together. It may be that several protons and several neutrons form a nucleus together.
Neutron instability. Physicists find that neutrons in the nucleus will decay into protons and electrons, and accordingly the number of protons in the nucleus will be+1. This process is called beta decay, where beta particles refer to electrons released from the nucleus. Physicists found that energy seems to be non-conservative in β decay, which led Pauli to guess that there is another neutral particle with almost zero mass in β decay that takes away some energy, that is neutrinos.
Physicists' research on cosmic rays has discovered positrons, which are particles with exactly the same mass and charge as electrons, but with opposite charges. Positron can be well explained by Dirac's relativistic quantum mechanics.
The concepts of boson and fermion were put forward by physicists when they studied the physical system composed of several particles. According to quantum mechanics, the same kind of microscopic particles have exactly the same charge, mass, spin and so on. They are indistinguishable. If we try to distinguish microscopic particles by "trajectories" like classical particles, it is an impossible task, because there is a position-momentum uncertainty principle for quantum mechanics. It is impossible for us to determine the position and momentum of microscopic particles at the same time with infinite accuracy, that is to say, infinitely fine orbits do not exist. In principle, we can't distinguish two microscopic particles of the same kind.
This requires a wave function to describe the particle system, that is, for the exchange of two particles, the wave function is either symmetric or antisymmetric. If the wave function is symmetrical, the corresponding particle is a boson, and vice versa.
Bosons in the above particles are photons, and the rest: electrons, positrons, protons, neutrons and neutrinos are called fermions.
It is worth mentioning that physicists have now put forward the concept of supersymmetry, thinking that every elementary particle as a fermion will have a supersymmetric boson corresponding to it, and every boson will have a supersymmetric fermion corresponding to it. For example, the supersymmetric companion of electrons is superelectrons, and the supersymmetric companion of photons is photons, and so on.
But unfortunately, we have not found any supersymmetric particles so far.
According to the standard model of particle physics (pictured), we can classify particles through interaction.
Quarks participate in strong interactions, including: U (upper), C (charm), T (upper), D (lower), S (odd) and B (lower);
The weak interactions are E (electron), μ meson, τ, electron neutrino, μ meson neutrino and τ neutrino.
These are fermions.
Gauge bosons are dielectric particles that transfer interactions: gluons transfer strong interactions, photons transfer electromagnetic interactions, and Z and W+- bosons transfer weak interactions. Of course, we can also say that gravitons transfer gravity, but the quantization of gravity has not been completed and gravitons have not been discovered.
Finally, there is a scalar boson-Higgs boson, which explains why some elementary particles have mass.