Particles exert attractive and repulsive forces on each other, mostly from their electrical charges, in part from other forces which control atomic behaviour.
There is a limited number of fundamental particles over and above the standard electron, neutron and proton, which set is all that most people know.
Most of chemistry can be explained with no more than the proton, neutron and electron. When it comes to physics, the behaviour of atoms needs more structure.
When particles collide at high speed, we can learn a great deal from the careful study of the fragments that are thrown off, and their energies.
Mesons are of medium mass, between the size of an electron and a proton, and they are very unstable, medium-mass elementary particles with short life spans.
Matter exists also in the form of antimatter, and when it comes in contact with ordinary matter, the two will annihilate each other, becoming energy.
In 1873, Johannes van der Waals wrote about intermolecular forces in fluids, and introduced the idea of weak attractive forces between molecules.
In 1930, Fritz London explained van der Waals forces in terms of their being caused by the interacting fluctuating dipole moments between molecules.
In 1911, Victor Hess discovered high altitude radiation from space after ascending in a balloon. At this time, cosmic rays were referred to as 'Hess rays'.
In 1912, Victor Hess used more ascents to show that the ionization of air increases with altitude indicating the existence of some form of cosmic radiation.
In 1927, Eugene Wigner concluded that parity is conserved in a nuclear reaction, that the laws of physics should not distinguish between right and left.
In 1958, Yang and Lee showed that, contrary to Wigner, certain types of reaction involving the weak nuclear force, such as beta decay, do not conserve parity.
The Standard Model says that there are hundreds of particles, but that these are all made up of various combinations of six quarks and six leptons.
In 1924, Edward Appleton demonstrated the presence of the ionosphere when he used radio ranging to measure the distance to the Heaviside layer.
The F-layer or Appleton layer (after Sir Edward Appleton) is a layer of ions about 200 km above the Earth by day, and 300 km above the Earth at night.
The Appleton layer reflects radio waves at frequencies up to about 50 MHz, and so allows radio signals below that frequency to travel around the world.
Ernst Rutherford predicted that there must be a neutron as early as 1920, but finding it was harder. Chadwick did not detect one experimentally until 1932.
In 1931, Wolfgang Pauli suggested that the neutrino could explain both the missing energy and spin in weak nuclear decay, starting the search for neutrinos.
In 1932, Werner Heisenberg suggested that nuclei are made of protons and neutrons, which would explain why there are isotopes, when the neutron number varies.
In 1923, Arthur Compton discovered the Compton effect which confirmed photons as particles. Compton and Debye provided the theory of Compton effect.
A lepton is a light-weight charged or uncharged particle: each with an anti-particle. They are the electron, the muon, the tau, and their associated neutrinos.
Within the nucleus, two main forces operate: the repulsion of the positively charged protons, and the strong nuclear force which pulls them together.
In 1934, Pavel Cherenkov discovered that what is now Cherenkov radiation was caused when very fast particles entered an optically dense medium.