2. Fundamental (Elementary) Particles

In particle physics, an elementary particle or fundamental particle is a subatomic particle with no sub structure that is not composed of any other particles. A particle containing two or more elementary particles is known as composite particle. An elementary particle can be one of two following groups; Fermions and Bosons.

Fermions are the building blocks of matter and have mass. They generally are matter particles and anti-matter particles. Fermions includes a set of quarks (up, down, charm, strange, top and bottom) and anti-quarks (antitop, antidown, anticharm, antistrange, antitop and antibottom) and another set of leptons (electron, muon, tau, electron neutrino, muon neutrino, tau neutrino) and anti-leptons (positron, antimuon, antitau, electron antineutrino, muon antineutrino and tau antineutrino).

Whereas Bosons are the force particles or force carriers that mediate interactions among fermions. Some of them have no mass. Bosons includes a set of Gauge bosons (gluon, photon, Z boson, W boson) and the Higgs boson (H).

Ordinary matter is composed of atoms which was once presumed to be elementary particle and hence the word 'atom' means 'unable to cut' in Greek. Later in 1930's, it was discovered that the atom contains more than two subatomic components and they are electrons, protons, neutrons along with the photon, which is the particle of electromagnetic radiation and also considered to be one of the gauge bosons.. Out of these particles in an atom, only electron is an elementary particle. Whereas protons and neutrons were made of quarks (up quarks and down quarks), which makes them composites particles within an atom.

There are three basic properties that describe an elementary particle and they are 'mass', 'charge' and 'spin'. Each property is assigned a numerical value. But for mass and charge, their numerical value can be zero. For example, a photon has zero mass and a neutrino has zero charge. These properties always stay the same for an elementary particle.

Mass : A particle has mass if it takes energy to increase its speed or to accelerate it. This comes from special relativity theory which shows that the energy equals the mass times the square of the speed of light. All particles with mass produces gravity. All particles are affected by gravity, even the particles with no mass like the photon.

Electric charge : Particles may have positive charge or negative charge or none. Two opposite charged particles attract each other where two same charged particles repel each other. An electron has a charge of −1. A proton has charge of +1. a neutron has an average charge of zero. Normal quarks have charge of ⅔ or ⁻⅓ .

Spin : The angular momentum or constant turning of a particle has a particular value called its spin number. Spin for elementary particles is one or ½.

Mass and charge are properties we see in our daily life, because gravity and electricity affect things that human can see and touch. But spin affects only the world of subatomic particles, so it cannot be directly observed.