ATOM

The fundamental unit of a chemical substance is called an atom. The word is derived from the Greek atomos, meaning ''uncuttable''. Atoms are extremely small neutral particles.

John Dalton in 1804 suggested the atomic theory of matter which indicates the existence of atoms. He suggested that atoms are extremely small indivisible particles. But atoms are no longer considered as indivisible. They are composed of subatomic particles like electrons, protons, neutrons, etc.

In an atom, a small, heavy nucleus is surrounded by a relatively large, light cloud of electrons. The nucleus consist of protons and neutrons which are called the nucleons. They are made up of even smaller particles. i.e; the elementary particles.

The elementary particles are fundamental objects which do not have a measurable internal structure. They are classified according to their spin as,

• FERMIONS (half integer spin)
• BOSONS (integer spin)

1. Fermions

They are particles with a half integral spin and anti-symmetric wave functions such as protons and electrons. They carry charge and spin angular momentum. They are classified as,

• QUARKS
• LEPTONS

Quarks

Quarks are one of the two basic constituents of matter. These are of six types:- up, down, top, bottom, charm, strange. The various types of quarks combine together in a specific way to form protons and neutrons.

Proton = 2 up quarks + 1 down quark

Neutron = 1 up quark + 2 down quarks

The antiparticles of quarks are known as anti quarks. Groups of quarks are called hadrons.

Gluons helps quarks to bind together and are indirectly responsible for binding of protons and neutrons in the atomic nucleus.

Leptons

It is the second basic constituent of matter which include the electrons and neutrino. Their respective antiparticles are known as anti leptons. They are of six types indicated below with their corresponding antiparticle.

• Electron and Positron
• Electron neutrino and Electron antineutrino
• Muon and Antimuon
• Muon neutrino and Muon antineutrino
• Tau lepton and Antitauon
• Tau neutrino and Tau antineutrino

2. Bosons

They are particles with integral spin and symmetric wave functions such as photons. They are classified as :

• Photon (spin=1)
• W boson (spin=1)
• Z boson (spin=1)
• Gluon (spin=1)
• Graviton (spin=2, existence not confirmed)
• Higgs boson (spin=0, existence not confirmed)