Introduction
The atoms of all elements are made up of three particles called protons, neutrons and electrons. To understand the fundamental forces existing in the universe, it is important to learn about particles, subatomic particles, and their properties. Radioactive decay of a nucleus and the nature of particles emitted in the decay are helpful in explaining nature’s most elementary reaction that involves energy and mass.
A. Quarks & Anti-quarks
In the classification of sub-atomic particles, Hadrons (Mesons and Baryons) are made up of three smaller particles called quarks. In this topic, some important properties of these subatomic particles, their antiparticles, and the law governing the properties during particle interaction, are explained.
Important Terms Explained:
Up quark, down quark, strange quark, conservation laws, charge, Baryon number and Lepton number.
B. Stable & Unstable Nuclei
The nucleus contains positively charged protons, which are closely packed together in a very small volume. According to the laws of physics, the protons exert strong repulsive forces on each other. This implies that there must be an additional attractive force similar in size to the electrostatic repulsion, which holds the nucleus together. In this topic, forces present in the nucleus, stable and unstable atoms and particles emitted in the decay of the unstable atoms are explained.
Important Terms Explained:
Electromagnetic force, strong nuclear force, radioactive decay, alpha particles, beta particles, gamma particles, neutrino and antineutrino.
C. Particle, Antiparticle & Photons
Every particle has an antiparticle according to the standard model for describing fundamental particles and interactions. For example, the positron is the antiparticle of the electron. A positron has mass identical to an electron, but has a positive charge. In this topic, properties of particles and their antiparticles are explained.
Important Terms Explained:
Electron, positron, proton, antiproton, neutron, antineutron, neutrino, antineutrino, photon, pair production and annihilation.
D. Particle Interaction
The fundamental particle interactions, also known as fundamental forces, are the interactions that do not appear to be reducible to more basic interactions. There are four fundamental interactions known to exist:
- Gravitational force
- Electromagnetic force
- Strong nuclear force
- Weak nuclear force.
The modern quantum mechanical view of the fundamental forces, other than gravity, is that the particles of matter do not directly interact with each other, but rather carry a charge and exchange virtual particles, which are the interaction carriers, or force mediators. In this article, the four fundamental interactions, exchange particles involved in these interactions and a graphical representation of these interactions, called Feynman diagrams, are explained.
Important Terms Explained:
Gravity, gravitons, electromagnetic force, virtual photons, strong nuclear force, gluons, weak nuclear force, bosons, exchange particles and Feynman diagrams.
E. Constituents of the Atom
The atoms of all elements are made up of three particles called protons, neutrons and electrons. The protons and neutrons are at the centre of the atom. The centre of the atom is called a nucleus. The diameter of the nucleus is about 
of the diameter of the atom. In this topic, these three elements that constitute the atom and their properties are explained.
Important Terms Explained:
Atom, nucleus, proton, neutron, electron, charge, mass, isotope and specific charge.
F. Classification of Particles
Subatomic particles are classified based on their mass, type of interaction and electrical charge. Classification based on the mass of the particles is the most basic way of understanding subatomic particles. Classification of subatomic particles is explained in this topic.
Important Terms Explained:
Hadrons, Baryons, Mesons, Leptons, proton, neutron, pion, kaon, electron, muon, neutrino.