A particle within physics is defined as an extremely small unit of matter. Despite the small size, a particle can still be assigned properties including volume and density.
Some elementary particles are widely known. These include protons, electrons and neutrons, the building blocks of an atom. All particles have energy and momentum, among other properties. Often, these properties are referred to as the particle's quantum number and are used to identify it. While all particles have mass, their mass cannot be calculated through equating the force to the product of the mass and acceleration. Instead, Einstein's equation for energy, E = mc^2, is used. The charge on a particle can be found by considering the electromagnetic forces associated with it. This charge, Q1, is related to the charge on another particle, Q2, by the following equation: F = (k)(Q1)(Q2)/r^2. In this equation, r is the distance between the two particles and k is a constant. From this equation, the force associated with the particle can also be determined.
Particles do have a limited lifespan. Through a process known as decay, a particle leaves behind two or more smaller particles. These remaining particles are called decay products. Not all particles decay, and the ones that do not are referred to as stable particles.