Subatomic Particles

Dalton’s atomic theory naturally led to the question of whether atoms themselves are divisible. A series of subsequent experiments demonstrated that an atom is in fact made of three fundamental particles called the subatomic particles – electrons, protons, and neutrons.

Electrons were discovered by J.J. Thomson, when he observed a beam of charged particles traveling in a straight line towards the positive electrode in a cathode ray tube and was able to deflect these beams using electric and magnetic fields. Thomson concluded that these particles were negatively charged, low mass constituents of an atom. These particles were later given the name electrons.

Robert Millikan determined that an electron has a charge of –1.6 × 10^–19 coulombs, where the coulomb is the SI unit of charge.

With the discovery of electrons, atoms were no longer viewed as indestructible, and models were proposed to describe the atomic structure.

The most successful was the nuclear model by Ernest Rutherford — based on his famous gold foil experiment, in which he directed positively charged alpha-particles at a thin sheet of gold foil. As most of the alpha particles passed through the gold foil undeflected, he c