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 concluded that most of the space inside an atom is empty. A few, however, were scattered at large angles and a small number bounced back, which confirmed that all of the positive charge and most of the mass of an atom reside in a tiny central core called the nucleus.

Rutherford also postulated that an atom has as many electrons, located in the empty space around the nucleus, as needed to counterbalance the positive charge of the nucleus and maintain the electrical neutrality of the atom.

Subsequently, Rutherford himself discovered protons as the positively charged particles in the nucleus, and James Chadwick discovered neutrons as the electrically neutral particles in the nucleus.

Charge and mass are two fundamental properties of subatomic particles.

The charge of the proton and the electron are equal in magnitude but opposite in sign. In relative units, the electron is assigned a charge of −1 and the proton is assigned a charge of +1. The neutron has no charge.

The mass of a proton is 1.673 × 10^–27 kg, while the mass of an electron is 9.1 × 10^–31 kg, and the mass of the neutron is 1.675 × 10^–27 kg.

A more common unit to express these extremely small masses is the atomic mass unit, or amu. 1 amu is equal to 1.66 × 10^–27 kg, or one-twelfth the mass of a carbon atom containing six protons and six neutrons. Thus, the mass of a proton or neutron is approximately 1 amu. Electrons have an almost negligible mass of 5.5 × 10^–4 amu.