Atomic Structure Explained: Subatomic Particles & Key Models

Dalton's theory, which posited atoms as indivisible particles, was eventually superseded by experiments revealing the electrical properties of matter. These groundbreaking studies demonstrated that atoms themselves contain smaller, subatomic particles responsible for electrical behavior: the electrons.

Early Atomic Theories and Electron Discovery

Thomson and the Electron

Towards the end of the 19th century, J.J. Thomson conducted groundbreaking experiments using cathode ray tubes (discharge tubes with a positive and a negative pole across which an electric current passed). He discovered a negatively charged particle within the atoms of chemical elements, which he named the electron. When an electrical discharge occurred between the plates, a beam of light appeared. This beam, attracted by the positive plate, clearly consisted of negatively charged electrical particles. These particles possessed mass, as evidenced by their ability to make a small paddle wheel spin.

Millikan's Electron Measurements

In the early 20th century, Robert Millikan accurately measured the mass and charge of the electron. Since matter typically appears electrically neutral, despite exhibiting electrical properties in specific situations, it was deduced that each atom must contain both positive and negative charges to balance each other.

Discovering the Nucleus and Other Particles

Rutherford and the Proton

Ernest Rutherford later discovered the proton, a positively charged particle. While the electron carries a negative charge, the proton's mass is significantly greater than that of an electron.

Chadwick and the Neutron

James Chadwick discovered a third subatomic particle: the neutron. This particle has no electric charge but possesses a mass approximately equal to that of a proton.

Evolution of Atomic Models

Thomson's Plum Pudding Model

J.J. Thomson's experiments revealed tiny, negatively charged particles (electrons) within atoms. Based on this, he proposed that the atom was a sphere of uniformly distributed positive charge, with electrons embedded within it, much like plums in a pudding.

Rutherford's Nuclear Model

Ernest Rutherford's gold foil experiment demonstrated that atoms are not solid, as previously thought, but are mostly empty space with a tiny, dense, positively charged nucleus at their center. He deduced that electrons orbit this nucleus, much like planets around a sun.

Bohr's Planetary Model

Niels Bohr proposed a revolutionary atomic model where electrons orbit the nucleus in specific, quantized energy levels or shells.

Atomic Structure and Terminology

Defining Atoms

Every atom belongs to a specific chemical element, represented by a unique symbol (e.g., ^A_ZX). The symbol is typically derived from the element's Latin name and may include a second letter if multiple elements share the same initial. The atomic number (Z) indicates the number of protons in the nucleus. The mass number (A) represents the total number of protons plus neutrons.

Isotopes Explained

Isotopes are atoms of the same element (same atomic number, Z) that have the same number of protons but differ in the number of neutrons. Consequently, they have the same Z but different mass numbers (A).

Ions: Charged Atoms

Ions are atoms or groups of atoms that have lost or gained one or more electrons, thereby acquiring a net positive or negative electric charge. When an atom loses electrons, it acquires a positive charge and becomes a cation. Conversely, when an atom gains electrons, it acquires a negative charge and becomes an anion.