6.2.1 JJ Thomson and the Electron

In this series of lessons, we explore a major paradigm shift in our understanding of matter and the fundamental nature of the atom.

We begin with J.J. Thomson’s discovery of the electron, progress to Ernest Rutherford’s revolutionary nuclear model, and conclude with James Chadwick’s discovery of the neutron—which appeared to complete the classical atomic model... or so they thought.

JJ Thomson

Following the development of the voltaic pile (battery) in the early 19th century, scientists began testing various materials to determine their electrical conductivity.

However, testing gases proved difficult until the invention of the Geissler tube in 1855, which made it possible to create a sealed glass tube containing gas at low pressure. In 1859, Julius Plücker successfully sealed electrode wires directly into these vacuum tubes, allowing electric current to pass through the partial vacuum.

Physicists observed that at specific low pressures, a glowing beam appeared inside the tube. This sparked a central debate: was this ray a form of electromagnetic radiation, as James Clerk Maxwell’s theories suggested, or was it a stream of moving charged particles?

Prior to 1897, the atom was considered the fundamental, indivisible unit of matter—in fact, the word atom comes from the Greek atomos, meaning "uncuttable." However, in 1897, J.J. Thomson conducted a landmark series of experiments using crossed electric and magnetic fields. He discovered that cathode rays were composed of previously unknown subatomic particles, which we now know as electrons. Thomson’s discovery revolutionized our understanding of atomic structure and paved the way for modern quantum mechanics

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