The electron, a negatively charged particle, plays a foundational role in modern science and technology. It is a fundamental component of atoms, influencing how they interact and form matter. Understanding this particle is central to comprehending the physical world. The journey to its discovery was a pivotal moment in scientific history, reshaping the understanding of matter itself.
Before the Electron
Prior to the late 19th century, the prevailing scientific view of matter was based on John Dalton’s atomic theory, proposed in the early 1800s. This theory posited that atoms were the smallest, indivisible units of matter, akin to solid, impenetrable spheres. Atoms of a specific element were considered identical in mass and properties, while atoms of different elements varied. This concept of the atom as an unbreakable particle was widely accepted in chemistry for decades.
Dalton’s model provided a framework for explaining chemical reactions and compound composition. It suggested chemical changes involved the rearrangement of indestructible atoms, not their creation or destruction. The idea that atoms possessed no internal structure remained unchallenged until new experimental evidence emerged.
The Cathode Ray Tube Experiment
The electron’s discovery began with experiments using cathode ray tubes. These were sealed glass tubes with most of the air evacuated. When high voltage was applied across two electrodes, a glowing stream, known as cathode rays, emanated from the negatively charged cathode and traveled towards the positively charged anode.
These rays traveled in straight lines, casting sharp shadows and causing the tube’s glass walls to fluoresce. Early investigations showed cathode rays could impart mechanical motion to a small paddlewheel, suggesting they possessed mass and energy.
Unveiling the Electron
British physicist J.J. Thomson began investigating cathode rays in 1897. He modified the cathode ray tube to control and observe the rays’ deflection. Thomson applied both electric and magnetic fields perpendicular to the cathode rays’ path.
He observed that cathode rays were deflected by both fields, indicating they were negatively charged particles. By balancing electric and magnetic forces to make the rays travel straight, Thomson determined their velocity. He then measured the deflection caused solely by the electric field.
Through these measurements, Thomson calculated the charge-to-mass ratio (e/m) of these particles. This ratio was significantly larger than that of any known ion, indicating the particles were much lighter than any previously known atom. He found them to be approximately 1,800 times lighter than hydrogen, the lightest atom. The charge-to-mass ratio remained constant regardless of the type of gas in the tube or the material of the electrodes, leading him to conclude these particles were universal constituents of all matter. Thomson initially called these subatomic particles “corpuscles,” but “electron” was later adopted.
The New Atomic Picture
The electron’s discovery altered the understanding of the atom, challenging Dalton’s belief in its indivisibility. Scientists now knew atoms contained smaller, negatively charged components. This necessitated a new model to explain how these particles fit within a neutral atom.
J.J. Thomson proposed his atomic model in 1904, known as the “Plum Pudding Model.” In this model, the atom was a sphere of uniformly distributed positive charge, with negatively charged electrons embedded like plums in a pudding. This arrangement allowed the atom to maintain electrical neutrality, as the positive “pudding” balanced the electrons’ negative charges. While an important first step in atomic theory, this model was later superseded.