By Robert M. Hazen, Ph.D., George Mason University
Even as scientists were confirming the existence of the atoms, they discovered another layer of complexity below atoms. Atoms are made of still smaller particles. Studies of electricity suggested there needed to be something that you could transfer, that carried charge. It was the work of J.J. Thomson, Ernest Rutherford among others that led to advancements in our knowledge of atomic structure.
Vacuum Tube Technology
Sparks occur when a high voltage causes electricity to jump across a gap in the air. Air, it turns out, is a very poor conductor. If the spark-producing device is sealed in a vacuum, these charges flow more steadily, more easily without sparks; just a steady stream of this electrical current flowing from one place to another. This flow of electricity is the principle behind every vacuum tube. And these vacuum tubes dominated electricity, all electronics, before 1960.
The idea of a vacuum tube is very easy. What you do is you have a filament that you heat to a high temperature. That filament, therefore, is glowing hot, and you put a negative voltage on the filament, and you put a positive voltage someplace else in the tube. This means that when electrons will be boiled off the filament, the negative electron, they’ll move toward the positive end. And by putting a fairly modest voltage—perhaps a few tens or a few hundreds of volts— across those filaments, you’ll get a steady flow of electrons from one side to the other.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
J.J. Thomson and Magnetic Deflection
A tube like this, a diode, or a rectifier tube, was used to control the flow of electricity in one direction in an electronic device. So you could have electrons coming in one side and they’d flow continuously in one direction from negative to positive, but never from positive to negative.
What J. J. Thomson discovered is—in the early days in discovering how these vacuum tubes might be used and how they might work—that a magnet deflects this invisible current. So, he concluded two things.
First, that the current has a negative charge. He also realized that because you’re deflecting the beam that’s coming from negative to positive, it also had to have mass. You’re applying a force, you see a bending, that’s an acceleration.
Learn more about magnetism and electricity.
So, you can say force equals mass times acceleration. You can determine the mass of this invisible fluid, indeed, the mass of the individual particles, which Thomson dubbed electrons. It turns out that electrons have a mass about one thousandth of the mass of the whole atom. And it was clear to Thomson that when electrons are stripped off an atom, the atom has a positive residual charge because you’ve pulled off the negative.
So, you’re learning much more about electricity by the study of this new particle called the electron, this mobile particle that can be moved from one atom to another through the movement of electric charge. Electrons are easily stripped off an atom, but what about that massive part that remains behind? What is the nature of that? The fragmentation of atoms by radioactivity led to the discovery of the atom’s surprising internal structure.
Lord Ernest Rutherford was a protégé of J.J. Thomson. He eventually rose to the directorship of the Cavendish Lab himself, the laboratory that Thomson had led for so many years. He was born in New Zealand. He received his undergraduate degree Down Under before winning a graduate scholarship to Cambridge.
From 1898 to 1907, he was professor of physics at McGill University in Montreal, Canada, and there he did Nobel Prize-winning work on the nature of radioactivity. In 1907, he returned to England to establish his own physics laboratory at the Victoria University in Manchester, and it was there that he did his most famous work of discovering the nature of the atom’s nucleus.
The Gold Foil Experiment
In 1909, Rutherford’s Manchester group began to use radioactivity to study the structure of the atom itself. The element radium emits alpha particles. These are like little atomic-scale bullets: high energy, fast moving particles. Think about these little atomic-scale bullets being fired out of a piece of radium, and then being directed at a piece of gold foil.
And this is the nature of Rutherford’s experiment. Almost all the alpha particles that were fired in that gold foil passed right through it or were deflected through very tiny angles. But about one in a thousand of these alpha particles was scattered backward, bounced right back toward the source.
Learn more about atoms.
Rutherford’s Idea of Atomic Structure
Rutherford concluded that atoms are not hard spheres that touch each other, but rather that the atom is almost entirely empty space, and has a tiny, hard nucleus that carries the positive charge and almost all the mass of the atom. The negatively charged electrons orbit around this nucleus, sort of like planets orbiting around the sun.
This is how the very first model of the atom’s inner structure was born. The Rutherford model is really not unlike a miniature solar system. You have a massive central nucleus, an analogue to the sun, and that central nucleus is fixed. And orbiting around it like the smaller planets and asteroids and so forth are the electrons in various orbits.
The outermost of these electrons can be stripped off, and those are the electric charges that are transferred from atom to atom, for example, in electricity. This was an amazing discovery, and the stage was set for the modern era of atomic studies.
Common Questions about Thomson, Rutherford, and Atomic Structure
J.J. Thomson discovered that the electric current flowing in a vacuum tube could be deflected by a magnetic field.
In Ernest Rutherford’s gold foil experiment, a thin gold foil was bombarded by a beam of alpha particles emitted by radium. In the experiment, most particles passed through the foil, but about 1 in a 1000 bounced back.
Ernest Rutherford concluded that the atom is almost entirely empty space, and has a tiny, hard nucleus that carries the positive charge and almost all the mass of the atom. The negatively charged electrons orbit around this nucleus.