By Robert Hazen, Ph.D., George Mason University
As scientists explored the properties of different elements and compounds in the late 19th century, they discovered the ability of certain elements to spontaneously emit nuclear radiation. The discovery of radioactivity, and the subsequent exploration of the atomic nucleus, eventually led to the fields of nuclear physics and nuclear chemistry. All these studies also led to the discovery of the neutron.
Discovery of Radioactivity
In the Bohr atom, there’s a central, massive, positive nucleus, a nucleus that contains positive protons, and orbiting around that nucleus are electrons. Scientists of the late-19th century discovered the astonishing, and the completely unexpected, ability of certain elements to give off nuclear radiation—intense amounts of radiation—spontaneously.
In this context, nuclear radiation is any energetic particle or wave that suddenly and spontaneously is emitted from an atom.
Learn more about the Bohr atom.
Rutherford’s Experiment
It was Ernest Rutherford, a British physicist, who discovered the nucleus of the atom.
Rutherford fired what one could call atomic bullets at a piece of gold foil. Most of those atomic bullets passed right through the gold foil and were collected by a detector.
However, about one in a thousand showed the remarkable and curious property of bouncing back towards the source; it deflected off something, and it was this unexpected recoil that led Rutherford to suggest that there must be some small, massive, charged particle at the center of every atom.
That’s the atomic nucleus.
Properties of Protons
Protons are positively charged particles. They have a mass of about 1,836 times the mass of the electron. Protons probably represent 99.99 percent of the mass of most of the objects.
The number of protons in an atom, then, is the atomic number, and the atom’s position in the periodic table is reflected by that number of protons. Hydrogen, element one, has one proton; gold, element 79, has 79 protons; and so on.
Single, isolated protons are also called hydrogen ions.
That may seem surprising, but think about what a hydrogen atom is: the hydrogen atom is a proton with one electron surrounding it. If the electron is stripped away, all that is left is a proton; so the hydrogen ions, H+, are simply protons.
Isolated protons are stable, and there are stable particles in many of our everyday experiences.
Learn more about the periodic table of the elements.
Discovery of the Neutron
It was relatively easy to discover the mass and the other properties of electrons and protons because they’re electrically charged particles. Electrically charged particles interact strongly with other charged particles. Moreover, they don’t penetrate matter very deeply and can be detected. Also, their paths can be bent in a strong magnetic field because magnetic fields influence electrically charged particles. So electrons and protons were easy to study.
However, it was found that atoms typically have a mass that’s more than twice the mass of the electrons plus the protons; and this led scientists to speculate about the existence of another massive nuclear particle, one that has no electrical charge, and therefore is much more difficult to detect.
It wasn’t until 1932 that British physicist James Chadwick discovered the neutron. He conducted experiments very similar to those of Rutherford. In Chadwick’s case, he fired his atomic bullets at the element beryllium, not gold. The resulting collisions produced a new kind of particle that was just as massive as the proton, but it penetrated through several inches of lead.
If you can take a particle and penetrate several inches of lead, it implies, first of all, that the particle is moving very fast; and second, that the particle is electrically neutral, it has neither a positive nor a negative charge. So Chadwick found that the neutron is indeed electrically neutral, and he also found that its mass is slightly greater than that of the proton.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Properties of Neutrons
Isolated neutrons are not stable; they spontaneously transform into a proton plus an electron, so there is a positive and a negative charge balancing each other. A neutron, then, becomes two new particles. If you have a collection of neutrons, about half of them will transform to a proton plus an electron every 11½ minutes; that’s called the neutron’s half-life.
While the number of protons for any given element is fixed (the number of protons defines what the element is), the number of neutrons can vary. So, for example, carbon is element six — carbon always has six protons—but it can have six neutrons, it can have seven neutrons; in some cases, it has eight neutrons.
Hydrogen’s another case, and this is a special case because in hydrogen these different numbers of neutrons give the hydrogen atoms different names. This is the only case where that’s true. Hydrogen is typically a single proton with no neutrons. Then there is deuterium, which is one proton and one neutron, and then there is tritium, which is one proton and two neutrons, essentially hydrogen with three nuclear particles.
What we find is that the heavier elements typically have far more neutrons than protons. You can think of this as a kind of spacer; the neutrons help to spread out the positive charges of the various protons, so you don’t have as much electrostatic repulsion.
Common Questions about the Discovery of Neutrons, and Their Properties
Rutherford fired atomic bullets at a piece of gold foil. Most of those atomic bullets passed right through the foil, but about one in a thousand bounced back, leading him to the conclusion that there must be something small and heavy at the center of the atom. His experiment was the first step towards the discovery of the neutron.
Since the discovery of the neutron, it has been found that although the number of protons in an atom is stable, the number of neutrons is not. Hydrogen’s different names are based on how many neutrons the hydrogen atom has.
The discovery of the neutron was hard because it doesn’t have a charge, so unlike discovering the proton and electron, magnetic pulses couldn’t be used to sway the particle.
