By Don Lincoln, Fermilab
We often see atom as a small nucleus, orbited by electrons. In that scenario, atoms are actually mostly empty space. How is it that we know that? Well, we didn’t always. The electron was discovered in 1897 by British physicist J. J. Thomson. Read on to know more.
Thomson’s Modus Operandi
What Thomson did was isolate an electron from an atom. And, if you can remove an electron from an atom, that means that you have to think about what it looked like when it was in the atom.
Thomson knew that electrons have a negative electric charge and that atoms are neutral, so he knew that there had to be some kind of positive electric charge in the atom. But he didn’t have any data that said how that positive charge was arranged. So, he invented a possible configuration.
Of Puddings and Cookies
Thomson’s mental image of atoms was that there was some sort of positively charged goo that was spherical in shape and the electrons were like little flecks embedded in the goo. He called his model the Plum Pudding model, after a British dessert.
Thinking of an analogous name, we can take up the chocolate chip cookie dough model, with the positive charge being the dough and the chocolate chips being the electrons.
Ernest Rutherford’s Discovery
In any event, Thomson’s model of how atoms made molecules involved little spheres that touched one another and held together, like a bunch of sticky marbles or something. However, we know that his idea was wrong.
How did that come to be? It actually was his student, New Zealand physicist Ernest Rutherford who figured it out in 1911. He used a radioactive source that emitted what are called alpha particles and shot them at a thin gold foil. Alpha particles consist of two protons and two neutrons and, in the world of radioactivity, they are very heavy—subatomic bowling balls, if you will.
This article comes directly from content in the video series The Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.
Rutherford’s Surprising Findings
What Rutherford expected was that the alpha particles would plow through the gold atoms and have their trajectory only slightly changed by the interactions and he found that to be true most of the time. However, rarely, just about one time in 8,000, he found that the alpha particles bounced back the way they came. And that was entirely inexplicable.
He is quoted as having said, “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” A 15-inch shell was one of the largest forms of naval artillery at the time, so the metaphor demonstrated his surprise.
It took Rutherford quite a while to figure out the meaning of this perplexing observation, but one day, many months later, he burst into his laboratory and exclaimed to his staff, “I’ve figured it out.”
Discovering the Nucleus
Rutherford had thought it through and realized that the only way an alpha particle could ricochet backward was not if it hit something soft and squishy like cookie dough, but if it hit something big and solid like a wall. And the only way that could happen was if all of the mass of the atom was concentrated in a small spot.
So that was the first mental advance—that the structure of atoms included concentrated mass. He then took that number of one alpha particle in 8,000 being reflected back, and he figured out what the size of the concentrated volume had to be. And, as they say, the rest is history.
Using Rutherford’s calculation, we now know that the atom consists of a small and dense nuclear center, about 10 femtometers in diameter. That’s 10 to the minus-14th meters, for a scientific notation person. This is to be contrasted to the size of the atom, which is about a 10th of a nanometer, or about 10 to the minus-10th meters.
Volume of Nucleus
This means that the diameter of the atom is 10,000 times bigger than the nucleus, and if we think not about diameter, but rather volume, that means that the nucleus of the atom is about one trillionth the size of the atom as a whole.
An atom is mostly empty space, with a few electrons swirling around it. If you’d like an analogy, if the nucleus of the atom was the size of a BB placed at the 50 yard line of a football stadium, the atom is the size of the entire stadium, with a handful of dust mote electrons swirling around inside. Of course, Rutherford’s discovery didn’t tell us much about the nucleus. We didn’t know about the proton and neutron at that time.
Common Questions about How the Nucleus of an Atom Was Discovered
The electron of the atom was discovered in 1897 by British physicist J. J. Thomson.
Using Rutherford’s calculation, we now know that the atom consists of a small and dense nuclear center, about 10 femtometers in diameter. That’s 10 to the minus-14th meters, for a scientific notation person. This is to be contrasted to the size of the atom, which is about a tenth of a nanometer, or about 10 to the minus-10th meters.
It was J.J.Thomson’s student, a New Zealand physicist Ernest Rutherford who figured out about the nucleus in 1911, when he used a radioactive source that emitted what are called alpha particles and shot them at a thin gold foil.