Thomson was venturing into the interior of the atom. At the Cavendish Laboratory at Cambridge University, Thomson was experimenting with currents of electricity inside empty glass tubes. He was investigating a long-standing puzzle known as "cathode rays. He called these particles "corpuscles," and suggested that they might make up all of the matter in atoms. It was startling to imagine a particle residing inside the atom--most people thought that the atom was indivisible, the most fundamental unit of matter.
It took more experimental work by Thomson and others to sort out the confusion. He found out that the rays are made up of electrons : very small, negatively charged particles that are indeed fundamental parts of every atom.
In Ernest Rutherford who performed many experiments to explore radioactivity did an experiment in which he discovered that the atom must have a concentrated positive center charge that contains most of the atom's mass. Though many chemicals were hard to get hold of, Chadwick even found a type of radioactive toothpaste that was on the market in Germany at the time, and managed to persuade the guards to supply him with it.
Using some tin foil and wood he built an electroscope and did some simple experiments. Chadwick was able to continue to work on radioactivity, now with more sophisticated apparatus than tin foil and toothpaste. In , Chadwick was appointed assistant director of Cavendish Laboratory. Rutherford had discovered the atomic nucleus in , and had observed the proton in However, it seemed there must be something in the nucleus in addition to protons.
For instance, helium was known to have an atomic number of 2 but a mass number of 4. Some scientists thought there were additional protons in the nucleus, along with an equal number of electrons to cancel out the additional charge.
In , Rutherford proposed that an electron and a proton could actually combine to form a new, neutral particle, but there was no real evidence for this, and the proposed neutral particle would be difficult to detect. Chadwick went on to work on other projects, but kept thinking about the problem. Around , several researchers, including German physicist Walter Bothe and his student Becker had begun bombarding beryllium with alpha particles from a polonium source and studying the radiation emitted by the beryllium as a result.
Some scientists thought this highly penetrating radiation emitted by the beryllium consisted of high energy photons. Chadwick had noticed some odd features of this radiation, and began to think it might instead consist of neutral particles such as those Rutherford had proposed.
They found that this radiation knocked loose protons from hydrogen atoms in that target, and those protons recoiled with very high velocity. American physicist Robert Millikan designed an experiment to measure the absolute value of the charge of electron which is discussed below. In , American physicist R. Millikan measured the charge of an electron using negatively charged oil droplets.
In his gold foil experiment, Rutherford bombarded a beam of alpha particles on an ultrathin gold foil and then detected the scattered alpha particles in zinc sulfide ZnS screen. From the previous discussion, we can see that the gold foil experiment gave a clear picture of the structure of an atom which consists of protons nucleus and same number of electrons outside of the nucleus.
Figure 6. Schematic diagram for the experiment that led to the discovery of neutrons by Chadwick.
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