Nuclear

Part 3 - Electrons

In 1874, the English chemist Sir William Crookes developed the cathode ray tube (the first electron accelerator); a sealed glass tube with an anode and a cathode inside connected across a battery. When most of the air was pumped out he found that the cathode rays had sufficient momentum to turn a small paddle wheel. He also noted that a magnetic field deflected the rays, behaving as though the rays were negatively charged. In 1879, he proposed that this could be explained if cathode rays were negatively charged gaseous molecules.

Arthur Schuster, in 1890, added two more plates to a Crookes tube. These were mounted between, and at right angles to, the anode and cathode so that the cathode rays passed between them. When he connected them to a battery they deflected the cathode rays toward the positively charged plate, indicating that the rays were negatively charged. Schuster estimated the charge-to-mass ratio of the ray components by measuring the amount of deflection as he varied the current across the second set of plates. In 1892 Hendrik Lorentz suggested that the mass of these particles (electrons) could be a consequence of their electric charge.

In 1896, at Cambridge University in Britain, J.J. Thomson proved that cathode rays were negatively charged particles, about 1800 times smaller than the smallest atom (hydrogen). And, they had to be a part of all atoms, since they were emitted from any element used as a cathode. They could be deflected with a magnetic field or an electric field and the mass of the electron relative to its charge (the charge-to-mass ratio) did not change with the material of the cathode. He repeated Shuster's work and then replace the flat anode plate with a tube which allowed the cathode rays to pass through the tube and hit the glass end of the tube causing it to fluoresce.

When the end of the glass tube was, later, enlarged and covered with phosphors to make a screen, and two sets of plates were positioned behind the tubular anode, the cathode rays could be steered to hit any point on the screen. And this became the cathode ray oscilloscope and TV tube.

Thomson also found that the same cathode rays were emitted by radioactive materials, by hot materials and by illuminated materials. He then suggested that atoms were made of the cathode ray particles (later called electrons) and that the atom could be divided. He postulated that the overall neutral charge of the atom was created by the electrons being, distributed in a mass of positive charge like raisins in a plum pudding (although in Thomson's 'plum pudding' model the electrons were not stationary.   We now know that electrons orbit a nucleus at speeds up to 2200 km/s).  

Uncharacteristically, Thomson failed to notice the x-rays that were emitted when the electrons collided with something. 

Meanwhile, in 1895, Wilhelm Conrad Röntgen had covered the end of his cathode ray tube with black paper and found that a nearby fluorescent panel glowed. The radiation passed not only through the air and the paper but also showed the bones of his hand! 

(X-rays are electro-magnetic radiation similar to light but have higher frequencies and thus more energy which allows them to penetrate solid objects. Gamma rays have even higher frequencies and thus greater energy and penetrating power which makes them useful for killing cancer cells inside the human body).

In 1896, in Paris, France, Henri Becquerel put an unexposed photographic plate wrapped in a protective envelope into a drawer and late covered it with uranium salts. Some time later he was surprised to find the plate had been brightly exposed. He had discovered that uranium emitted radiation that had easily penetrated the protective envelope.

Marie Curie, also in Paris, was intrigued. Using an electrometer she found uranium made the air around it conductive. Later, she showed that of two uranium ores, pitchblende was four times, and chalcolite twice as active as uranium, indicating that the ore contained a substance that was much more radio-active than uranium.

In 1898, Marie Curie and her husband Pierre Curie, tried to find the source of the radiation but it was present in such minute quantities they had to grind down tons of the pitchblende ore to find only infinitesimal traces of two new elements, radium and polonium. They were also the first to use the term "radioactivity." Marie Curie also found another radioactive element, thorium.

Despite its expense, radium was used almost immediately to treat cancer and was also used by many researchers as a source of radiation.

(Electro-magnetic radiation with frequencies above those of light destroys all cells but, as the cancer is typically denser -- because cancers cells are typically smaller -- more of them are killed than surrounding cells. Low energy particle radiation like electrons, alpha particles (helium nuclei), protons and neutrons are typically less penetrating).

Meanwhile, Marie Curie also discovered a radioactive gas, emanating from radium, which remained radioactive for a month. This was later named Radon. (222-Radon has a half-life of 3.8 days meaning that half of the radiation remains after 3.8 days. This makes it a useful tracer for research as it quickly decays to the stable isotope 206-lead. It is also inert to most chemical reactions, including combustion, because its outer valence shell contains eight electrons).

Note that the number in front of the element is the mass number which is equal to the total number of protons and neutrons in the nucleus.  A number after the element is the atomic weight which is equal to the number of protons in the nucleus.  The mass number is sometimes placed behind the element.  Thus 222-Radon has 222 nucleons while 206-lead has only 206.