By Robert Hazen, Ph.D., George Mason University
An atom’s nucleus has two kinds of particles: protons and neutrons. An isotope is an atom in which we know both the number of protons and the number of neutrons. The isotopes can experience a number of different fates, including radioactivity. It is the protons that determine the chemical behavior of an element.
Table of Isotopes
All known combinations of protons and neutrons in an atomic nucleus are summarized on something called the table of the isotopes. The table lists the number of protons on the vertical scale, so as you go from the first to the second to the third row, you’re going up the periodic table.
The first row at the bottom of the table is hydrogen, the second one is helium, and so forth. The number of protons tells the name of the element. The horizontal scale indicates the number of neutrons. For any given element, if you have several different possible numbers of neutrons, then you have several different boxes on this chart of the isotopes.
Indeed, as you go towards elements with 40, 50, or 60 protons, you see numerous combinations of neutrons; sometimes thirty, sometimes forty different isotopes for a given element. All told, there are about 2,000 known isotopes shown on this chart of the isotopes.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Number of Protons and Neutrons in Isotopes
It’s interesting that for lighter isotopes, the number of protons and the number of neutrons are usually quite similar. For example, you have carbon-12 as a common element, six plus six; oxygen-16, eight and eight, and so forth.
When you get to heavier isotopes, they usually have far more neutrons than protons. For example, gold-197 has 79 protons and 118 neutrons.
This tendency means that for light elements, all of the items on the chart of the isotopes seem to lie close to a 45-degree line, but as you get to heavier and heavier elements, they fall off that 45-degree line.
One way to think about this situation is that the positive protons repel each other; as you put more and more protons into a nucleus, you need more neutrons to help stabilize that nucleus.
Radioactive Element in an Isotope
Almost every atom around us is a stable isotope. These are atoms whose nuclei are going to remain unchanged for billions upon billions of years. For example, hydrogen is a stable isotope, as are carbon-12, oxygen-16, silicon-28, and iron-56.
It’s certainly true that isotopes can be altered in highly energetic nuclear reactions, and so we have fission and fusion reactions. But there is a small fraction, perhaps one atom in a million in our everyday surroundings, that are radioactive.
These atoms can transform suddenly as the result of spontaneous changes in the nucleus. Indeed, radioactivity is defined as the ability of some nuclei to change spontaneously, emitting high-energy radiation.
Learn more about isotopes and radioactivity.
The surprising phenomenon of radioactivity was discovered, quite by accident, by the French chemist Antoine Henri Becquerel, who lived from 1852 to 1908. He was one of the many chemists who analyzed rare minerals in the search for new elements. Becquerel focused on ores of uranium from mines in Saxony and Bohemia.
One day he put some of his mineral specimens in a drawer. Also in that drawer was a photographic plate, an undeveloped plate, and coincidentally, there was a coin sitting on top of the photographic plate. When he eventually went to develop that photographic plate, he found that the film had been fogged up, but there was a clear silhouette of the round coin on top of the photographic plate.
The inevitable conclusion was that there’s some substance in the mineral that emits a strange form of energy, something that would fog the plate and pass right through black paper but wouldn’t pass through the coin. Once that strange phenomenon was discovered, there was an intensive effort undertaken by chemists around the world to isolate the radiation-producing substance.
Learn more about nuclear fission and fusion reactions.
The Curies: Discovering Radioactive Elements
By far the most famous and successful of these chemists was Marie Sklodowska Curie, who lived from 1867 to 1934. She was born Marie Sklodowska in Poland. She was a brilliant student, but she was unable to advance in the rigid Polish science hierarchy; she was denied the chance to do research in that country. Her fortunes changed dramatically when she came to live with her sister in Paris.
There she met Pierre Curie, and their careers were intertwined until his tragic and untimely death in a carriage accident in 1906. Marie worked with her husband in his laboratory in Paris; she conducted her research under terrible conditions, it’s said. The lab was cold, leaky, and improperly ventilated, partly because of the resistance of her male colleagues to provide her with any support.
Through the years, the Curies refined tons of high-grade uranium ores from Bohemian mines. Eventually, in 1898, they extracted small quantities of previously unknown elements polonium and radium, two radioactive elements. Her remarkable sample of 22 grams of radium chloride became the international standard for radiation units. Indeed, that unit is now called the Curie, in their honor.
Marie Curie was awarded the Nobel Prize in both physics, in 1903, and chemistry, in 1911. In fact, she’s the only person ever to win both physics and chemistry Nobels. Her husband also received the Nobel Prize in physics, as did their daughter and their son-in-law, in separate discoveries. Marie Curie died in 1934 of cancer that was undoubtedly caused by her extensive, cumulative exposure to this high-energy radiation source.
Common Questions about the Story of Radioactive Elements
No, almost all isotopes in everyday life are stable and will remain stable for the rest of our lifetime. We rarely come in contact with radioactive elements; only a few isotopes that we come in contact with, maybe one in a million, are radioactive.
Henri Becquerel discovered the phenomena of radioactivity by accident while storing a radioactive element in a drawer that also contained a coin and a photographic plate. When the plate was developed, he understood that something had to have happened to fog the plate and pass right through black paper but not pass through the coin.
Marie Curie’s efforts led to her astonishing sample of 22 grams of radium chloride, containing radioactive elements. It is now used as the international standard for radiation units, and named Curie in her and her husband’s honor.