One element cannot change into another, at least not in normal chemistry. But the atoms of some elements are unstable and can split to form other elements. Elements that do this spontaneously are described as radioactive.
During radioactive decay an unstable atom’s nucleus loses one or more of its particles (protons or neutrons). At the same time energy is released. This is the source of heat for atomic power, and the destructive force of atomic bombs.
Low-levels of radioactive decay are everywhere and you should not be worried by it. High levels of radiation might be cause for concern, but radioactive decay has many beneficial uses in medicine and technology.
With the exception of technetium, all of the objects described in this tour can be seen in the Elements gallery at the Ulster Museum.
Bismuth: The last stable element, or the longest half-life?
Elements and isotopes
Every atom of an element has a particular number of protons in the nucleus - for carbon it is 6 – but the number of neutrons can vary, from zero up to more than 150 in some of the heaviest elements. Carbon may have 6, 7 or 8 neutrons, and these different varieties of an element are called isotopes.
Some isotopes are stable but many are not. Some unstable isotopes decay into a different isotope of the same element. In others parent elements decay into new daughter elements.
Hydrogen, element number one, mostly has just one proton and no neutrons, but it has two isotopes called deuterium, with one proton and one neutron, and tritium with one proton and two neutrons.
Hydrogen and deuterium are stable but tritium is radioactive and decays to helium, element 2. The low energy radiation that is emitted can be used to make the ‘phosphor’ coating of a tritium-filled tube glow green.
Radioactive decay and half-lives
Each radioactive isotope has a particular half-life, the time it takes for half of the atoms in a sample to decay into another isotope or element.
After two half-lives just a quarter of the original atoms remain, after three half-lives it is just an eighth, and so on. After eight or nine half lives virtually no atoms of that isotope remain.
The half-lives of some isotopes are vanishingly short, for others they are immensely long.
Radium, element 88, has a half life of 1,600 years. Almost 15,000 years would pass before it had completely decayed to radon, element 86. Mixed with a zinc sulfide ‘phosphor’ radium makes a luminous paint, but the high energy radiation it emits damages this phosphor. It will cease to glow within a few decades even though the radium continues to emit radiation for thousands of years.
Promethium, element 61, is one of only two elements that in the Periodic Table nestle between stable elements yet have no stable isotopes themselves (the other is technetium, element 43). One isotope promethium-147, has been used for the luminous dials of watches. Unlike radium, the low energy radiation it emits does not damage the phosphor in the luminous paint. But with a half-life of just 2.6 years virtually all will have decayed to samarium, element 62, in less than twenty years.
Bismuth: The last stable element, or the longest half-life?
From polonium, element 84, onwards there are no known stable elements. All are radioactive and decay into other elements.
Bismuth, element 83, was thought to be the last stable element but we now know that it too is radioactive. It slowly decays into thallium, element 81, but this takes a while. Bismuth has a half-life of twenty billion billion years so to decay entirely would take more than 10 billion times the age of the Universe, at a mere 13.7 billion years!
Radioactivity: Good or evil?
Atomic bombs and reactor meltdowns give radioactivity a bad press, but it has many beneficial uses in technology and medicine.
Heat from the decay of plutonium, element 94, to uranium, element 92, has been used to generate electricity on many space-craft, from the Voyager probes launched in 1977 to explore the Solar System, to NASA’s Curiosity rover exploring the surface of Mars since 2012.
Closer to home a tiny amount of americium, element 95, is the key ingredient in every smoke detector and decays to neptunium, element 93. These are the only synthetic element, not found naturally on Earth, that most people will ever encounter.
Radioactivity in medicine
Technetium, element 43, is the lightest element for which no stable (non-radioactive) isotopes are known. Nearly all of the technetium on Earth is made artificially, most of it in nuclear power stations.
One isotope, technetium-99m, is used in medicine. It emits high energy gamma rays and, when injected into the body, can help to locate cancerous bone tissue. With a half-life of just 6 hours, it has entirely decayed within two days to technetium-99. This isotope has a half-life of more than 210,000 years but it is much less radioactive, emits only low-energy beta radiation, and is quickly removed from the body. Technetium poses little risk to health, and considerable benefits in detecting cancers.
Technetium is unusual in being surrounded by stable (non-radioactive) elements in the Periodic Table. It eventually decays to ruthenium, element 44.
Senior Curator of Natural Sciences, Dr Mike Simms, checking his radiation levels about 10 hours (almost two half-lives) after injection with Technetium-99m.
Radioactive rocks and radon
Some rocks, such as granite, contain small amounts of radioactive elements like uranium, element 92, and thorium , element 90. This decays very slowly to lead, element 82, and by measuring how much there is of each element geologists can measure the time that has elapsed since the granite first solidified. For the Mourne Mountains this was 56 million years ago.
The Mournes granites still contain tiny amounts of uranium and thorium. One of the daughter elements produced from their decay is the highly radioactive gas radon, element 86. This has a very short-life (half life of just 3.8 days) but good ventilation is needed to ensure the gas does not build up inside buildings and cause health issues.
Uranium oxide was used as a glaze for ceramics long before its radioactive properties were known. This is what gives ‘Fiesta’ dinnerware, produced since 1936 by the Homer Laughlin China Company in West Virginia, USA, its striking colour.
In 1944 the company’s stock of uranium was seized by the US government for its atomic bomb programme. The company resumed manufacture in 1959, using depleted uranium that contained less of the highly radioactive uranium-235, but it ceased using uranium altogether in 1972.
Adding particular metal oxides to glass can give it a distinctive colour. Adding uranium oxide gives it a pale yellowish-green colour, sometimes called ‘Vaseline glass. This was particularly popular from the 1880s to the 1930s but by the 1940s glass manufacturers found it difficult to get supplies of uranium as other, more strategic, uses were discovered. Uranium glass was also used for beads and inexpensive jewellery, and still is today.
Uranium glass seldom contains more than a few percent of the element and is barely radioactive, but it glows bright green under ultraviolet light.
Spark plugs and dusty records
Polonium, element 84, is horribly poisonous and highly radioactive yet it has been put to some odd uses.
For a time in the 1950s polonium was used on the tips of spark plugs! It was believed that the ionizing radiation would improve sparking performance, but a little knowledge of the element would have convinced motorists to buy something cheaper. With a half-life of just 138 days pretty much all of the Polonium-210 would have decayed into lead-206, element 82, in less than 2 years.
The same ionizing effect has been used since the 1950s to remove static from vinyl records and photographic film. Staticmaster brushes contain a minute amount of polonium-210 but, like the spark plugs, the effect soon wears off. To get around this problem the polonium-containing cartridges could be replaced each year.
Adding particular element oxides to glass can change its optical properties. Some camera manufacturers in the 1950s and ‘60s added thorium oxide to the glass to make very high-quality lenses. Thorium, element 90, like all of the elements on the Periodic Table beyond Bismuth, element 83, is radioactive so this was perhaps not a great idea for something you hold close to your brain!
Fun with fission!
You might have come across chemistry sets for children, even if today they are much watered-down versions of those from the 1960s and ‘70s. But the early 1950s saw the brief appearance of the remarkable Gilbert U-238 Atomic Energy Lab.
At the dawn of the Atomic Age this ‘toy’ aimed to inform and entertain children about nuclear physics. The packaging assured us that it was ‘Exciting!’ and ‘Safe!’, but this claim was perhaps only partly true. The uranium ore samples and four radioactive sources were certainly less dangerous than you might think, but the concept was a lot more exciting than the reality.
At the end of the day there really wasn’t that much to see; just a few faint lines in the cloud chamber, some tiny flashes in the scintillometer, and maybe a few clicks on the Geiger counter. It had none of the bangs and sparks of a normal chemistry set and it was expensive too ($50 in 1950, equivalent to around $540 today. Few were sold and the Gilbert Company discontinued it after just two years. Those few kids that did get one for Christmas or birthday probably found it quite baffling, so most were little used and soon consigned to the attic. Which is very fortunate because it means that there are still a few of these amazing sets around today.