By Ron B. Davis Jr., Georgetown University
The discoveries of strontium and barium were closely related. In fact, they were initially believed to be a single element. In 1790, Adair Crawford discovered the pair of elements in a mineral sample named after where it was found—Strontian, Scotland. It was then up to Humphrey Davy to isolate pure strontium using his electrolysis method.
Strontium’s Stable Isotopes
Strontium has four stable isotopes, strontium-84. -86, -87 and -88. The emission spectrum of these atoms creates a vibrant red color. That’s why salts of strontium are commonly included in fireworks—to produce that vibrant red color.
But strontium’s undeniable claim to fame comes from another group 2 association predicted by the periodic table. Just above strontium is calcium, the element that makes up 40% of the bone-forming mineral apatite.
Strontium’s similar valence and size, being a 5s2 element whereas calcium is a 4s2 element, allows it to impersonate calcium in our bodies, inserting itself into our bones. Fortunately, stable isotopes of strontium are relatively harmless when they do this.
Unfortunately, however, there are unstable isotopes of strontium that you absolutely do not want replacing the calcium in your body—especially in your bones. These include the heavier isotope strontium-90.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Strontium-90 Is Radioactive
Strontium-90 is a significant product of nuclear fission that takes place in nuclear bombs and power plants. Those extra neutrons push strontium-90 away from the nuclear stability curve and make it radioactive. It has a half-life of about 28.9 years, so when released into the environment, it has plenty of time to move around in our environment, and ultimately find its way into us.
Unlike cesium-137, strontium-90 doesn’t emit dangerous gamma radiation that can cause health problems when external exposure takes place. Unfortunately, strontium moves through the environment readily, just like calcium, and when it is inhaled or ingested it can take up residence by displacing calcium in the bones.
The strontium-90 tied up in bone mineral then slowly decays through beta emission, converting into yttrium-90, stepping to the right across the table as it seeks to increase its proton-to-neutron ratio. As it decays, it releases high-energy electrons that can damage healthy cells and lead to cancer and other chronic health problems.
Not All Is Lost
In a one-two punch, the yttrium-90 can itself undergo a second decay, with a half-life of just 64 days. This releases a second, even more energetic dose of beta radiation as it steps to the right on the table once again to become stable zirconium-90.
And yet, there is also a silver lining to this apparently nefarious isotope. Due to its lack of especially dangerous gamma radiation, medical science has harnessed strontium-90 with technologies that dispense safe, controlled, localized doses of beta radiation to help fight bone cancer and to improve radiographic images.
Barium and Its Dense Compounds
Yet another alkaline earth element obtained by Davy in 1808 is barium, on row six. In spite of its modest density for an element of its size, barium gets its name from the Greek term meaning ‘heavy’ in recognition of the fact that many of its compounds are extremely dense.
Barium sulfate, for example, is added to slurries that are pumped into wells during drilling. A slurry is a way to transport solid materials in a liquid. And because barium sulfate is nearly twice as dense as many rock-forming minerals—at 4.5 g/cm^3—most rock chips generated during drilling will float to the surface of the barium sulfate slurry, where they can be removed and allow drilling to progress.
Barium has a larger ionic radius than strontium, as we’d expect from its location lower in the periodic table. This fact makes barium sulfate more difficult for the body to absorb when swallowed. Instead, the barium sulfate coats the lining of the esophagus and digestive tract, where it can absorb x-rays, increasing the contrast in x-ray images. This can help physicians to get a better picture of a patient’s digestive system.
Discovery of Radium
It was a long wait for the largest of the alkaline earth metals, radium, which was discovered in 1898 by Marie Curie—90 years after Davy isolated four of the alkaline earth elements. Curie had been working with a uranium-containing mineral called pitchblende, and uranium itself had been isolated in 1841, so many of its properties were well understood.
One day, having already extracted the uranium from her sample, she noted that her sample of pitchblende actually continued producing radiation. This led her to conclude that there must be a second radioactive element within the pitchblende. Her hypothesis proved correct when she finally isolated that element in 1910, naming it radium.
No Stable Isotopes of Radium
Like all nuclei from the seventh row of the periodic table, there are no stable isotopes of radium. But unlike francium, with its maddeningly short half-lives, radium has some staying power. Radium-226, for example, forms from the decay of uranium-238 and boasts a half-live of one thousand six hundred years, giving it plenty of time to accumulate in pitchblende.
As radium decays, it produces not just alpha and beta radiation, but also what scientists call ‘ionizing radiation’—gamma radiation intense enough to rip electrons from air molecules. This ionizing radiation can be used to excite some compounds to the extent that they emit visible light. In other words—when radium is combined with the right materials, such as zinc sulfide—the mixture will glow.
So radium is long-lived enough that it can be collected and used to make products that glow. Shortly after its discovery manufacturers began producing paints containing radium and zinc sulfide that would glow in the dark for years or even decades until the light-emitting compounds degraded.
This technology was quickly embraced by companies around the world to make products that could benefit from glow-in-the-dark components, the most notable example being watch dials.
Common Questions about Strontium, Barium, and Radium
Strontium and barium were initially considered one element, so their discoveries were closely related. In 1790, Adair Crawford found these two elements in a mineral sample in Strontian, Scotland, and gave it the name strontium.
Since strontium-90 does not emit hazardous gamma radiation, it can be used in medicine, although it is a radioactive strontium isotope. The controlled and localized dose of beta radiation emitted by strontium-90 is used efficiently in medicine, helping to cure bone cancer and improve radiographic images.
Barium means heavy in Greek. The naming is due to the fact that many barium compounds have a high density, although barium itself has a modest density for an atom of this size.