Ethan Shaw Mr. Carrell 10B Chemistry May 17, 2013
Throughout today’s society the mention of radioactivity largely occurs as a political scare tactic or environmentalist jargon. Despite the many negative connotations, radioactivity is still a very new and largely unexplored concept. The principle was first observed by French scientist Henri Becquerel in 1896. He hypothesized that that phosphorescence might be related to the effect of X-rays in cathode ray tubes because both caused materials to glow in the dark after exposure to radiation. While testing various phosphorescent salts, he noticed that the uranium salts—unlike all the others—blackened the photographic plate which held them. This same phenomenon was later observed with non-phosphorescent uranium salts, and thus Becquerel’s hypothesis proved wrong. His experiment, however, paved the way for further research over the next few decades. Although the dangers of radiation were not initially realized, Becquerel’s findings nonetheless paved the way for the dawn of the nuclear era.
Technically, all elements undergo radioactive decay. For the vast majority, however, an element’s common isotopes are stable and this process is very slow and thus negligible. There are thirty-eight elements classified as radioactive, meaning they have no truly stable isotope or are synthetic and always unstable. Generally, heavily radioactive elements are found toward the very bottom of the periodic table. Among these actinium is generally considered one of the most dangerous radioactive substances. It is the first element in row seven, and therefore this row is termed as the actinoid series. Actinium’s significance lies in its status as the first non-primordiali radioactive element to have been isolated. The key data points concerning actinium are as follows:
- Atomic number: 89
- Family: N/A (technically group #3, but this is debated due to its actinoid status)
- Atomic mass: 227
- Description: soft; silvery-white color
- Solid at room temperature
- Default oxidization state: +3
- Location in periodic table: f-block, period #7
Interestingly, the account of actinium’s discovery raises much scholarly contention. André- Louis Debierne, a French chemist studying under the famous Pierre Curie, credited with the element’s name. While experimenting with the separation of rare earth oxide minerals from pitchblendeii around 1899, he discovered an unknown substance which he observed to be a hundred thousand times more radioactive than uranium. Although similar to lanthanum, its properties did not fully match up, and so Debierne decided to christen a new element, naming it actinium. Three years later, though, the German Friedrich Giesel—ignorant of Debierne’s claim—independently discovered the element and named it emanium. In placing radium impurities on a zinc sulfide screen, he noticed its emanationiii moving across the screen and giving off a unique phosphorescence. He thus named his discovery emanium, from the Latin “emanare” (“to flow out”). Although Debierne’s elemental name remains today and he has the chronological advantage, many modern scholars question the validity of his discovery and find more reliable proof in Giesel’s account. Regardless, both men made key contributions toward laying the foundation for the emerging field of nuclear engineering.
Though not universally synthetic, the vast majority of actinium is artificially produced through the nuclear irradiationiv of radium-226 and is therefore largely unstable. As a result, it has few practical uses and is generally created for laboratory research purposes. Actinium’s most common isotope is Ac-227, traces of which are found within uranium ore as a result of radioactive transmutationv (as Debierne observed). However, the quantities are very small (0.2
mg Ac per metric tonne U) and separating out the actinium is thus a very inefficient means of collection. This impracticality led to the current radium irradiation method. Notably, actinium itself is a source of neutrons just like the radium used to create it. Alpha, beta, and gamma particles make up the observed radiation spectrum and are named in order of penetration capabilityvi. Actinium provides beta particles with a source of neutrons to be converted and emitted as electrons. A similar process occurs in the release of alpha particles, which are pertinent in the treatment of cancer using radiation therapy. Even so, actinium—like all radioactive elements—is unstable and thus prone to transmute into various other elements and isotopes as a result of certain processes (such as fission). In truth, as German chemist Otto Hahn observed, actinium’s radioactivity seems to come from its decay products. Perhaps actinium’s actual significance lies in its propensity to create other, more useful radioactive substances.
Figure: an illustration of the actinium decay chain, involving radioactive transmutation.
i Also, synthetic or artificial. A primordial element is one that is stable and thus its current isotopes are hypothesized to have been in existence since the earth’s formation.
ii A common name for the uranium-rich mineral uraninite.
iii In chemistry, the gaseous product of radioactive disintegration.
iv Refers to the process of exposing a substance to neutrons.
v The process in which one radioactive element is changed into another.
vi The illustration given is that a sheet of paper will stop an alpha particle and a sheet of aluminum shielding a beta particle, but even a thick block of lead will only slow down a gamma particle due to its immense penetration power.
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