Radio elements and their Families

Radio elements and their Families

Before the discovery of polonium and radium there were seven empty slots in the periodic system between bismuth and uranium. While the number of newly found radioactive elements was small there were no problems with their location in the periodic system. But emanations were a baffling problem. They had identical properties and therefore could not be assigned to different boxes of the periodic system, for instance, to the two empty boxes corresponding to the unknown heavy analogues of iodine and cesium. This would be an unnatural thing to do.

But even if we leave the enigmatically radon family alone the situation still remains unclear. In 1900 W. Crookes observed a strange phenomenon. After fractional crystallization of a uranium compound he obtained a filtrate and a precipitate. Uranium remained in the solution but it exhibited no activity. On the contrary, the precipitate did not contain uranium but exhibited a high–intensity radioactivity. On the strength of his observations Crookes made a paradoxical conclusion that uranium was not radioactive by itself, and its radioactivity was due to some admixture which he managed to separate from uranium. As if he had ill premonitions, Crookes refrained from giving the admixture any definite name and referred to it as uranium–x (UX). Later it was found that uranium restores its activity after separation of UX which was just a much more active substance. Thus, UX could be regarded as a new radioactive element.

Two years later E. Rutherford and F. Soddy discovered similar temporary disappearance of activity in thorium. The respective admixture was named (by analogy) thorium–x (ThX). Rutherford and Soddy attempted to find an answer to the fundamental question: what happens with a radioactive element in the process of emission of radiation? Does the chemical nature of the element remain unchanged or does it change? They made a valuable observation that the emanation of thorium was produced by ThX rather than by thorium itself. In other words, they identified the first step of radioactive transformations:

Th → ThX → EmTh

This was the event that played the decisive part in developing the theory of radioactive decay. According to Rutherford and Soddy, the mechanism of radioactive decay consists in transformation of chemical elements and in their natural transmutation. This was particularly clear in the case of radium, which converted into radon after emission of alpha radiation. Somewhat later, the alpha particle was found to be a doubly ionized helium atom. The decay of radium gave rise to two new elements, namely, radon and helium:

Ra → Rn + He

This suggestion was soon verified in the experiments of Ramsay and Soddy.

Rutherford and Soddy argued further that all the known radioactive elements were not absolutely independent but were generically linked to each other (converted successively one into another). These elements can be said to make up three radioactive families–the uranium, thorium, and radium families named after the originating element of the respective family. Many questions still remained unanswered. How many radioactive elements make up a family? What elements end the families? And finally, what kind of a “material entity” is a radioactive element and what is its real nature?

The last question was not just an abstract one since starting from the early years of the 20th century the number of radioactive substances started snowballing and the problem of their arrangement in the periodic system became very acute.

New radioactive substances became known under a variety of names such as radioactive bodies, activities, and radioactive elements. Scientists were aware that they encountered new, unknown material entities. Most of them manifested their existence only by their radioactive properties, namely, the radiation intensity, the decay type, and the half–life. But nothing or almost nothing could be said about their chemical nature. The old classical chemistry of elements always dealt with weighed quantities of substances so that a new element (or its compound) could be extracted in a material form, its reactions could be studied and its spectrum could be recorded. For most newly discovered radioactive elements all this was unfeasible. Hence, it was not unreasonable to ask whether they were elements in the proper chemical sense of the word.

The first researchers of radioactivity disagreed on this account. The Curies and Debierne assumed that all new radioactive substances were elementary in nature, and, hence, were new chemical elements. The discoveries of polonium, radium, and actinium, apparently, supported this viewpoint and these scientists stubbornly adhered to it even when numerous reports on discoveries of new radioactive substance started to pour in. But this stubbornness only fuelled the controversy.

Rutherford and Soddy held another viewpoint. In their opinion, radioactive substances could have different natures. Proceeding from their concept of radioactive families they argued that there exist relatively stable radioactive elements, that is, uranium, thorium, and radium, which give rise to the families or series of radioactive substances. Their chemical nature is well known and, thus, they can be classified as ordinary elements with only the property of radioactivity distinguishing them from other elements. The elements which close the radioactive families are normal stable elements (it was already vaguely surmised that lead had to close the radioactive families). According to Rutherford and Soddy, between these two types of elements there exist intermediate substances whose main feature in instability and which cannot be described in chemical terms. They are not elements in the conventional sense of the word, they are just something like atomic fragments. It was suggested to name them “metabolons” (from the Greek for transforming bodies). This approach did away with the problem of location of these substances in the periodic system.

But the name “metabolon” was not widely accepted. Soddy himself soon came to regarding metabolons as chemically individual substances, just like normal radioactive elements. In 1902 the British physicist G. Martin introduced the term radioelement which will be explained below. Here we shall just emphasize that the terms radioelement and radioactive element are by no means identical though they are sometimes confused in literature.

The entire history of radiochemistry in the first two decades of the 20th century is essentially the search for new radioelements and their genetic links to the earlier discovered ones. The compositions of radioactive families became increasingly clear and the families were acquiring features of systems of radioelements just as the periodic system classified the stable elements. The former radium family proved to be a part of the uranium family but there emerged the new actinium family whose originator could not be identified for a long time (this was definitely done only in 1935). Most radioelements were short–lived products whose half–lives were measured in seconds or, at best, in minutes. It was extremely difficult to determine the chemical natures and the places of radioelements in their radioactive families; even the cumbersome and monotonous work on separation of the rare–earth elements could not be compared to this task, which would need an entire book to describe it. Therefore, we have just to present here the chronological data on the discoveries of radioelements (see Table 1-3). The current composition of the three radioactive families is shown in Diagram 1.

Each radioactive family contains two characteristic groups of elements. The radioelements preceding the emanations are comparatively long–lived; on the contrary, the elements following the emanations have very short half–lives. Special notation was worked out to identify them using

Uranium–238 family

Radioelement Date of discovery Discoverers
Uranium–I

Uranium–X1

Uranium–X2

Uranium–II

Ionium

Radium

Emanation of radium

Radium–A

Radium–B

Radium–C

Radium–C′

Radium–C′′

Radium–D (radiolead)

Radium–E

Radium–F (polonium)

1896*

1900

1913

1911

1907

1898

1900

1903

1904

1903

1903

1909

1912

1900

1904

1905

1898

A. Becquerel

W. Crookes

K. Fajans, O. Göhring

H. Geiger, J. Nattal

B. Boltwood

The Curies, G. Bemont

E. Dorn

E. Rutherford, H. Barnes

P. Curie, J. Danne

P. Curie, J. Danne

P. Curie, J. Danne

O. Hahn, L. Meitner

K. Fajans

K. Hofmann, E. Strauss

K. Hofmann, L. Gonder,

W. Wölf

E. Rutherford

The Curies

* The date of discovery of uranium radioactivity

letters A, B, and C, alongside the symbols of respective elements (Ra, Th and Ac). The groups of these short–lived elements were known as active sediments; they were the most difficult elements to analyse and served as a source of much confusion and numerous errors. But it was their study that made a significant contribution to the development of the new science of radiochemistry.

Uranium–235 family

Radioelement Date of discovery Discoverers
Uranium–235 (AcU)

Uranium–U

Protactinium

Actinium

Radioactinium

Actinium–K

Actinium–X

Emanation of actinium

Actinium–A

Actinium–B

Actinium–C

Actinium–C′

Actinium–C′′

1935

1911

1918

1918

1899

1902

1906

1939

1900

1904

1905

1902

1911

1904

1904

1908

1913

1914

A. Dempster

G. Antonov

O. Hahn, L. Meitner

F. Soddy, J. Cranston

A. Debierne

F. Giesel

O. Hahn

M. Pereil

A. Debierne

F. Giesel

T. Godlewski

F. Giesel

H. Geiger

A. Debierne

H. Brooks

O. Hahn, L. Meitner

E. Marsden, R. Meitner

E. Marsden, P. Perkins

Thorium–232 family

Radioelement Date of discovery Discoverers
Thorium

Mesothorium–I

Mesothorium–II

Radiothorium

Thorium–X

Emanation of thorium

Thorium–A

Thorium–B

Thorium–C

Thorium–C′

Thorium–C′′

1898*

1907

1908

1905

1902

1899

1910

1899

1903

1909

1906

H. Schmidt, M. Curie

O. Hahn

O. Hahn

O. Hahn

E. Rutherford, F. Soddy

E. Rutherford

H. Geiger, E. Marsden

E. Rutherford

E. Rutherford

O. Hahn, L. Meitner

O. Hahn

* The date of discovery of thorium radioactivity.

As the composition of radioactive families approached the one we know the need for reasonable placement of radioelements in the periodic system became increasingly evident. After all, each of radioelements manifested chemical similarity to one or another conventional element occupying a certain box in the system.

But the number of radioelements was too large. Ramsay described the prevailing situation by the French saying embarrass de richesses (confusing abundance). By the beginning of the second decade of this century about 40 radioelements had been discovered. Some groups of elements were so similar in their chemical properties that they could not be separated with any of the available methods. (For instance, all three emanations, then thorium, ionium and radiothorium, and finally radium and thorium–X).

But the atomic masses of radioelements in each of such groups differed considerably, sometimes by a few units. The situation was indeed confusing. Some scientists suggested leaving many radioelements outside the periodic table, but more creative people were not satisfied with such a solution. In 1909 the Swedish scientists D. Strömholm and T. Svedberg suggested placing several radioelements into one box of the table (soon it was clear that they were right). The British radiochemist A. Cameron supported the idea of the Swedes in 1910.

Though back in 1903 radioactivity was proved to be accompanied with transformation of elements scientists for a long time could not give a definite answer to the question what exactly happens with a radioelement when it emits the alpha or beta particle. An answer to this question would allow to understand where in the periodic system a given radioelement is shifted owing to radioactive decay. The structure of an atom was still unknown and any changes in the nature of a radioelement could be identified by comparing its chemical properties to the properties of its product. But this was often extremely difficult to do since radiochemists had to work with exceedingly small amounts of substances. In many instances the chemical “portrait” of a radioelement had to be drawn from the secondary features.

Tenacious work of scientists and accumulation of experimental data made it possible to formulate the law of radioactive displacement. Though many scientists took part in this work the main contributions were made by F. Soddy and the Polish chemist K. Fajans and therefore this law is known as the Soddy–Fajans law. According to it, alpha decay gives rise to a radioelement displaced two boxes to the left from the starting position in the periodic table while beta decay displaces the product one box to the right. When it was shown that the charge of an atomic nucleus equals the number of the respective element in the periodic system the above empirical law was explained in the following way: an alpha particle removes from a nucleus a position charge of two and therefore the number of the starting element (the charge of its nucleus) is decreased by two while emission of a beta particle increases the positive charge of the nucleus by one.

The displacement law provided for harmonious relationship between radioactive families and the periodic system of elements. After several successive alpha and beta decays the originators of the families converted into stable lead giving rise in the process to the natural radioactive elements found between uranium and bismuth in the periodic table. But then each box in the system had to accommodate several radioelements. They had identical nuclear charges but different masses, that is, they looked as varieties of a given element with identical chemical properties but different masses and radioactive characteristics. In December 1913 Soddy suggested the name isotopes for such varieties of elements (from the Greek for the “common place”) because they occupy the same box in the periodic system.

Now it is clear that radioelements are just isotopes of natural radioactive elements. The three emanations are the isotopes of the radioactive element radon, the number 86 in the periodic system. The radioactive families consist of the isotopes of uranium, thorium, polonium, and actinium. Later many stable elements were found to have isotopes. An interesting observation may be made here. When a stable element was discovered this meant simultaneous discovery of all its isotopes. But in the cases of natural radioactive elements individual isotopes were discovered first. The discovery of radioelements was the discovery of isotopes. This was a significant difference between stable and radioactive elements in connection with the search for them in nature. No wonder that the periodic system was badly strained when accommodation had to be found for the multitude of radioelements, –it was a classification of elements, after all, not isotopes. The discovery of the displacement law and isotopy greatly clarified the situation and paved the way for future advances.

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