Radioactivity and the Periodic Table

[fog37]

TL;DR Summary

Elements, radioactivity, periodic table, stability

Hello,
The periodic table organizes all known chemical elements (total of 118) based on their atomic number and properties. My understanding is that the first 92 elements are commonly found in nature while the other 26 are either highly radioactive and/or artificially made. Radioactive elements are elements that change identity (decay) over time by a change in their number of protons inside the nucleus. Some radioactive elements decay faster than other (lifetime).

The first 92 elements are "stable" which, I believe, means that these elements are not radioactive at all. Or are all elements, even the first 92, somewhat slightly radioactive and called stable simply because they have extremely long lifetimes?

The periodic table does not include the isotopes of the elements. Why not? For example, C-12 is stable while C-14, also a naturally occurring element, is radioactive but not in the table...

Finally, elements generally combine to form molecules and compounds. The reason for combining is the search for stability. However, some of those compounds are radioactive, hence unstable. So what is the point of combining if the end result is an unstable product?

Thanks!

 

[Vanadium 50]

My understanding is that the first 92 elements are commonly found in nature while the other 26 are either highly radioactive and/or artificially made.

Not true. Technitium, Promethium, Astatine. Small amounts of Plutonium and Neptunium.

Since your premise is not true, there's little point discussing conclusions.

 

[Astronuc]

Summary:: Elements, radioactivity, periodic table, stability

Finally, elements generally combine to form molecules and compounds. The reason for combining is the search for stability. However, some of those compounds are radioactive, hence unstable. So what is the point of combining if the end result is an unstable product?

One is mixing nuclear stability with chemical stability, which are two different phenomena.

We have a chart of the nuclides, which lists all the isotopes, both stable and unstable.
https://www.nndc.bnl.gov/nudat2/

 

[fog37]

I looked technetium (atomic number 43) up. All its isotopes are all radioactive and all available technetium is produced as a synthetic element. To be correct, I read that elements 1 through 92 (except for elements 43 and 61, promethium) occur naturally on Earth.

In regards to nuclear stability and chemical stability, thanks for pointing out the two different types of stability. From the band of stability, elements with atomic number greater than 70 are never stable...

 

[phyzguy]

The first 92 elements are "stable" which, I believe, means that these elements are not radioactive at all. Or are all elements, even the first 92, somewhat slightly radioactive and called stable simply because they have extremely long lifetimes?

(1) As @Vanadium 50 pointed out, not all of the first 92 elements have stable isotopes.
(2) It is not known whether the stable isotopes are truly stable or whether they just have extremely long lifetimes. Attempts have been made to measure the lifetime of the proton, but without success. So all we can say is that the lifetimes of the stable elements are unmeasurably long.

 

[dRic2]

To be correct, I read that elements 1 through 92 (except for elements 43 and 61, promethium) occur naturally on Earth.

Fun fact. Technetium and Promethium are sometimes called lost elements because you could have found them on Earth in the past. They just have a mean life very much smaller than the actual age of the Earth so they "got extinct" long time ago.

*Post edited, see below the comment by @hutchphd
**Note that there are no natural occurring nuclear reaction that can lead to the formation of those two isotopes (unlike for example for the short lived ${14}^C$ which is continuously produced by cosmic rays reaction with nitrogen). See @Borek comments below

 

[phyzguy]

From the band of stability, elements with atomic number greater than 70 are never stable...

This is still not right. I think the heaviest stable isotope is Bismuth, with atomic number 83.

 

[hutchphd]

They just have a mean life smaller than the actual age of the Earth so they "got extinct" long time ago.

Misleading. This needs to say "very much smaller".

.

 

[dRic2]

Misleading. This needs to say "very much smaller".

.

Yes, sorry for being sloppy. The difference is more than 3 orders of magnitude: from $10^6$ years for the mean life of the most stable isotope of Tc to $10^9$ years for the age of the Earth (Pm should have an even smaller mean life).

 

[TeethWhitener]

This is still not right. I think the heaviest stable isotope is Bismuth, with atomic number 83.

Used to be. The last “stable” isotope of bismuth was recently found to have a half life of something like 10¹⁹ years. So now the heaviest stable element is lead to the best of our knowledge.

 

[Borek]

They just have a mean life very much smaller than the actual age of the Earth so they "got extinct" long time ago.

Somehow even with the correction it doesn't sound entirely right. There are unstable and short living isotopes that are all the time present on the Earth either because they are produced by the cosmic radiation or as intermediates in longer decay chains. It happens it doesn't eliminate any element from being present on the Earth, but for the sake of precision I would try to be very clear whether I mean isotopes or elements with such general statements.

 

[phyzguy]

Used to be. The last “stable” isotope of bismuth was recently found to have a half life of something like 10¹⁹ years. So now the heaviest stable element is lead to the best of our knowledge.

I didn't know that! Thanks for the correction.

 

[Vanadium 50]

Tc, Pm, Pu, Np and At (fixed, thanks Astro!) are all present in the crust at gram-level (plus or minus a few orders of magnitude) quantities. There is no reason to treat them differently when determining what elements are "natural" and what are "artificial".

It is true that many heavy isotopes are theoretically unstable. However, at some point you need to be realistic: ²⁰⁸Pb → ²⁰⁴Hg + α is energetically allowed, but has a lifetime somewhere in the 10⁴⁵ year ballpark. That is so long that no atom of lead on Earth has ever decayed that way.

 

[Vanadium 50]

Forgot a couple in the same category: Fr and Ac.

 

[Astronuc]

Interesting point about At is it's rarity. It's a natural decay product (see below), but both longest-lived isotopes, ²¹⁰At and ²¹¹At with half-lives of 8.1 hours and 7.214 hours, respectively, and they are artificially made.

According to the Royal Society, "top kilometre of the Earth's crust contains less than 50 mg of astatine making a Guinness world record's rarest element." See https://www.rsc.org/periodic-table/element/85/astatine (Podcasts).

JLAB provides a larger number, "Small amounts of astatine exist in nature as a result of the decay of uranium and thorium, although the total amount of astatine in the Earth's crust at any particular time is less than 30 grams. Due to its scarcity, astatine is produced when it is needed. A total of 0.05 micrograms (0.00000005 grams) of astatine have been produced to date." https://education.jlab.org/itselemental/ele085.html

Other sites have numbers in between
https://www.americanelements.com/at.html - "less than one gram present on Earth at any given time"
https://www.livescience.com/39514-facts-about-astatine.html - "only about 25 grams of naturally occurring astatine in Earth's crust at any given time"
https://www.chemicool.com/elements/astatine.html - "About 25 grams exists in Earth's crust at any given time."

At occurs naturally from the decay of transuranic elements, but the lightest isotopes is ²¹⁵At in the decay chain of ²³⁹Pu, ²³⁵U
http://hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/radser.html#c1

 

[Vanadium 50]

Interesting point about At is it's rarity. It's a natural decay product (see below)

Just like polonium. Both decay almost immediately. For that matter, I'd put radium in the same category: with a 1600 year half life, all of it is from decays. But for some reason, radium is "natural". (Of course, with a half-life ~1000x longer than polonium, there's ~1000x as much of it in the ground)

Technitium and Promethium are presumably fission fragments.

 

[Astronuc]

Technitium and Promethium are presumably fission fragments.

Yes. Basically, the lanthanides are all fission products, transmuted (n-capture) fission products, or successive neutron capture of lighter elements.

Lanthanides

in particular, with more than 140 neutrons and protons in each nucleus, require a special environment, with lots of neutrons to spare. This type of nucleosynthesis in a neutron-rich environment is called the r-process, for “rapid neutron capture”.

https://astrobites.org/2014/02/23/making-lanthanide-metals-or-not/

Either side, Nd and Sm have 5 stable isotopes, and Nd has 2 long-lived isotopes and Sm has 3. Sm-poisoning is a consideration in reactor behavior.

https://ansn.iaea.org/Common/documents/Training/TRIGA Reactors (Safety and Technology)/chapter2/physics145.htm

 

[snorkack]

Elements 1 to 82 except 43 and 61 have stable isotopes.
Element 83 has half-life far in excess of Earth age (and decays to a stable daughter).
2 elements (90 and 92) have isotopes whose halflife is comparable to that of Earth.

Elements 91, 89, 88, 86 and 84 have isotopes that are major daughters of the 3 longlived series mothers.

However, elements 87, 85, 61 and 43 only form in minor decay branches of element 92.

 

[Astronuc]

I looked technetium (atomic number 43) up. All its isotopes are all radioactive and all available technetium is produced as a synthetic element. To be correct, I read that elements 1 through 92 (except for elements 43 and 61, promethium) occur naturally on Earth.

From the Wikipedia article on the element Tc,

Nearly all available technetium is produced as a synthetic element. Naturally occurring technetium is a spontaneous fission product in uranium ore and thorium ore, the most common source, or the product of neutron capture in molybdenum ores. The silvery gray, crystalline transition metal lies between manganese and rhenium in group 7 of the periodic table, and its chemical properties are intermediate between those of both adjacent elements. The most common naturally occurring isotope is ⁹⁹Tc, in traces only.

Note that Tc is a fission product, but spontaneous fission of naturally occurring isotopes of U and Th are rather rare. On the other hand, there have been times in the past where some region of the Earth with sufficient ²³⁵U develops self-sustaining 'chain reaction' as in the Oklo natural reactor. Such events would have been more common billions of years ago before the natural ²³⁵U decayed.

Regarding neutron capture in natural Mo ores, the source of neutrons would mostly likely be some self-sustained neutron reaction. One of the more probable pair of fission products is ¹⁰⁰Zr + ¹³⁴Te, and Zr will decay by beta decay to Nb, which decays to stable Mo, which may absorb neutrons and become less stable. The most stable isotope ⁹⁹Tc forms from beta decay of ⁹⁹Mo, which forms from n-capture in ⁹⁸Mo. This is the process used to artificially produce ⁹⁹Tc for medical use. Tc decays by beta decay to Ru. Ru is also a fission product.

https://www.nndc.bnl.gov/nudat2/reCenter.jsp?z= 42&n= 56 (select Zoom 1).
https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor#Ruthenium

 

[mjc123]

The periodic table does not include the isotopes of the elements. Why not? For example, C-12 is stable while C-14, also a naturally occurring element, is radioactive but not in the table...

Because it's a periodic table of the elements, not the isotopes. It is based (or was originally) on chemical properties, which don't usually vary with isotopes of the same element. And isotopes weren't understood when the PT was first constructed. Besides, it would make the PT (in traditional printed form) too complicated. However, there are online interactive versions of the PT that give information on isotopes, e.g. https://ptable.com/#Isotopes

 

[Rive]

The periodic table does not include the isotopes of the elements.

Also about this one: many elements has multiple stable (or at least: with notable abundance) isotopes, with different atomic mass.
The periodic table has the atomic mass of an element as average of the atomic mass of its isotopes, weighted by their abundance.
So, this way the periodic table does contain the isotopes.

 

[phyzguy]

Interesting point about At is it's rarity.

The rarity of At-215 is the subject of Asimov's entertaining essay titled "Only a Trillion".

 

[snorkack]

Out of a million U-238 nuclei, 200 decay through At-218... and 0,54 undergo spontaneous fission (all fragments combined).
Out of a million U-235 nuclei, 13 800 decay through Fr-223, 0,83 through At-219 and 2,3 through At-215.

 

[cmb]

Also fun facts considering OP meaning of 'found in nature'.

Presumably this means 'abundance on Earth'?

The helium that occurs on Earth is from nuclear decay, usually mined where there is granite (with uranium inclusions), it is the alpha particles, this will all bleed off the Earth over time if not replaced by such decay.

Similarly, all the argon in the atmosphere is from the decay of naturally occurring, but radioactive, potassium 40, which happens to have a half-life somewhere close to the age of the Earth so can still be found (being left over from the super nova remnants the solar system was formed from).

Without radioactive uranium and potassium, it is possible we might not have helium and argon (either!). One might say these elements are 'artificial', albeit being 'naturally' made from radioactive processes (rather than those made from artificial processes).

Further, such super nova remnants include plutonium which would have been a 'primordial' naturally occurring element of young planet Earth, Pu 244 lasting some 100 million years or so half life.

There is also the case of spontaneous fission of uranium, whose neutrons would then activate trace levels of that uranium into neptunium and plutonium. There must be 'some' atoms of these elements in existence on Earth, due to natural processes, and possibly higher atomic masses too for the same reason.

Any of the current Earth-bound elements being radioactive and still here is, therefore, down to whether their half life is comparable to the age of the solar system, or whether they are decay products from some originally-primordial element that is no longer with us.

 

[shjacks45]

Not true. Technitium, Promethium, Astatine. Small amounts of Plutonium and Neptunium.

Since your premise is not true, there's little point discussing conclusions.

Funny you didn't mention that Vanadium50 is a Natural (primordial because it would exist at the birth of the Universe) isotope with a half life of 10^17 years which is many times the age of the Universe. Bismuth209 also has a 10^16 year half-life, would have to be created by the first supernova.
Wasn't noted was that like electron shells that the lowest energy states for atoms also follow quantum numbers (the most stable are even or paired). Tritium is radioactive, but anomalously stable as the energy neede to knock off a neutron is similar to Deuterium. Why? Because the neutron is paired. Calcium40 is anomalously stable because 20 nucleon isotopes are more stable and Ca is double 20. Isotopes with odd number of Protons tend to be stable with an even number of Neutrons (e.g. Sodium23, Aluminum27, Vanadium51), and less stable with both being odd, like Vanadium50.
Also note the "stability curve" and that isotope energy can decrease with more nucleons ends at Ni60/Fe58 which seems to end stable elements at Lead element 82. Although Uranium238 is primordial, it is radioactive (half life > age of Universe.)