Is lead (Pb) radioactive? I have heard about it being radioactive but have also heard it referred to as the heaviest stable element. Radioactivity refers to release of alpha, beta, gamma rays.

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    $\begingroup$ The major isotopes of lead are not radioactive. $\endgroup$ – Klaus-Dieter Warzecha Apr 23 '15 at 7:44
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    $\begingroup$ Because this is a popular question with an incorrect premise, i have started a discussion regarding the validity of closing down this question in meta. Kindly please give your views regarding this question there. $\endgroup$ – Eka Apr 24 '15 at 3:18
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    $\begingroup$ The question ‘Why is lead radioactive?’ is ambiguous, but it is not necessarily based on a false idea. Any ordinary lead sample is indeed radioactive, which makes the question ‘why?’ valid. Maybe the question could be edited to clarify the problem. $\endgroup$ – Faded Giant Apr 24 '15 at 8:07
  • $\begingroup$ @Loong In the spirit of the OP's message on the meta post you should go ahead and extend/ clarify the question. I believe it would be not conflicting with the author's intent - I believe the main driving force for the question was to get some clarification on the matter. A little context can never hurt, and the OP can always roll back, if she/he thinks the edit is inappropriate. $\endgroup$ – Martin - マーチン Apr 27 '15 at 3:47
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    $\begingroup$ Saying "The major isotopes of lead are not radioactive" is sort of circular reasoning. The major isotopes are the major isotopes precisely because they are not radioactive. Lead is the highest atomic numbered element that has stable isotopes. $\endgroup$ – MaxW Feb 13 '17 at 3:42

Important naturally-occurring radioactive lead nuclides are:

$\ce{^{214}Pb}$ $\left(t_{1/2}=26.8\ \mathrm{min}\right)$ from $\ce{^{238}U}$

$\ce{^{210}Pb}$ $\left(t_{1/2}=22.3\ \mathrm{a}\right)$ from $\ce{^{238}U}$

$\ce{^{211}Pb}$ $\left(t_{1/2}=36.1\ \mathrm{min}\right)$ from $\ce{^{235}U}$

$\ce{^{212}Pb}$ $\left(t_{1/2}=10.64\ \mathrm{h}\right)$ from $\ce{^{232}Th}$

On a geological time scale, their half-lives are very short. Nevertheless, these lead nuclides are members of the $\ce{^{238}U}$, $\ce{^{235}U}$, and $\ce{^{232}Th}$ decay series. Thus, they are constantly reproduced by decay of their respective mother nuclides. Therefore, all environmental samples (including soil, water, air, plants, and animals) naturally contain significant amounts of radioactive lead nuclides. However, in various materials (e.g. residues from mining or oil and gas production, fertilizers, building materials), the concentration of natural lead nuclides can be strongly enhanced by technology.

When lead is freshly refined (chemically purified), most of the mother nuclides are removed and the short-lived lead nuclides quickly decay. Nevertheless, the lead still contains significant amounts of $\ce{^{210}Pb}$ $\left(t_{1/2}=22.3\ \mathrm{a}\right)$. The presence of $\ce{^{210}Pb}$ is a source of background mainly through the lead X-rays and bremsstrahlung caused by high-energy beta radiation emitted by its decay product $\ce{^{210}Bi}$. This background is a real problem when lead is used as a shielding material for low-background detectors. Therefore, some shields are made from selected lead with certified low $\ce{^{210}Pb}$ content. Preferably, very old lead is used. Since $\ce{^{210}Pb}$ decays with a half-life of 22.3 years, samples of lead that are many decades old are relatively free of this activity. In one extreme example, 2000-year-old lead was salvaged from a sunken Roman ship.

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    $\begingroup$ what is "a"...? $\endgroup$ – Anixx Apr 23 '15 at 14:28
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    $\begingroup$ @Anixx ‘a’ is the unit symbol for the unit ‘year’ according to ISO. (However, ‘year’ is not an SI unit.) $\endgroup$ – Faded Giant Apr 23 '15 at 14:45
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    $\begingroup$ @Anixx In particular, the "a" comes from Latin "annum", which means "year". $\endgroup$ – senshin Apr 23 '15 at 16:39
  • $\begingroup$ "all environmental samples [...] naturally contain significant amounts of radioactive lead nuclides" seems to be an extreme exaggeration, if not outright incorrect. You're claiming there's a "significant" amount of radioactive lead in every coffee I ever drink? $\endgroup$ – Emilio Pisanty Feb 13 '17 at 1:04
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    $\begingroup$ I agree that the low levels of radioactive lead should be described as "detectable" not "significant." $\endgroup$ – MaxW Feb 13 '17 at 3:32

One radioactive isotope of lead is $\ce{^{210}_{82}Pb}$, which has a half-life of 22.20 years. The only reaction that $\ce{^{210}_{82}Pb}$ undergoes is:

$$\ce{^{210}_{82}Pb ->^{210}_{83}Bi +^{0}_{-1}e}$$

The $\ce{^{0}_{-1}e}$ released in the reaction is a high speed electron, otherwise known as a $\beta^-$ particle. The fact a $\beta^-$ particle is released by the $\ce{^{210}_{82}Pb}$ nucleus would suggest that it is an excess number of neutrons causing the instability.

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As noted in the comments, the premise is wrong.

Actually: Lead is the heaviest stable element.

Why are heavier elements unstable?

The atomic nucleus consists of neutrons and protons.

Without going into detail, these are held together by short range nuclear forces.

The protons also repel each other due to their positive charge.

As we add more protons, the charge repulsion continues to build. Each pair of protons generates a repulsive force, so this grows in total as the square of the number of protons.

The short range attractive force does not grow quadratically in the same way, as 'distant' nucleons do not feel as much of an effect.

The overall effect is that above lead the repulsion is strong enough that the overall energy of a lower number and the radiated particle is less than that of the heavy nucleus.

The difference in total energy drives the radioactive decay.

As the elements get heavier this energy difference increases resulting in faster decays. This is complicated somewhat by the nuclear structure (the same concept as the electronic structure) in that completed nuclear shells create stability.

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Before explaining why lead is radioactive, I would like to briefly explain what radioactivity is.

According to Wikipedia:

Radioactive decay, also known as nuclear decay or radioactivity, is the process by which a nucleus of an unstable atom loses energy by emitting ionizing radiation. A material that spontaneously emits this kind of radiation — which includes the emission of alpha particles, beta particles, gamma rays and conversion electrons — is considered radioactive.

That is, radioactive decay occurs because an unstable atom "prefers" to release extra energy to become more stable. If you consider lead, it has different isotopes. A large percentage (>98%) of lead ordinarily found in nature is of stable isotopes whereas the rest of its isotopes are unstable. It is the unstable isotopes of lead that are radioactive.

Another example is carbon.

Carbon-12 with six neutrons is a stable isotope of carbon which has no radioactivity and carbon-14 with eight neutrons is not stable and shows radioactivity.

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  • $\begingroup$ "Which means": I cannot see how the preceding sentence could in any way be used to infer any percentage at all. And if you mean the linked table, I think all isotopes there are either marked "observationally stable" (and their mole fractions add up to 1) or have a mole fraction that is marked "trace" at best, so 98.6% seems just an error on your part; if anything it would be 100% $\endgroup$ – Marc van Leeuwen Apr 23 '15 at 10:59
  • $\begingroup$ @MarcvanLeeuwen thanks for showing the mistake I would correct that error. $\endgroup$ – Eka Apr 23 '15 at 11:43

All isotopes of lead are metastable and radioactive (although nobody, so far as I know, has invented a sufficiently sensitive detector to confirm this quantum mechanically certain uncertainty).

Alpha particle decay of lead results in stable isotopes of mercury. So the rather improbable answer to the question "Which is the heaviest stable element/nucleus?" is, of all things Thallium. That is not what I learned at school. The article Barely Radioactive Elements leads you, very gently and humbly, so it seemed to me, to Avogadro's Number of years.

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  • $\begingroup$ Actually, taking into account isotopes which are theoretically unstable but experimentally extremely long-lived, every nucleus beyond $\ce{^92_40Zr}$ is radioactive. $\endgroup$ – Nicolau Saker Neto Aug 24 '16 at 10:39
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    $\begingroup$ Please don’t add any variations or permutations of ‘please Google X’ to your answers. Rather, summarize and quote your sources and — most importantly — cite them. This is a scientific site, after all, and I have yet to read a paper saying ‘as the reader may confirm by Googling/SciFinder searching …’ ;) $\endgroup$ – Jan Aug 24 '16 at 11:06
  • $\begingroup$ Touche, Jan! No offence intended. I am neither chemist nor physicist,nor (I trust) an unwelcome intruder - merely a medical specialist who tries to remain eclectic in my winter burrow in Darkest Australia. Oh, and I was not purporting to produce a "paper". On a point of order, why is a "scientific site source" any less of a source solely because (if I understand you correctly) it can be accessed by not solely the cognoscenti? $\endgroup$ – lyn barruga Aug 25 '16 at 10:42

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