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Why is radioactive strontium-89 is considered medically useful for bones (along with other benefits) whereas strontium-90 (also being radioactive) is harmful for human body?

How does the addition of one atomic mass has adverse effects on human body?

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    $\begingroup$ "Medically useful" is an awfully vague term. A scalpel is medically useful, come to think of it. A scalpel can kill, too. $\endgroup$ Jun 10 '19 at 21:32
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    $\begingroup$ Strontium 89 (or 90) have zero benefit for human bones or other tissue. Sr-89 is used to treat (=destroy) bone metastasis. Normal, non-radioactive strontium is used for treating osteoporosis. $\endgroup$
    – Karl
    Jun 10 '19 at 21:57
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The half-life of $\ce{^{89}Sr}$ is 50 days compared to a half-life of 29 years for $\ce{^{90}Sr}$. So a large enough dose of $\ce{^{90}Sr}$ to be useful would leave the patient radioactive for the rest of his/her life.


Edited for use of $\ce{^{89}Sr}$. In a comment Karl pointed out that the isotope is used to treat bone cancer.

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    $\begingroup$ Not so much detectable, mind you. To treat cancer you want to hit it with a nice big dose of radiation in a short period of time to beat it down. $\endgroup$
    – Jon Custer
    Jun 10 '19 at 21:55
  • $\begingroup$ The physical half-life of Sr-90 is indeed 28.79 years; however, the biological half-time of strontium is shorter. In particular, the removal half-time of strontium from exchangeable bone volume is about 80 d. After ingestion of Sr-90, about half of the resulting total effective dose occurs within the first 4 years after intake. $\endgroup$
    – user7951
    Jun 12 '19 at 20:07
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The specific radioactive properties of an isotope are very different even from other isotopes of the same element. The chemical properties will be very similar.

The issue for using isotopes for medical purposes is that you need a combination of specific chemical properties and the right radioactive behaviour. So one strontium isotope will not be a remote substitute for another.

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Both isotopes decay by beta emission, but strontium 90 has a decay energy of 0.546 MeV, while strontium 89 has a decay energy of 1.463 MeV. (CRC Handbook)

The range of the electron from Sr 90 is better suited to putting a strontium atom in the middle of a tumor without irradiating too much of the surrounding flesh.enter image description here

https://images.search.yahoo.com/yhs/search?p=range+of+beta+particles&fr=yhs-Lkry-SF01&hspart=Lkry&hsimp=yhs-SF01&imgurl=http%3A%2F%2Fwww.alpharubicon.com%2Fbasicnbc%2Fimages%2Farticle16radiological7104.gif#id=1&iurl=http%3A%2F%2Fwww.alpharubicon.com%2Fbasicnbc%2Fimages%2Farticle16radiological7104.gif&action=click

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