Radioactivity - lead stable?

[alext87]

My question: When a radioactive element decays it changes to other elements: why is it that most (if not all) radioactive materials decay to lead after giving off beta or alpha particles?

 

Why should lead be the most stable element it decays into - what makes it special!?

[greg1917]

As you'll now, certain numbers of electrons make an atom quite unreactive.

 

2, 10, 18, 32

 

There also appears to be 'magic numbers' of stable protons and neutrons. 82 happens to be one of the numbers in this series - lead is element number 82.

 

the reasons behind nuclear stability are pretty advanced.

 

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch23/natural.html

 

Same but reworded:

 

http://ottawa.rasc.ca/observers/an9506p3.html

 

A pretty big page on all sorts of nucleosynthesis and nuclear stuff. peruse at your leisure.

 

http://www.astro.queensu.ca/~hanes/p014/Notes/Topic_058.html

[swansont]

As you'll now' date=' certain numbers of electrons make an atom quite unreactive.

 

2, 10, 18, 32

 

There also appears to be 'magic numbers' of stable protons and neutrons. 82 happens to be one of the numbers in this series - lead is element number 82.

 

[/quote']

 

As is 126, which is the number of neutrons in Pb-208, so it is "doubly magic." Not surprisingly, it has the highest abundance of the lead isotopes, and is the heaviest stable isotope.

 

Note that lead is not the "most stable." You have just reached the point where it is no longer energetically advantageous to give off an alpha particle, so it is stable vs. alpha decay, but lead is less tightly bound than iron (and elements nearby), based on binding energy/nucleon. Beta decay is all about having too many or few neutrons for the number of protons in the nucleus.

 

(edit- fix typo)

[YT2095]

as a general rule of thumb, and element from #84 and upwards are considered unstable, but fitting in with the above post (and I`m sure Greg will correct me if I`m wrong) Cerium is considered quite stable in terms of the naturaly occuring salts and minerals, as does Thorium but to a lesser extent. these two would probably fit in that stability series (and I`ve not looked, I just know this from practical info).

there CAN be isotopes of lead sure, but so can many other elements, but those all decay back to the stable form of whatever the isotope is made from, Lead, Cobalt, Strontium etc...

[greg1917]

Aside from the magic number rule, there is another principle that applies to radioisotopes.

 

Isotopes with an odd number of protons are likely to be more unstable than those with an even number.

 

Isotopes with an odd number of neutrons are more likely to be unstable than those with an even number.

 

Thus isotopes (like Pb208) with an even number of both protons and neutrons are likely to be very stable.

 

Note Ive said 'likely' in all those statements - there are exceptions to each.

[swansont]

as a general rule of thumb' date=' and element from #84 and upwards are considered unstable, but fitting in with the above post (and I`m sure Greg will correct me if I`m wrong) Cerium is considered quite stable in terms of the naturaly occuring salts and minerals, as does Thorium but to a lesser extent. these two would probably fit in that stability series (and I`ve not looked, I just know this from practical info).

there CAN be isotopes of lead sure, but so can many other elements, but those all decay back to the stable form of whatever the isotope is made from, Lead, Cobalt, Strontium etc... [/quote']

 

₈₃Bi²⁰⁹ is naturally occurring but with a reeaaally long half-life, so technically it's unstable, but it's damn close to being stable. It has the filled shell of neutrons, as you might expect.

 

The lighter radioactive isotopes won't decay back to a stable form of their own element - beta decay always involves a change of element, since you gain or lose a proton.

[swansont]

Aside from the magic number rule' date=' there is another principle that applies to radioisotopes.

 

Isotopes with an odd number of protons are likely to be more unstable than those with an even number.

 

Isotopes with an odd number of neutrons are more likely to be unstable than those with an even number.

 

Thus isotopes (like Pb208) with an even number of both protons and neutrons are likely to be very stable.

 

Note Ive said 'likely' in all those statements - there are exceptions to each.[/quote']

 

And of course there is the fourth combination: odd-odd is always unstable once you get heavier than N-14.

[alext87]

GREAT! Thanks alot guys!