Island of nuclear stability?

[Positron137]

I've heard this expression in nuclear physics: the "island of stability." I know it has to do with the stability of a heavy transuranium atom (at least i think so), but what precisely does that expression mean? And what does it have to do with quantum mechanics? Why is this "island" there? Because I know that several artificial elements decay very quickly. Thanks!

 

[SteamKing]

See this article: http://en.wikipedia.org/wiki/Island_of_stability

Briefly, with a few exceptions, most of the transuranic elements (Z > 92) are relatively short-lived. The 'island of stability' hypothesizes that when Z is around 118 to 126, the elements will become relatively stable, with half-lives on the order of minutes rather than minute fractions of a second. So far, none of these elements have been synthesized.

 

[snorkack]

Uranium is on an island of stability.

Look at the longest-lived isotopes of lighter elements:
Pb-208 - stable
Bi-209 - 2*10^19 y
Po-209 - 102 y
Heavier isotopes are extremely unstable:
Po-210 - 138 d
Po-211 - 520 ms
Po-212 - 300 ns
Now going to heavier elements, they are increasingly shorter-lived:
At-210 - 8 h
At-213 - 125 ns

Rn-211 - 15 h
Rn-214 - 270 ns

Fr-212 - 20 min
Fr-215 - 90 ns

Ra-213 - 2,7 min
Ra-216 - 180 ns

Ac-214 - 8,2 s
Ac-217 - 70 ns

Th-215 - 1,2 s
Th-218 - 110 ns

Pa-216 - 100 ms
Pa-219 - 50 ns

U-219 - 55 ms
U-220 - 60 ns

See how unstable the elements get?

With a catch.

It is actually a band of instability.

Po-212 does have half-life of 300 ns.
But Po-218 has 3,1 minutes.

With heavier elements:
At-220 - 3,7 min
Rn-222 - 3,8 days - and that is longer than the 15 h of Rn-211
Fr-223 - 22 min
Ra-226 - 1600 y
Ac-227 - 22 y
Th-232 - 14*10^9 y
Pa-231 - 33 000 y
U-238 - 4,5*10^9 y
Np-237 - 2,1*10^6 y

As you see, the island of stability is separated by a deep and wide instability strip.

That island goes on quite far to higher elements. But what happens beyond? Is there another island or islands beyond instability strip/s?

 

[Positron137]

Ah ok. That's amazing! So I guess the research going on now is to see how far this island goes, where it stops, whether there are more islands, etc.?

 

[Jolb]

As the other posters have explained, different neutral atoms have different lifetimes. For example, there are Helium-4 atoms (two neutrons and two protons) throughout the universe that have been around since shortly after the Big Bang--they're extremely stable. But if we built the another isotope of Helium, Helium-5 (three neutrons and two protons), it would shoot out its extra neutron within 10^-20 seconds--unlike Helium-4, it is unstable.

If we were to write down all the possible isotopes of elements in terms of their proton number Z and neutron number N, then each pair of numbers (Z,N) gives us a different isotope of some element. We could associate each pair (Z,N) with a spot in a two-dimensional grid, with Z on one axis and N on the other axis. Then, for each pair (Z,N), we could either measure the stability of the atom or try to theoretically predict it--then we could color that atom's square a dark red for very stable atoms and a blue for unstable atoms, which yields the following plot:

The bottom left stable atoms are what appear in our periodic table. The gap before the "peninsula" is the "band of instability" snorkak points out. The upper-right region of stability corresponds to elements that we have not been able to create yet but are predicted to have a relatively long lifetime, and it is called the "island of stability" because it does not "connect" to the "continent" via other stable atoms.

One example that I like to remember as a sort of opposite phenomenon is that the relatively light element Technetium (Tc, Z=43) actually lies in a "pond of instability" right among a bunch of other stable elements. It is the lightest radioactive "element" on the periodic table!