Ask A Scientist Chemistry Archive

name         Becky E
status       educator
grade        6-8
location     MS

Question -   I have taught the synthetic elements are 43, 61,
and 93 and up.  I had a student question that 85 Astatine was
one.  I searched the Internet and one web site said it was.  Is
this true?  Do I need to include 85 as synthetic because they are
coloring the synthetics on their periodic table?
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Becky-
     Do as your common sense leads you,
because "synthetic" can be a fuzzy question, a matter of human 
opinion more than science fact.
If you manage to describe your criterion in passing, you will do the 
kids no harm.
They can refine their views later on, and your intuitive 
classification may add interest for them.

Facts would be:
    "Does this element have a stable isotope?"
          (Yes  to all elements (83)Bismuth and below,
                except (43)Technetium and (61)Promethium, and
           No  to all elements above 83: (84)Polonium, etc...)
or
    "Even though all this elements isotopes are radioactive and 
 decay eventually,
       is the longest-lived isotope's decay time long enough to last 
 the age of the earth?"
       (Yes  for Th232 at 14 billion years & U238 at 4.5 billion years,
        No    for all others above 83)

Yes to either one would make it a naturally occurring element.
A third "Yes" case is "naturally occurring" but very temporary:
the "daughter isotopes" in the diverse decay sequences
of the primary naturally remaining radiative elements, U238 and Th232.

(85)Astatine and (86)Radon are in this category.
Astatine isotopes are all very unstable, all live less than 10 hours.
If you wanted some, in practice you would not mine it,
you would briskly refine a little out of the decaying radiative 
material you presently have in stock.
Or maybe do it by exposing some stable element
to neutrons from a nuclear reactor.  That would be synthetic....

But, like (86)Radon which is in the news as a natural hazard even 
though it lives less than 4 days,
traces occur in nature so we do not usually classify it as a 
synthetic element.

Another way to look at it is: how was it first seen (discovered)?
(The human-history perspective.)
Natural radioactive elements were first isolated and examined by 
determined scientists.
from natural stockpiles.
They only needed a tiny amount for a fraction of a day or less
to see it in a mass spectrometer or some other sensitive instrument.
  A few elements were so completely missing
that only by deliberately arranging nuclear reactions did they get 
enough to see in a mass spectrometer.
This natural absence seems to me to be more random coincidence than 
inherent specialness,
kind of like a small spot on the wall being completely missed
amidst the scatter of pellets from a shotgun.

As scientists continued the search,
they found trans-uranium elements in cosmic radiation, evidence of 
one atom at a time.
(Or was it holding heavy elements up high to be bombarded by 
medium-weight cosmic ray particles? I forget.)
Does this make them all "naturally occurring"?
A matter of opinion; choose and define your own criterion.

If it were my choice I would merely color all the elements with no 
stable isotope (43,61,83+),
and then tell them some stories about how some of the unstable ones 
manage to be encountered in nature,
and that many of them are deliberately synthesized by man.
Informed citizens would be well served if they noticed:
"Those guys are talking about Astatine or Thorium;
wasn't that one of the colored ones on my chart?

A second decoration might be added to indicate presence in our planet's crust,
  maybe intensifying the color for the radioactive elements that 
 have not been seen in ores,
or maybe fading the few which you know can be derived from Uranium 
ore stockpiles.
Radon, Radium, and Francium would be examples of that.
U and Th would be greatly faded, because they are more abundant and 
less intensely radioactive.
"Almost" stable.

Color intensity inversely proportional to the shortness of the 
half-life sounds good to me too.
Use log10(half life), with white being 10^11 years, and black being 
perhaps hours (10^-3 year).
Then Radium, highly dangerous but marginally mine-able, with 
half-life 1600 years,
would be medium-dark gray.
16 shades of gray can show 16 orders of magnitude well enough.
Every element with any shade less than white needs some particular 
excuse to exist,
some moment of nuclear reaction, whether man-made or natural.

The particular excuse for the "natural" Uranium in the earth
would be in the extreme nuclear reaction at the moment of supernova 
explosions.
Same as the heavy-but-stable elements like gold.

Jim Swenson
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