Hi, Mongolia:
You asked: Are there any elements with Atomic number LESS than 92 which are not
found naturally on Earth?
Of elements below uranium (92), only promethium (61) has _not_ been
detected on Earth occurring naturally:
[Promethium]
http://www.scescape.net/~woods/elements/promethium.html
"In 1945, Marinsky, Glendenin, and Coryell made the first chemical
identification by use of ion-exchange chromatography. Their work was
done by fission of uranium and by neutron bombardment of neodymium.
Searches for the element on earth have been fruitless, and it now
appears that promethium is completely missing from the earth's crust.
Promethium, however, has been identified in the spectrum of the star
HR465 in Andromeda. This element is being formed recently near the
star's surface, for no known isotope of promethium has a half-life
longer than 17.7 years."
[One, Two, Three, Many]
http://www.todayinsci.com/stories/story033b.htm
"The first transuranium elements, neptunium and plutonium, have also
been found to exist on Earth. They are born in nature as a result of
very rare nuclear reactions between uranium and free neutrons. These
ghosts can "boast" of hundreds and thousands of tons. But as to
promethium and technetium, which are also due to uranium (the latter
is capable of spontaneous fission, with its nuclei breaking up into
two approximately equal fragments), there is nothing that can be said
of them.
"Scientists have found hardly perceptible traces of technetium, and
are still looking for promethium in uranium minerals. The balance has
yet to be invented on which the Earth's "reserves" of promethium and
technetium could be weighed. "
Although technetium (43) was the first element produced artificially,
it has since been detected in trace amounts in the Earth's crust.
Since that fact is not duly recorded in every Web site, allow me to
elaborate:
[Technetium]
http://www.scescape.net/~woods/elements/technetium.html
"The element was actually discovered by Perrier and Segre in Italy in
1937. It was found in a sample of molybdenum, which was bombarded by
deuterons in the Berkeley cyclotron, and which E. Lawrence sent to
these investigators. Technetium was the first element to be produced
artificially. Since its discovery, searches for the element in
terrestrial material have been made without success. If it does exist,
the concentration must be very small."
[Technetium]
http://en.wikipedia.org/wiki/Technetium
"Dmitri Mendeleev predicted many of the properties of element 43,
which he called ekamanganese, well before its actual discovery (see
Mendeleev's predicted elements). In 1937 its isotope Tc-97 became the
first element to be artificially produced, hence its name (from the
Greek ????????, meaning "artificial"). Most technetium produced on
Earth is a by-product of fission of uranium-235 in nuclear reactors
and is extracted from nuclear fuel rods. No isotope of technetium has
a half-life longer than 4.2 million years (Tc-98), so its detection in
red giants in 1952 helped bolster the theory that stars can produce
heavier elements. On earth, technetium occurs naturally only in
uranium ores as a product of spontaneous fission; the quantities are
minute but have been measured.
" ... In 1962, technetium-99 was isolated and identified in
pitchblende from the Belgian Congo in very small quantities (about 0.2
ng/kg); there it originates as a spontaneous fission product of
uranium-238. This discovery was made by B.T. Kenna and P.K. Kuroda.
There is also evidence that the Oklo natural nuclear reactor produced
significant amounts of technetium-99, which has since decayed to
ruthenium-99."
Note that the second link above also answers the last part of your
Question: Are there any elements with atomic number GREATER than 92
which have been found naturally on earth?
Plutonium is atomic number 94; neptunium is atomic number 93.
* * * * * * * * * * * * * * * * * * * * *
You also asked: What three elements with atomic number LESS than 92 are found
naturally with the least quantity? (include elements which may not
have been found in any measurable quantity)
The Comment by pjay1437-ga is relevant. Taking technetium as the
rarest of all, due to its extreme trace quantities (estimated by David
Curtis of Los Alamos as 1 nanogram per kilogram of uranium), broad
concensus exists that the two next rarest elements are astatine (85)
and francium (87):
[Astatine]
http://www.scescape.net/~woods/elements/astatine.html
"The longest-lived isotopes, with naturally occurring uranium and
thorium isotopes, and traces of 217At are equilibrium with 233U and
239Np resulting from interation of thorium and uranium with naturally
produced neutrons. The total amount of astatine present in the earth's
crust, however, is less than 1 oz."
[Francium]
http://www.scescape.net/~woods/elements/francium.html
"While it occurs naturally in uranium minerals, there is probably less
than an ounce of francium at any time in the total crust of the
earth."
* * * * * * * * * * * * * * * * * * * * *
A relevant treatment from the converse point of view is in this earlier GA thread:
[Google Answers: How many of the elements has man made by atomic reactions?]
http://answers.google.com/answers/threadview?id=321247
Hlabadie-ga notes "All the elements can be synthesized, but only four
of the first 92 are commonly regarded as synthetic; and two of those
exist only in
infinitesimal quantities of extremely short-lived naturally occurring
isotopes... It is easy to see why there is some disagreement about
Francium and Astatine."
A good general survey on abundance/rarity of elements on Earth is the
Wikipedia article here:
[Abundance of the chemical elements]
http://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements
"The six very rare, highly radioactive elements (polonium, astatine,
francium, radium, actinium and protactinium) have such low natural
abundances that they are not included or their natural adundances are
not known as they are so low."
regards, mathtalk-ga |
