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Knowing Synthetics are Elements
Name: Christi V.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 10/8/2004
Question:
One of my students asked me the following question: If
elements are the simplest form of a substance, how do you know if a
synthetic element is in the simplest form if it did not exist before?
Replies:
I wish I had more students like yours who are willing to ask insightful
questions!
While it is true that in the early history of chemistry many substances that
were deemed elemental turned out to be compounds that at that time simply
could not be decomposed further into their elemental form, even then there
were already many guides to deciding that a new substance was in fact
elemental. One of the key guides is how they fit into Mendeleev's Periodic
Table predictions. You will recall that Mendeleev was astute enough to leave
spaces for elements that had not yet been discovered, but which properties
could be predicted from the group they were to belong. Moreover, the
compounds
that these heretofore undiscovered elements were expected to form could be
predicted, and by using the available techniques for determining molar masses
of molecules and elements, a new substance could be established as being of
the appropriate mass to be a candidate for the missing element's spot on the
periodic table.
In the modern era, however, the synthetic elements are made deliberately by
some fusion process (since all these elements are higher in mass than any
known elements), and the way it can be established that a new element has in
fact been made is similar to how new subatomic particles have been detected -
by their patterns in cloud chambers, etc. Thus, the new elements are not
collected by decomposition of compounds but by deliberately building them
from
smaller nuclei.
Greg (Roberto Gregorius)
A chemical element is defined by its atomic number (the number of protons)
= the number of electrons in the neutral species. Back in the 1800's and
before when we did not have the periodic table filled in there could, in
principle, be molecule whose collective number of protons matched a "hole"
in the periodic table. I do not think there is a real example, but it would
be possible. Now, however, all the elements with their defining atomic
number fill the periodic table. Currently transuranic elements are being
created, albeit in nano quantities. Is it possible that element A(216) is
actually a molecule M(216-n)Xn where n>0? On paper, yes, but in practice
the experimental setup (high energy accelerators with the ancillary
detection equipment) makes it highly unlikely. These elemental atoms are
very unstable and their nuclear decay products are followed in the
accelerator experiments. So a molecular species would become very evident
because the energy of the decay process is so high that any chemical bonds
would be broken and a "hidden" molecule would show itself clearly.
Vince Calder
Christi-
No offense meant, but the phrasing of this question gives me
difficulties.
I may ramble, trying to sort it out.
A list of statements which I subscribe to:
----------
A molecule is the smallest possible piece of a substance.
A molecule is made of atoms.
An element is a substance made with only one kind of atom.
An element is the simplest kind of substance, (not the "simplest form of a
[specific] substance")
A compound is a single substance made with two or more kinds of atoms.
An alloy or composite is a material of mixed elements which may be of two
or more compounds.
An alloy can be refined into compounds by "thermal/physical" means.
A compound can be broken into elements by "chemical" means.
Elements can be changed to other elements only by "nuclear" means.
----------
I guess "simplest kind of substance" means to me that you cannot break it
down any further.
Now that we know of nuclear technology, that is no longer strictly true of
any substance.
"You cannot break it down by any non-nuclear (chemical or mechanical, or
"common") means" ?
That is about right, though a bit fuzzy and circular.
Today, we know of atoms.
With any handful of substance, we can put it in a machine which takes out
a few atoms,
flings them as individuals through empty space, and measures how hard they
are to turn.
Knowing the exact weights of the atoms, we know which elements they are.
If we have never seen atoms of that weight in any rock or other thing
found in nature,
then the stuff in our handful must be a synthetic element.
If you saw a glowing purple bird, you might likewise assume it was a
synthetic life form.
That's how it was back in the beginnings of nuclear science.
By now we have synthesized atoms of every element-number from 1 to 110 or so.
We know their weights now, and we know that for some elements, all
reasonable weights decay radioactively.
These do not last more than some thousands of years, some far less than a
second.
So if we have some of that element, it must have been synthesized recently.
And it probably makes a Geiger Counter click dangerously, too.
We also know all the atoms by their favorite colors of light, called
spectroscopy.
I.E., which wavelengths of light does a single atom strongly absorb? and
which does it emit when energized?
We know this kind of "fingerprint" for the synthetic ones and natural ones
alike.
To me, "the simplest form of a substance" means a gas of free molecules of
that substance,
or a perfect crystal, all its atoms or molecules stacked up just right.
We never have perfect crystals; they are always slightly imperfect.
But since we have measured almost all possible atoms,
we do not need to make crystals to recognize any of the elements.
I think your student put some abstract (ancient Greek?) philosophical
definitions
together with a modern nuclear phenomenon, the "synthetic element".
It's very difficult to make ideal definitions cover all real facts.
Jim Swenson
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