Posted on 12/15/2010 5:23:20 PM PST by smokingfrog
For the first time in history, a change will be made to the atomic weights of some elements listed on the Periodic table of the chemical elements posted on walls of chemistry classrooms and on the inside covers of chemistry textbooks worldwide.
The new table, outlined in a report released this month, will express atomic weights of 10 elements - hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium - in a new manner that will reflect more accurately how these elements are found in nature.
"For more than a century and a half, many were taught to use standard atomic weights — a single value — found on the inside cover of chemistry textbooks and on the periodic table of the elements. As technology improved, we have discovered that the numbers on our chart are not as static as we have previously believed," says Dr. Michael Wieser, an associate professor at the University of Calgary, who serves as secretary of the International Union of Pure and Applied Chemistry's (IUPAC) Commission on Isotopic Abundances and Atomic Weights. This organization oversees the evaluation and dissemination of atomic-weight values.
Modern analytical techniques can measure the atomic weight of many elements precisely, and these small variations in an element's atomic weight are important in research and industry. For example, precise measurements of the abundances of isotopes of carbon can be used to determine purity and source of food, such as vanilla and honey. Isotopic measurements of nitrogen, chlorine and other elements are used for tracing pollutants in streams and groundwater. In sports doping investigations, performance-enhancing testosterone can be identified in the human body because the atomic weight of carbon in natural human testosterone is higher than that in pharmaceutical testosterone.
(Excerpt) Read more at eurekalert.org ...
Relative atomic mass is a synonym for atomic weight and closely related to average atomic mass (but not a synonym for atomic mass), the weighted mean of the atomic masses of all the atoms of a chemical element found in a particular sample, weighted by isotopic abundance.[5] This is frequently used as a synonym for the standard atomic weight and it is correct to do so since the standard atomic weights are relative atomic masses, although it is less specific to do so.
“How did you go about doing it? Did you count the number of molecules?”
Obviously. Isn’t that what you do with a mole?
Acid sample 1, Acid sample 2
Crystal sample 1, Crystal sample 2.
Weigh both crystal samples. Measure volumes of both acid samples.
Add acid 1 to both crystal sample 1 and 2. Measure volumes needed to dissolve crystals.
Repeat with Crystal sample 1 and crystal sample two with acid sample 2.
Solve for the 4 variables and pray you don’t disturb the scale.
Atomic mass has been on the periodic tables since Mendeleev made the first one. But then he didn't know about isotopes. The periodic table was reorganize early on with the valence numbers but the atomic mass remains. It's convention.
Wow, you can quote wikipedia.
Amazing expertise.
The old definition has the atomic mass of every element as a ratio of C-12. Now, this is no longer the case. Which is exactly the point I was trying to hammer home earlier.
PLEASE show me where in the article that is stated! Here is what the article says ... MORE ACCURATELY. It says nothing about a change in definition. NO IUPAC definitions are being changed. NONE, NADA, ZIP, NIL.
"The new table, outlined in a report released this month, will express atomic weights of 10 elements - hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium - in a new manner that will reflect more accurately how these elements are found in nature."
“Atomic mass has been on the periodic tables since Mendeleev made the first one.”
You’re right. It has been. Periodic tables cited the mass of the most common isotope, and have done so until now.
“But then he didn’t know about isotopes. The periodic table was reorganize early on with the valence numbers but the atomic mass remains. It’s convention.”
The problem is that this change introduces information that has no relevance to the purpose of the Periodic table. Why was Mendeleev able to make predictions? He hit upon the tables and the columns by arranging them according to their physical principles.
Why is it relevant to the chemistry of Chlorine to know that Cl-37 is one quarter as abundant as Cl-35? You see what the fellow above posted, that expressing it as a ratio of C-12 links the whole table together.
Do you agree with it?
Because it changes the whole relationship of atomic mass as a ratio of C-12. That’s the point.
It’s not more accurate. They are fudging the numbers to make them mean something else quite entirely.
Found in nature. Where? In the universe? Or just here on Earth? Or just in some laboratory in North America? Europe? Russia?
What?
No. They have cited the mass of the common element.
“Do you agree with it?”
That relative atomic mass is quite different than atomic mass and that they do not mean the same thing? Yes.
I’ve kinda been saying that all along. Glad to have you on board.
‘common element’
What on earth do you mean by ‘common element’. Aren’t Elements supposed to be the same no matter where you go?
Hydrogen is Hydrogen, Helium is Helium.
You totally ducked responding to the highlighted portion that was relevant to the previous post. Thank you.
Ah, well. Ze times zey change, non?
I didn’t ignore your reference. The reference quotes what I said. Atomic masses are expressed as a ratio of C-12.
Relative atomic masses express the ratio of the isotopes as found here on earth. They are not the same thing. They do not mean the same thing. One references an intrinsic property of the element, and one references experimental data which is sometimes right.
We’ve come to agreement on pretty much everything. You like the change, although I’m not sure why. I don’t. I like my periodic table as is.
No. Elements have different isotopes according the different areas they are found in or as a result of man's intervention. It is called the "PERIODIC TABLE OF THE ELEMENTS" and thus the atomic weight it displays is the weight of the ELEMENT, NOT the isotope.
Which has the ELEMENT atomic mass just as the new tables will have.
And the atomic masses expressed on the periodic table of ELEMENTS are the atomic masses of the ELEMENTS, not the atomic masses of the isotopes.
Well sure, yours does now. Mine doesn’t. :) Had it for a long time.
So what atom of Cl has a mass of 35.45 amu?
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