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 ...
CL 35 the 35 means that it has 17 protons and 18 neutrons. Why isn’t this 35.000 instead of 35.453? Because the mass of the neutron is the combined mass of a proton plus an electron. That’s where you get the extra bit on.
An amazing theoretical feat, without even an underlying theory.
It’s scary that several here claim to work with mass calculations yet say they are using isotopic mass of the ONE most stable isotope and say that they are getting their isotopic weight off the atomic weight chart.
NO NO NO!
“Only when there are two isotopes. And only when you’re dealing with a lot of significant figures.”
So how is knowing the atomic mass of the mixture at all helpful in solving this problem? It isn’t.
For all of these problems I need to know the atomic mass of the most common occurring Isotope. Not the mixture. The mixture is useless to me, although I can derive from it the expected ratios, which isn’t the point of the periodic table in the first place.
That’s because we aren’t using your charts. Thank God for that.
The atomic weight is 35.5 because of isotopes.
So what’s the atomic mass of CL 35 then.
I’m waiting.
Atomic mass of CL 35 please.
Go look it up. It will do you good to actually research something. Your 35.453 is the ATOMIC mass of chlorine which is a mixture of CL-35 and CL-37, roughly 75/25 respectively.
Instead of weighting, how about googling Chlorine and looking up the atomic and isotopic waits.
Whether you can understand it or not, it shows how the elements relate to one another.
“The most stable one? Either an isotope is stable, or it isn’t”.
Radioactive elements? Not all isotopes are stable and not always is the most abundant isotope stable.
Um, yeah, those "aren't stable." As in, isotopes are stable or they AREN'T STABLE. I stand by what I wrote.
“It just says the atomic weight is 35.453, as a result of it being a mixture of 35 and 37”
Wrong. You fail chemistry. The atomic mass of CL 35 is 35.453 as a result of CL 35 having 17 protons and 18 neutrons.
See what I mean? You’re already confused as to what atomic mass means because of this bullshit change. I rest my case.
Nope, you're wrong. I have tried to correct you several times, but you have not only resisted every effort, but you have adopted such an attitude that I'm done with you. Scroll down to chlorine on this page. http://www.chemguide.co.uk/analysis/masspec/elements.html Or maybe this page: http://www.webelements.com/chlorine/isotopes.html Or just search for "Chlorine isotopic abundance."
And then apologize.
Until then, goodbye.
“Go look it up. It will do you good to actually research something. Your 35.453 is the ATOMIC mass of chlorine which is a mixture of CL-35 and CL-37, roughly 75/25 respectively.”
So you don’t know. Awesome. Thanks.
The format of your post is a little confusing. Are these your words? If so, you are incorrect. It is because it is a mixture of 35CL and 37CL and is a result of CL 35 having 17 protons and 18 neutrons CL 37 having 17 protons and ____ neutrons. I will let you fill in the blank.
Yes, I mean Cl-35.
“If you had a sample of pure 35Cl which had no 37Cl, then you could say it had an atomic weight of 35.”
Ok good. You at least know what you are talking about.
“The purpose of using the given atomic mass (35.453) is to accurately know the weight of one mole of chlorine”
You’re switching between atomic weight and atomic mass? Interesting.
Use ‘relative atomic mass’ and ‘atomic mass’. Atomic mass should be almost or exactly the same as ‘mass number’, because that is what they are supposed to represent. That’s why we use the 35 and the 37 as shorthand.
Relative atomic mass is useful, if and only if you are assuming that the sample has the expected ratio. It’s not useful on a periodic table. Nor is it useful when you are trying to determine the experimental ratio between two isotopes of the same element.
Can you understand why assuming that the ratio is going to be the bearing takes away from the understanding that an element is defined by the number of protons it has?
At least two people have tried to show you the light as to where your error is and instead of taking two minutes to research (I gave you the hint ... Google Chlorine atomic weight) that will back up our facts as presented to you and what do you do? You make some outrageous attack. I really hope that you reconsider your position, fix your attitude, do the research and come back and acknowledge your error. I don't expect you to apologize for the above snip but it would be refreshing.
I take it that you have never taken a quantitative analysis class. The 35.453 is the most important number because if you have one mole of chlorine gas, it will weigh 70.906 grams. Not 70 grams. Not 74 grams. But 70.906 grams.
OK. One more lesson in chemistry. Are you getting tired from digging that hole?
Definition: Atomic mass or atomic weight is the average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally-occurring element.
Also Known As: Atomic Weight
“If so, you are incorrect. It is because it is a mixture of 35CL and 37CL and is a result of CL 35 having 17 protons and 18 neutrons CL 37 having 17 protons and ____ neutrons. I will let you fill in the blank”
19.
Relative atomic mass has nothing to do with the actual properties of Chlorine. This is why it shouldn’t be on a periodic table.
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