# Ten Tricky Elements Force a Periodic Table Upgrade

By Andrew Moseman | December 17, 2010 5:22 pm

The weights, they are a-changin’.

What we’re taught in school science classes is a streamlined version of a muddier and more complicated reality, and it’s no different with something as iconic as the periodic table of elements. This week the venerable chart’s overseers decided to fiddle with the atomic weights of 10 elements, changing their values from a single set number to a range of numbers, which is messier but more accurately resembles the messy real world.

The reason for the change is that atomic weights are not always as concrete as most general-chemistry students are taught, according to the University of Calgary, which made the announcement, and the snappily named International Union of Pure and Applied Chemistry‘s Commission on Isotopic Abundances and Atomic Weights, which oversees such weighty matters. [CNET]

Pull up a handy copy of the periodic table like this one and you’ll see two numbers to each element. The first, which is always a clean integer with no annoying decimal places, is the atomic number: it’s the number of protons in the nucleus, and it also serves to give the element its place on the table. Copper has 29 protons, so it’s number 29. Simple.

The second is the atomic weight. This tells us, in grams, how much a mole (6.02 x 1023) of atoms will weigh, and is thus proportional to the weight of a single atom. These numbers are not integers, and that’s not because atoms come in fractions. Instead, atoms have different isotopes that contain different numbers of neutrons in the nucleus. A pure isotope will have a well-defined weight, but in the natural world, most elements appear as a mixture of isotopes. A mass of hydrogen, for example, will mostly contain atoms with a single proton and no neutrons, but will have a few with one or two neutrons mixed in. As a result, when you have over 1023 atoms of hydrogen around, these heavier isotopes ensure that it weighs a touch more than just a gram. [Ars Technica]

The problem here is that some elements have more than one stable isotope. As a result, the element’s atomic weight could skew slightly lighter or heavier depending on which isotopes are present in the sample you’re testing. Given that knowledge, the chemistry powers that be decided it was improper to stick with a single number as the atomic weight for these elements, and instead assigned them a range of values.

So, now instead of carbon listed as being 12.0107 atomic mass units with a measurement uncertainty of about 0.0008, it has an official atomic weight of [12.0096; 12.0116], where the brackets and semicolon indicate an interval of atomic weights. The interval doesn’t reflect an uncertainty in measurement precision but rather a real variation of atomic weight from substance to substance. [Scientific American]

Hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium are all getting the switch to the new way of stating atomic weight. Small potatoes, perhaps. But when Pluto was booted from the planet club, we all saw what happens when you mess around with what people learned in science class.

Image: Wikimedia Commons

CATEGORIZED UNDER: Physics & Math
• GHJ

I never realised there was a distinction between atomic mass and atomic weight before this entry prompted me into googling it to clarify. I seem to have been misusing ‘atomic mass’ when I really meant ‘atomic weight’. So thanks for that.

• Lee Elfenbein

This article says ” pull up a copy of the periodic table like this one and you’ll see TWO numbers to each element” But this not true. Right there on the chart, there is only ONE number for each element. Why? Why do you say there are two?

• Philip

@ Lee
Perhaps next time check the linked page before making a fool of yourself? But bad form also for the Discover sub-editor for not finding a periodic table graphic with the weights (one which had not been dumbed down to the level of morons)

• http://discovermagazine.com Andrew Moseman

@Lee @Philip
Apologies for any confusion about that. Unfortunately it’s not easy to quickly find a table that has the atomic weights listed and is also clearly marked as being OK for commercial re-use.

• Chris

Yet another way to confuse the undergraduates!
One thing I would like to point out, since C-12 is used to define the amu, even if you had a sample of pure hydrogen (no deuterium or tritium) one mole of it would still weigh a little over a gram (1.007825 grams to be precise)

• Matt B.

While they’re at it, why don’t they just put lutetium and lawrencium up next to hafnium and rutherfordium, where they belong? I think this was avoided originally because rutherfordium and elements beyond hadn’t been discovered.

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