QUOTE(Limey @ Sat 16th January 2010, 2:25pm)
Well I did come up with
these. They weren't as bad as the history ones I found, I must admit, but they still seemed like decent errors to me.
QUOTE(Limey @ Mon 11th January 2010, 2:02pm)
QUOTE(Milton Roe @ Mon 11th January 2010, 2:35am)
Okay, move on to the physics and chem! Please!
[What follows is an addendum where Limey does this, which I’d missed. My answers follow]
"However, the hydrogen-1 atom has no neutrons and a positive hydrogen ion has no electrons." If you're going to open the can of worms about the positive hydrogen ion, it seems that it would be appropriate to also mention things like alpha particles, but I'm not sure I would count this a "significant" omission.
Answer. The quote from the article reads:
"Though the word atom originally denoted a particle that cannot be cut into smaller particles, in modern scientific usage the atom is composed of various subatomic particles. The constituent particles of an atom are the electron, the proton and the neutron. However, the hydrogen-1 atom has no neutrons and a positive hydrogen ion has no electrons."
I think it’s a good sentence, as the only nuclide which needs to be qualified under this definition is hydrogen-1, which has no neutrons. If an atom requires at least one electron, then the hydrogen ion is not an atom. In that case, it’s the only singly ionized atom which isn’t an atom. This could be more clear, or (even better) the clause about hydrogen should be left out.
QUOTE(Limey)
"with a negative electrical charge and a size that is too small to be measured using available techniques." Untrue, and misleading. In Demelt, Hans "A Single Atomic Particle Forever Floating at Rest in Free Space: New Value for Electron Radius" (1988), for example, the electron's radius was measured through indirect techniques as less than 10^-20 cm. That was 20 years ago, and is actually mentioned, in a somehwat misleading way, in the Wikipedia article on the electron.â€
Answer: this is not “untrueâ€. It’s still fair to say the electron has a size too small to be measured by available techniques. A size of LESS than 10^-20 cm (10^-22 m) means just that: we have the upper bound, but not the lower one. It might be zero for all we know. But though we know it’s less than 10^-20 cm, it’s too small for us to say what it is, if anything. Whatever the size actually IS (if it is anything), it’s too small to measure with available techniques. How much more clear does one have to be?
BTW my guess is that electrons are not points. A point would have more than the mass of a galaxy, of the universe, of infinity. I doubt that infinities exist in nature
QUOTE
There is also an omission here in that the concept of the classical electron radius is not mentioned. The whole issue of electron size, a basic literature review reveals, is a challenging one, and it should either not be mentioned or it should be discussed in enough depth to do the issue justice.
Well, that’s probably the reason the classical radius isn’t mentioned. It opens a can of worms and it ultimately is not a useful concept, anyway. This much is explained in the article on [[electron]] but it’s too detailed for the article on [[atom]]. What isn’t even in the electron article is that even the Penning trap results may be too much detail, for it gets into concepts from QED and how g results place (lower) bounds on the bare mass of the electron (just as very high energy scattering results do), which in turn corresponds to limits on its bare radius, and so on.
http://cerncourier.com/cws/article/cern/29724. A point electron of course has infinite bare mass and radius, but again we have no idea if electrons are points, strings, whatever. For all we know, the bare electron mass is the mass of a large automobile, and thus still has a radius, albeit smaller than we can (ever) measure.
QUOTE(Limey)
" Neutrons and protons have comparable dimensions—on the order of 2.5 × 10^−15 m—although the 'surface' of these particles is not sharply defined." Is downright meaningless in that is entirely unclear what it is referring to. What is on the order of 2.5*10^-15? It's generally accepted that the diameter of a proton is about 10^-15 (see, e.g.,
http://hypertextbook.com/facts/1999/YelenaMeskina.shtml ) but also that protons/neutrons do not have a radius as such (http://hypertextbook.com/facts/1999/YelenaMeskina.shtml). The wording here makes it impossible to know just what is being claimed, but I feel confident in labeling this an error.
Well, first, if you found a text which says the diameter is on the order of something, then you can’t at the same time claim that these things don’t have radius as such. The radius is obviously just half the diameter (this is all Euclidean, with no GR problems), and if one measure is meaningful the other is just as meaningful! Can’t have it both ways. Yes, the text should have said whether it was diameter or radius.
Proton radius is almost universally defined as root mean square (rms) radius, calculated from the radially dependent charge density, which in turn is integral (rho*r^2)dr/integral (rho)dr, where the integrals are definite with r running from 0 to infinity, and rho, the charge density, is a function of r, determined experimentally, ala Robert Hofstadter’s Nobel prize-winning work in high energy electron scattering off protons. Why the rms value is used rather than some other, is a matter of convenience, and is mostly due to the fact that <r^2> is available as a value which comes out of the Mott scattering equation’s “form factor†term, so it’s easily extracted and fitted. You can google “proton radius†and this measure is about all you find for protons. Hofstadter thought r(rms) was 0.8 fm, and I can find various values from 0.79 all the way up to 0.89 fm.
However, explaining all this is a bit much for an article this general. Even explaining rms takes some calculus, and is distracting. Graphs of proton radial charge density are available, and aside from a small central depression, are mostly exponential. The concept of a “radius†does make more sense for larger nuclei, which have a region of semi-uniform density before you reach an outer skin. Still protons are not point particles anymore than hydrogen atoms are, and somehow this needs to be communicated. The electron density of a neutral H atom is exponential, too. Same for helium. Do they have radii? For that matter, what about the radius of our Sun?
Does it have one? What about Jupiter?
Truth and clarity are conjugate variables, you know.
QUOTE(Limey)
"The radius of a nucleus is approximately equal to [1.07*A^1/3] fm, where A is the total number of nucleons." is very wrong. The conventional formula is Rsub0 * A^1/3 (if anyone knows how to make subscripts and superscripts, please tell me), but the constant conventionally used is not 1.07, instead it decreases from around 1.3 for light nuclei to around 1.2 for heavier nuclei ("Improved Z^1/3 Law of Nuclear Charge Radius", see
http://www.iop.org/EJ/abstract/0253-6102/51/1/23) . Older studies put the value at 1.2 to 1.5, but have since been dismissed. I've managed to find a few references to a constant of 1.07 but they're quite old. More recent scholarship has also departed from the A^1/3 law (which is a weak estimate at best even after the constant is varied). Lei, Zhang, and Zeng find that the Z^1/3 law is substantially superior to the A^1/3 law. Either way, there is a fairly substantial error here.
If you use google for “1.07 nucleus radius half-density†you get hundreds of results, the latest I see offhand is a 2005 text by Hooshyar on nuclear fission (Google [Hooshyar proton “half-density radiusâ€] to see the intro)
http://www.google.com/search?hl=en&q=proto...h&aq=f&oq=&aqi=Most of this is whose experimental data you believe. The article on [[atomic nuclei]] actually does have a charge half-density radius formula with a constant of 1.25 fm*A^1/3, and a note that this isn’t constant. The formula in the article on atom was added by another person. They disagree as does the literature. I’ll agree that the formula in [[atom]] needs qualification, with the various definitions (as well as assumption of 2-plus parameter Fermi charge distribution for larger nuclei, which neither protons nor alphas have) discussed. It should be added in a subarticle that half-charge radii tend to be a little smaller than inelastic hadron/hadron scattering radii, which probe strong force interactions, and make nuclei bounce off each other a bit farther out than what you calculate from a number that is half-way through the “charge density†outer skin.
So again, qualification needed, but at another level of complexity, and can’t put easily put in the primary article.
QUOTE(Limey)
"This is much smaller than the radius of the atom, which is on the order of 10^5 fm" Atomic radius varies by element. For smaller atoms, it is on the order of 10^4 fm, for larger ones it is on the order of 10^5 fm. Clearly, this is in error, and it would be easy enough to correct.
Well, it’s 31,000 to 298,000 fm from He to Cs (the largest I can find):
http://en.wikipedia.org/wiki/Atomic_radius There are only four elements (He, O, F, Ne) under 50,000 fm, which are thus closer to 10^4 than 10^5 fm in radius. So for a “generic atomâ€-- almost all the elements-- it’s true. That’s a quibble, since the nature of the statement is a broad generality. “With a few exceptions†could be added as a qualifier.
QUOTE
Anyway, I haven't the time nor the inclination to make my way through the whole article, but having reviewed the first four paragraphs, there are two clear errors (in the nuclear radius formula and with regard to the radius of the atom) as well as a problematic statement almost certainly in error, but rather ambiguous (with regard to the dimensions of protons and neutrons) and another seriously problematic statement with elements of both error and omission (with regard to the size of an electron). Finally, there's the omission when discussing hydrogen ions, but I'm willing to say that's more of a taste issue. On the whole, I'd say that the science isn't nearly as a bad as the history, but it's still full of flaws.
Meh, the nuclear radius formula is close enough and is supported by some literature (it needs regularizing) and the atomic radius comment is correct for >95% of the elements and makes the point it intends to. The omission and commission errors are matters of taste and as you see from the above discussion, would thoroughly confuse any intended high school reader of [[atom]]. The most we can see is we need some qualification language and more in the sub-articles. I think the worst error you found is the proton/neutron “dimension†statement of 2.65 fm, which even if intended to refer to diameter, would suggest a radius of 1.325 fm, which is (still) pretty far outside the reported radii for these particles from any choice of measurement I know if. So that number needs fixing.
On the whole I don’t think you’ve found too many ZOMG problems. If you reviewed Britannica with that amount of vigor I’d be interested in the result. Though the Britannica article on atoms is probably written much more vaguely, making it less susceptible to quantitative criticism (but on the other hand, a lot less informative.)
Care to keep going? Feel free to skip the dicey stuff and nail just the “clear slap your head wrong†errors.
Milton