What are the inconsistencies and errors in common measurement units?

[Meduza]

i know the rules of increasing and decreasing atom sizes down and across the periodic table.
however, i have met one strange thing when i was given data to plot a graph.
the hydrogen atom radius was stated as 32 pm, but helium was 50 pm. when i searched the internet, i found few websites that state radius of helium 31, but one of them stated it as 128 pm!
why is that?
had someone made a mistake?

http://periodic.lanl.gov/elements/2.html

 

[Borek]

Won't hurt to check what they mean by atomic radius. It can be defined in many ways. Their radius for hydrogen is given as 78 pm, that's large too.

Then, there is always a possibility they took the data in furlongs or feet and forgot to convert to meters. If NASA can, Los Alamos Lab can too.

 

[Meduza]

Won't hurt to check what they mean by atomic radius. It can be defined in many ways. Their radius for hydrogen is given as 78 pm, that's large too.

Then, there is always a possibility they took the data in furlongs or feet and forgot to convert to meters. If NASA can, Los Alamos Lab can too.

even if they ment the radius as diameter, it still is over twice as i have got in my task and that does not correspond with the most of other sources which estimate 31 pm.

i know that there are different methods or approaches to measure these values, but they should not be that different. radius is radius, from the centre to the boundary, isn't it?

 

[Meduza]

i have just sent them an e-mail. i hope it will not take too long to get the answer.

 

[Borek]

Atom doesn't "end" abruptly. More like electron density goes exponentially down - so even two meters from the nucleus it has some non zero value. It is all in the way boundary is defined.

 

[granpa]

http://en.wikipedia.org/wiki/Atomic_radius#Definitions

Widely used definitions of atomic radius include:

* Van der Waals radius: in principle, half the minimum distance between the nuclei of two atoms of the element that are not bound to the same molecule.[3]

* Ionic radius: the nominal radius of the ions of an element in a specific ionization state, deduced from the spacing of atomic nuclei in crystalline salts that include that ion. In principle, the spacing between two adjacent oppositely charged ions (the length of the ionic bond between them) should equal the sum of their ionic radii.[3]

* Covalent radius: the nominal radius of the atoms of an element when covalently bound to other atoms, as deduced the separation between the atomic nuclei in molecules. In principle, the distance between two atoms that are bound to each other in a molecule (the length of that covalent bond) should equal the sum of their covalent radii.[3]

* Metallic radius: the nominal radius of atoms of an element when joined to other atoms by metallic bonds.[citation needed]

* Bohr radius: the radius of the lowest-energy electron orbit predicted by Bohr model of the atom (1913).[4][5] It is only applicable to atoms and ions with a single electron, such as hydrogen, singly-ionized helium, and positronium. Although the model itself is now obsolete, the Bohr radius for the hydrogen atom is still regarded as an important physical constant.

 

[alxm]

i found few websites that state radius of helium 31, but one of them stated it as 128 pm! why is that?
had someone made a mistake?

Yes, you. Because you're comparing covalent radii with van der Waals radii, which are not the same thing.

i have just sent them an e-mail. i hope it will not take too long to get the answer.

Guess that next week you'll be e-mailing a greengrocer, demanding an explanation of why their apples don't look like your oranges. It 'shouldn't be that different', right? It's all fruit!

 

[Meduza]

Yes, you. Because you're comparing covalent radii with van der Waals radii, which are not the same thing.

Guess that next week you'll be e-mailing a greengrocer, demanding an explanation of why their apples don't look like your oranges. It 'shouldn't be that different', right? It's all fruit!

this wasnt very nice.

what puzzled me, was that different sources listing different atomic radii for the same element. alone. none of these mentioned different forces, just a simple atomic radius.

also: one source says that radius of helium cannot be larger than the one of hydrogen due to the higher amount of protons in helium. but my task gave me radius of hydrogen 32 and radius of helium 50. it did not say anything about different forces or bonding with different atoms to form molecules. that was the primary reason why i started to look into this deeper. from my own interest.

i understand that you are pretty advanced in this area, while i am not. and for that matter you should have understood my position without using this rather inappropriate tone.

 

[Meduza]

thank you grandpa and Borek,

i briefly looked at the atomic radius in wikipedia yesterday. i know that scientists do not use exact definition of what they mean when they use term "atomic radius", which they should, i think. just to make things clear.

i only guessed that when they list atomic radius of some element alone, they mean all the same, therefore the value should be the same, (once someoene came into a conclusion that we will use 31 for helium and so on). otherwise they should specify what their value actually means. we do not use metres and yards interchangeably. everybody knows how long is meter and how long is yard. just as an example.

 

[sjb-2812]

...we do not use metres and yards interchangeably. everybody knows how long is meter and how long is yard. just as an example.

But do you?

Is a yard 36 inches, or 3600/3937 of a metre? (see e.g. http://en.wikipedia.org/w/index.php?title=United_States_customary_units&oldid=396647605 and related files) ?

Or, something with more error - what volume is a gallon?