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isolated atom

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why is it not possible to isolate an atom to measure its radius? There are many works in chemistry which require isolated atom for example in wave functions. 

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if we can isolate an atom then why do we first find the distance between nuclei of two atoms and divide it by 2? 

Edited by mundane

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4 minutes ago, mundane said:

if we can isolate an atom then why do we first find the distance between nuclei of two atoms and divide it by 2? 

Do we always do this?

And by the way, it is usually Physicists that are interested in the wave functions of atoms, isolated or not.
Chemists are more usually concerned with the wave functions of molecules.

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Searching for the phrase "isolate an atom to measure its radius" from the OP brings up results (including several text books) saying  things like "Since we can not isolate an individual atom and measure its diameter the way we can measure the diameter of a golf ball."

For example: https://books.google.it/books?id=HfVKD4o8UToC&pg=PA31&lpg=PA31&dq=isolate+an+atom+to+measure+its+radius&source=bl&ots=pbWiNjYnKW&sig=ACfU3U3SSEU_NFxeSnYwlY3nGWOGdVOSKQ&hl=en&sa=X&ved=2ahUKEwibkOuE54LnAhWGLMAKHR40AhEQ6AEwEnoECA0QAQ#v=onepage&q=isolate an atom to measure its radius&f=false

It is certainly possible to isolate a single atom so I guess the problem is with measuring the diameter of an isolated atom. I have no idea how one could attempt to do that.

19 minutes ago, studiot said:

That is not very helpful:

  • It is not clear which of the approximately 20 million results you think might be most relevant (especially as they may be ordered differently for each user)
  • The rules say you should not post links without comment/explanation.
  • I can't see how a field ion microscope is relevant to the question of "isolating a single atom"

Could you expand on this for us?

8 minutes ago, studiot said:

Do we always do this?

I don't know. I (as someone who has studied a little physics and chemistry) am only familiar with this as a way of measuring atomic radius. What other methods are used?

You have a habit of answering questions from people who want to learn with your own (often cryptic) questions. As you have a lot of knowledge and experience, why not provide a more helpful answer?

Are there any ways of measuring the size of a single, trapped atom? Are they more difficult and/or less reliable than measuring atomic spacing? What other methods are there? How do they compare (both in ease of use and the results they give)?

 

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2 minutes ago, Strange said:

Searching for the phrase "isolate an atom to measure its radius" from the OP brings up results (including several text books) saying  things like "Since we can not isolate an individual atom and measure its diameter the way we can measure the diameter of a golf ball."

It is certainly possible to isolate a single atom so I guess the problem is with measuring the diameter of an isolated atom. I have no idea how one could attempt to do that.

That is not very helpful:

  • It is not clear which of the approximately 20 million results you think might be most relevant (especially as they may be ordered differently for each user)
  • The rules say you should not post links without comment/explanation.
  • I can't see how a field ion microscope is relevant to the question of "isolating a single atom"

Could you expand on this for us?

Well I'm just packing to go to the clinic so I thought a quick response was better than no response.

The field ion microscope produces photmicrgraphs of individual atoms (The Wiki article shows the original one)
The device is a measuring microscope which means that its version of a microscope graticule has a measuring scale.
So we can measure the radii of the atoms.

Though the textbook is quite right this is not the same as picking up a golf ball and using calipers.

 

But it also depends upon what is meant by isolate.

So I would invite mundane to expand a little on his query.

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13 minutes ago, mundane said:

if we can isolate an atom then why do we first find the distance between nuclei of two atoms and divide it by 2? 

I assume because it is easier. The atoms in a crystal, for example, don't move much when we fire X-rays at them to perform crystallography. A single isolated atom is unlikely to have much effect on X-rays (and the radiation hitting it could knock it out of place) and so would be a hard thing to measure.

(That is a bit of a guess.)

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~Consider some helium atoms bouncing around in a container.

Each is about as isolated as you can get, but would you measure the (instantaneous) distance between two atoms and divide by two?

Which two would you choose?

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so I put it in this way- I have read in that we can't isolate an atom like to find its radius. like, hey, come here alone, lil guy, I have to measure your radius. We always measure the distance between two nuclei of two atoms and divide the it by 2. 

on the other hand, in wave functions we take two or more isolated independent atoms. 

Ψ=Ψa+Ψb

Ψa- atom a     Ψb- atom b

so my question is when we can get an atom isolated then why can't we take out its radius without having it been bonded (covalent, metallic and ionic)? 

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1 hour ago, studiot said:

Do we always do this?

And by the way, it is usually Physicists that are interested in the wave functions of atoms, isolated or not.
Chemists are more usually concerned with the wave functions of molecules.

As we say in the atomic physics biz, "one atom good, two atoms bad"

41 minutes ago, mundane said:

so I put it in this way- I have read in that we can't isolate an atom like to find its radius. like, hey, come here alone, lil guy, I have to measure your radius. We always measure the distance between two nuclei of two atoms and divide the it by 2. 

on the other hand, in wave functions we take two or more isolated independent atoms. 

Ψ=Ψa+Ψb

Ψa- atom a     Ψb- atom b

so my question is when we can get an atom isolated then why can't we take out its radius without having it been bonded (covalent, metallic and ionic)? 

Couple of things to unpack here. The first is, what radius? The electron's distance to the nucleus is not a fixed value. What you end up finding is the most probable distance, but it's not like it's a fixed value, it's an average value 

Second is how would you isolate a single neutral atom? That's not an easy thing to do. You can trap neutral atoms, but the same force that lets you trap them (e.g. radiation pressure) affects all of them, so it's not easy to interact with one atom but not another. Even so, people have done it, but it's not easy. And then after doing something hard to get the atom by itself, it's another level of complexity to do an experiment on the atom — interactions tend to send the atom out of whatever confinement it's in. That's one reason why measuring the separation of a covalent bond, or a lattice separation, are used. They are things that you can do.

It's a lot easier to trap ions, but then you aren't finding out information about the atom when you do that. The radius of an ion tends to be bigger or smaller, depending on whether you've added or removed an electron.

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if it's difficult to isolate an atom then how do we find wave function of an isolated one? is it just a hypothesis? 

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1 hour ago, mundane said:

if it's difficult to isolate an atom then how do we find wave function of an isolated one? is it just a hypothesis? 

I don't think it is difficult to isolate a single atom. https://www.newscientist.com/article/2161094-a-single-atom-is-visible-to-the-naked-eye-in-this-stunning-photo/

There are many experiments done on single atoms. The wave function comes from theory and is confirmed by experiment (on particles of various types, including atoms, and the interactions of particles).

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Just now, mundane said:

in that case finding radius of isolated atom shouldn't be a big deal 

I'm not sure why you would conclude that. I have no idea how one could make that measurement (which doesn't mean it can't be done).

Also, the radius is not a well defined value in the same way that the diameter of a golf ball is. There is no hard boundary to an atom. So the radius probably depends, to some extent, on how you choose to define the radius, how you measure it and what environment the atom is in. In other words, I wouldn't expect exactly the same radius for an isolated atom and one in a crystal. In fact, I would expect slightly different values for the radius depending on what compound the atom is in.

It is an interesting question, so I hope someone else can shed more light on it!

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55 minutes ago, Strange said:

 

"photo, Single Atom In An Ion Trap"

I had specified the difficulty is with neutral atoms. But if it's easy, go ahead and do it. 😉

 

 

2 hours ago, mundane said:

if it's difficult to isolate an atom then how do we find wave function of an isolated one? is it just a hypothesis? 

There is the theoretical basis, and there is also no need for you to have a single atom. You can have a collection of them as long as they are not interacting with each other very much, and/or the interactions tell you something about the wave function. 

53 minutes ago, mundane said:

in that case finding radius of isolated atom shouldn't be a big deal 

How would you measure it?

(the underlying point here is that measurement requires a reaction or interaction of some sort)

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11 minutes ago, swansont said:

"photo, Single Atom In An Ion Trap"

I had specified the difficulty is with neutral atoms. But if it's easy, go ahead and do it.

Doh. I am annoyed with myself now for not spotting their sloppy use of the word "atom".

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2 minutes ago, Strange said:

Doh. I am annoyed with myself now for not spotting their sloppy use of the word "atom".

Understandable.

One situation where you get single atoms is in optical cavities, but you don't typically get pictures of them because the only optical access is for the confining light. Also that traps for neutral atoms tend to not be as deep as for ions, so it's easier to kick the atoms out, such as from collisions with background atoms. Ions can be confined for a lot longer. For some experiments it doesn't matter if you use ions, but specifically for atomic radius, as I noted, you get a different answer for ion vs neutral.

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2 hours ago, Strange said:

Also, the radius is not a well defined value in the same way that the diameter of a golf ball is. There is no hard boundary to an atom. So the radius probably depends, to some extent, on how you choose to define the radius, how you measure it and what environment the atom is in. In other words, I wouldn't expect exactly the same radius for an isolated atom and one in a crystal. In fact, I would expect slightly different values for the radius depending on what compound the atom is in.

 

At last a sensible answer to a smarty-pants question. +1

5 hours ago, mundane said:

We always measure the distance between two nuclei of two atoms and divide the it by 2. 

I disagree with this most strongly for the reasons Strange has already outlined, plus the reason I already outlined.

A better statement would be

We very occasionally measure the distance between two nuclei of two atoms and divide the it by 2.
and then outline the conditions under which this statement holds true.

 

For example why are the atomic radii in ethane, ethylene, and acetylene different if measured by this halving the interatomic distance method ?

Would you apply the same method to the radii of oxygen and carbon in a carbonyl group, making them equal ?
Or perhaps you would suggest that the radii of hydrogen and flourine are equal in hydrogen flouride?

But of course none of these atoms are isolated, they are part of a molecule.

So do what I said and take a  two metre cube of helium gas and take one atom on one side and one on the other.

Their interatomic distance is thus two metres.
Half that is exactly one metre.
So, according your claim, the atomic diameter of helium atoms in helium gas is one metre.

 

 

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8 hours ago, mundane said:

if we can isolate an atom then why do we first find the distance between nuclei of two atoms and divide it by 2? 

It might be more accurate to say that one definition of the radius of the atom is half the interatomic spacing.

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