# Interatomic spacing concept confusion

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1. Oct 3, 2015

### engineering810

I am trying to understand the concept of interatomic spacing. Below is a link to three pictures, and my questions is: why are they different? I thought atoms were packed together touching eachother like in the first picture but the second and third imply otherwise. Any explanation or help is appreciated as I have been trying to find the answer for hours on end now. Thank you for any help in advance!

http://s1300.photobucket.com/user/waveriderr810/library/

2. Oct 3, 2015

How so?

3. Oct 4, 2015

### engineering810

In the second and third pictures the atoms appear to be spaced out and have space in between them, which represents their interatomic distance. Aren't atoms all packed together therefore there would be no spacing between them?

4. Oct 4, 2015

### DaveC426913

Atoms are not hard billiard balls. They interact via the charge from the electron shell. This charge drops off with distance.
When diagramming an atom, it is arbitrary how large you make the sphere, since it has no real-world counterpart.
How one represents atoms in a diagram is a matter of taste and of clarity.

5. Oct 4, 2015

### engineering810

Thank you for responding, this actually makes sense to me but my teacher has us calculating the density in unit cells depending on how much space the atom is occupying and how much space is vacant. Wouldn't this imply they are spheres packed together also touching eachother? Maybe the atomic radius implys something else I'm not understanding?

6. Oct 4, 2015

### Staff: Mentor

Atomic radius is calculated by diving the distance between centers of neighboring atoms into two. As Dave wrote atoms don't have a well defined surface, so they don't "touch" each other like balls do. However, atoms do interact - if you squeeze them too closely they will repel each other, if you separate them, they will have tendency to attract each other. These forces keep atoms spaced in the crystal lattice despite the fact they don't "touch".