# How many Rb atoms to fit on a 9Cm^2 square?

1. Oct 13, 2013

### UnD3R0aTh

1. The problem is in the attachments

2. Basic chemistry knowledge

3. The radius of one Rb atom is 4.95/2 time 10^-8 cm, we calculate the area then divide 9 into the area of one atom, we get the number of atoms, why is 4.7 x 10^15 atoms wrong?

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2. Oct 13, 2013

### Staff: Mentor

There are empty spaces between the atoms - they need more space than just pi*r2.

3. Oct 13, 2013

### UnD3R0aTh

exactly my thought, how to calculate that?!

4. Oct 13, 2013

### Staff: Mentor

Does this sketch help?

#### Attached Files:

• ###### atoms.png
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5. Oct 13, 2013

### UnD3R0aTh

excellent, i solved the first one, but the second one i'm not really sure how to calculate the side of the triangle that's enclosing one atom!!!!

6. Oct 13, 2013

### Staff: Mentor

You can draw an equilateral triangle in the lattice.

7. Oct 13, 2013

### UnD3R0aTh

yes but it looks like it's side is longer than the diameter, how to calculate it's length?!

8. Oct 13, 2013

### UnD3R0aTh

no wait, the triangles do not work, they overlap! parallelograms work! how to calculate the area of a parallelogram using it's sides?

9. Oct 13, 2013

### Staff: Mentor

Then you are drawing the wrong triangles. Use the centers of the atoms, this is easier. Afterwards, find out how many atoms per triangle (or triangles per atom) you have.

10. Oct 13, 2013

### UnD3R0aTh

can u plz draw a diagram like u did with the first one?

11. Oct 13, 2013

### Staff: Mentor

12. Oct 13, 2013

### UnD3R0aTh

i don't know to how to draw this, using what program?

13. Oct 13, 2013

### Staff: Mentor

Every drawing program can draw lines. Windows always comes with Paint, for example.

14. Oct 13, 2013

### UnD3R0aTh

pffffft i've been thinking about this problem for far too long i'm at my wits end, i really need to stop wasting my time with this and study, i feel nervous already cause i wasted so much time, plz give me a more strong hint, save me some time, thank you

15. Oct 13, 2013

solved it