Let's consider the triangle shown below:

- This is an isosceles triangle with a side-length of 2d and an angle of 60°.
- Drop a bisector to the uppermost angle so that it bisects the opposite side, the result is a right triangle with two known side lengths.

The diameter of a rubidium atom is 4.95 Å. We will consider two different ways of placing the atoms on a surface. In arrangement (a), all the atoms are lined up with one another. Arrangement (b) is called a *close-packed * arrangement because the atoms sit in the "depressions" formed by the previous row of atoms:

How many Rb atoms could be placed on a square surface that is 5.5 cm on a side, using arrangement (b)?

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