Answer
$ N = 4.79 \times 10^{7}$ atoms
Work Step by Step
The diameter of each cesium atom is twice the size of its radius.
$d = 2r$
$d = 2 \times 2.65 $ Å
$d = 5.30$ Å
If you place spheres next to each other, the number you can fit on a 1-inch long string is determined by their diameters.
$ l = 1 $ inch $ = 0.0254$ m
$ 1 $ Å $= 10^{-10}$ m
$ N = \frac{l}{d} = \frac{0.0254}{5.30 \times 10^{-10} } = 4.79 \times 10^{7} $ atoms
