Chapter 2 - Atoms, Molecules, and Ions - Worked Example - Page 49: 5

The Rutheford's foil is $1.7 \times 10^4$ atoms thick.

$1$ $mm$ = $10^{-3}$ $m$ $0.005$ $mm$ = $0.005$ $\times 10^{-3}$ $m$ = $5 \times 10^{-6}$ $m$ = Foil thickness. Thus, it follows: $\frac{5 \times 10^{-6}m}{Foil} \times \frac{1-atom}{2.9 \times 10^{-10}m} = 1.7 \times 10^4 \frac{Atoms}{Foil}$