Question

The radius of a palladium atom is 137 pm. How many palladium atoms would have to be laid side by side to span a distance of 3.87 mm?

Answer 1

Call the radius of the palladium atom r and the distance 3.87 mm d.

Then the number of atoms is just d/r.

The trick is to write both of these quantities in the same units. Right now they are in differenct units. Can you see what to do now?

I would choose (but this is completely arbitrary) the units meters.
Then \[ r = 137 \ 10^{-12} m \] and \[d = 3.87 \ 10^{-3} m \]

So d/r = ....