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# A ##1/4## inch thick lead sheet is used for protection from medical x-rays. If a single lead atom has a diameter ##350## pm, how many atoms thick is the lead sheet?

##1/4## ##"inch"## ##=## ##6.35## ##xx## ##10^-3## ##m##.

##1## ##"pm"## ##=## ##1## ##xx## ##10^-12## ##m##

We assume that the lead atoms are close packed. Therefore we divide the sheet thickness by the diameter of of a single lead atom.

##(6.35xx10^-3cancelm)/(350xx10^-12cancelm)## ##=## Something large. The difficulty with doing these problems is getting such a large number that intuitively you have no idea how feasible the number is.

PS can you help this gentleman, . There are ##454## ##g## in ##1## ##"pound"##.