Question

How many atoms we have in a cylinder with the diameter of 2µm and height of 1 µm of gallium (Ga).

Answer 1

First, find the volume, then mass, then change to mol, finally to atoms

Volume = A*h

A = PI*(D^2)/4 = 3.141592*(2*10^-6)^2 / 4 = 3.1415*10^-12

h = 1*10^-6

V = ( 3.1415*10^-12)(1*10^-6) =  ( 3.1415*10^-18) m^3

D = 5.91 g/ml = 5910 g /L = 5910000 g/m3

mass = D*V = 5910000*( 3.1415*10^-18) = 1.85662*10^-11 g

mol = mss/MW

MW of Gallium = 69.723

mol = (1.85662*10^-11)/(69.723) = 2.66285*10^-13 mol

1 mol = 6.022*10^23 atoms

then

2.66285*10^-13 mol = (2.66285*10^-13)(6.022*10^23) = 160356827000 atoms of Gallium