The diameter of a rubidium atom is 4.95 amu. Considering two different arrangements of atoms, the first be?

Question

The diameter of a rubidium atom is 4.95 amu. Considering two different arrangements of atoms, the first being where the atoms are lined up side by side, and the second being where the atoms sit in the "depressions" made by the two atoms beneath it....
a) how many Rb atoms could be placed on a square surface that is 1.0 cm on a side?
b)How many Rb atoms could be places on a square surface that is 1.0 cm on a side, using the second arrangement of atoms (sitting in "depressions")?
c)By what factor has the number of atoms on the surface increased in going to arramgement 2 from arrangement 1?

Answer 1

amu is an unit of mass , not length.