49.1740 g (6.02 x 1023 atoms) / (91.22 g) = 3.25 x 1023 atoms
The fourth ionization energy of zirconium is:Zr3+(g)→Zr4+(g)+e−
The number of atoms of lead is 6,68.10e23.
According to http://en.wikipedia.org/wiki/Zirconium (whether you're willing to trust that), it seems the density of Zirconium around room temperature is 6.52 g/cm^3.
The density of zirconium (pure metal) is 6.511 g/cm3. The apparent density of zirconium sponge or powder is of course lower.
149 g of calcium contain 22,39.10e23 atoms.
6,687.1023 chlorine atoms
27,30.10e23 atoms
The number of atoms of lead is 6,68.10e23.
78.96 g of selenium will have 6.023 x 1023 atoms. So, 30.4 g of selenium will have 2.32 x 1023 atoms.
5 g of sulfur contain 0,94.10e23 atoms.
156,86.1020 atoms of uranium in 6,2 g uranium
Zirconium has a concentration of about 130mg/kg within the Earth's crustand about 0.026μg/L in sea water. So it's not very abundant, though not rare.