How will derive the formula of density of a unit cell ?

Explanation

Using the edge length of a unit cell, we can calculate the density `(rho)` of the crystal hv considering a cubic unit cell as follows .
Density of the cell `rho=(" Mass of the unit cell")/ ("Volume of the unit cell") " "...(1)`
Mass of the unit cell ={Total number of atoms belong to the unit cell } `xx` mass of one atom
Mass of one atom `= ("Molar mass"(g mol^(-1)))/("Avagardo number "(mol^(-1)))" "....(2) `
`m=M/N_A" "....(3)`
Substitute the value (3) in (2)
Mass of the unit cell `=nxxM/N_A`
For a cubic unit cell , all the edge lengths are equal .i.e , a = b = c
Volume of the unit cel `=axxaxxa=a^3`
`:.` Density of the unit cell `=rho = (nM)/(a^3N_A)`