Number of atoms : 1.996 x 10²⁴
Further explanation 
The mole is the number of particles(molecules, atoms, ions) contained in a substance 
1 mol = 6.02.10²³ particles 
Can be formulated 
N=n x No 
N = number of particles 
n = mol 
No = Avogadro's = 6.02.10²³ 
mol of Uranium with mass=789 g (Ar U=238,02891 u) :

Number of atoms :
