How many atoms are in 2.12 grams of ni?

Adopting the number of avogrado 6.02 * 10²³ / mol
Nickel Molar Mass = 58.7 g / mol
We will first have to find the number of moles in 2.12 grams of the element, like this:
 
1 mol ----------------- 58.7 g
X ---------------------- 2.12 g
 
58.7 * x = 2.12 * 1
58.7x = 2.12 

X ≈ 0.036 mol
 
Now we will find how many atoms there are in 0.036 mol of this element, like this:
 
1 mol -------------------- 6.02 * 10²³ atoms
0.036 mols ----------------- X
 
X = 0.036 * 6.02 * 10²³
X ≈ 0.21 * 10²³ atoms
or 

Exothermic,  physical change after it is already baked