Mole (mol) is the number equal to the number of carbon atoms in exactly 12 g of pure carbon-12.
Photo: In this Periodic Table of Elements you will find a number in ^superscript in front of each element name.
* Above the element carbon is printed 12.0. That means 1 mol C atoms weighs 12.0 g.
* Above the element magnesium is 24.3.      That means 1 mol Mg atoms weighs 24.3 g
   (The isotope Mg-23 is mixed with two more isotops: Mg-24 and Mg-25. 
* Above the element aluminium is 27.0.         That means 1 mol Al  atoms  weighs 27.0 g
Molecules of elements and compound as well as lattices are formed by reaction of a characteristic number of atoms. So you can easily calculate how much 1 mol of these elements and compounds weighs:
* 1 mol hydrogen molecules   (H₂): 1.0 + 1.0 g                =      2.0 g
* 1 mol water molecules      (H₂O): 1.0 + 1.0 + 16.0g     =    18.0 g
* 1 mol sulfur molecules         (S₈):   8 x 32.1 g                =  256.8 g
* 1 mol sodium chloride (NaCl):  23.0 + 35.5 g               =   58.5 g