Nthe balanced equation 4fe + 3o2à 2fe2o3 , how many atoms of iron (fe) are reacting with every three molecules of oxygen (o2)?

Answer is: four iron atoms are reacting with every three molecules of oxygen.
Balanced chemical reaction: 4Fe + 3O₂ → 2Fe₂O₃.
From chemical reaction: n(Fe) : n(O₂) = 4 : 3.
Ratios of atoms of iron and molecules of oxygen is 4 : 3..
There is four atoms of iron and six atoms of oxygen on both side of reaction.

Exothermic,  physical change after it is already baked