Chlorine reacts very violently. What about its electron shells makes this the case?

it readily gains another electron. the outer electron shell is closer to the nucleus, than iodine for example, which makes it less shielded (greater attraction for another electron = violent reaction to get it)

Explanation:

2fe + 3h2so4 -> fe2(so4)3 + 3h2

it is necessary to balance equations in order to get the proper amount of reactants as well as products in order not to make a miscalculation.

you cannot change the subscripts as that would change the substance itself. for instance, diatomic elements such as h2 only have the subscript two when a gas. another example would be so4, if you were to change the subscript, the element would no longer be sulfate.

we change the coefficients instead as that only changes the number of molecules rather than the substance itself.

hope this !