The reaction you've provided, SO2 + H2O -> H2SO2, is not balanced because the number of atoms of each element on the reactant side (left) does not equal the number of atoms of each element on the product side (right). Here's a breakdown:

Reactants:

* S: 1 atom

* O: 3 atoms (2 from SO2 and 1 from H2O)

* H: 2 atoms

Products:

* S: 1 atom

* O: 2 atoms

* H: 2 atoms

The Problem: There are 3 oxygen atoms on the reactants side and only 2 on the products side.

To balance the equation, you need to adjust the coefficients in front of the chemical formulas:

SO2 + H2O -> H2SO3

Explanation of the balanced equation:

* SO2: 1 molecule (1 sulfur atom and 2 oxygen atoms)

* H2O: 1 molecule (2 hydrogen atoms and 1 oxygen atom)

* H2SO3: 1 molecule (2 hydrogen atoms, 1 sulfur atom, and 3 oxygen atoms)

Now the equation is balanced:

* S: 1 atom on each side

* O: 3 atoms on each side

* H: 2 atoms on each side

Important Note: The product of the reaction is actually sulfurous acid (H2SO3), not H2SO2.