Answer:
When the number of protons in an atom changes, it changes what element we're talking about. The number of protons defines the element.
When the number of neutrons in an atom changes, it changes what isotope of an element we're talking about. Isotopes have similar chemistries (they bond to other atoms in pretty similar manners), but their nuclear stabilities can differ greatly. U-232 is a synthetic uranium isotope with a 68.9 year half-life, U-235 makes good bombs, while most uranium is U-238, which is very slow to decay, and helps us establish things like the age of the planet. The difference between the 3? How many neutrons are stuck to our 92 protons.
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