I've always heard that different isotopes of the same element have exactly the same chemical properties.
But for example, I've read that some compounds which have hydrogen atoms interchanged by deuterium become odorless (this is one of the arguments for the vibrational theory of olfaction).
A 2001 study by Haffenden et al. showed humans able to distinguish benzaldehyde from its deuterated version. In addition, tests with animals have shown fish and insects able to distinguish isotopes by smell
So, are there any situations in which neutrons do matter (besides nuclear decays)?
Another curious effect is that water is slightly blue, due to vibration. But heavy water is not.
Heavy water is colorless because all of its corresponding vibrational transitions are shifted to lower energy by the increase in isotope mass. For example the H2O band at 760 nm is shifted to approximately 1000 nm in D2O.
(Left: tube if filled with (light) water. Right: empty tube.)