Chemical properties of isotopes 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)?

Addendum 2013/10/28

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.)
Short answer: nuclear neutrons greatly affect chemistry involving hydrogen.

Longer answer

Hydrogen is the lightest element, and deuterium is just less than twice as heavy. This makes a pretty massive (pun intended) difference to any vibrational modes involving moving hydrogen/deuterium. This means that, for example, the deuterated equivalents of weak acids are less acidic, and any reaction involving the loss of a proton will be significantly slower with the deuterated version.

Since helium is pretty unreactive, and by the time you're at lithium an extra neutron is only increasing the mass by about 14%, the effects on reaction energetics are much, much smaller for anything else.

I'd suggest looking up the "Kinetic Isotope Effect" for more information and more complex effects the mass change on substituting isotopes can cause to chemical activity.

As Eric Brown said in his answer, different isotopes may also have very different spectroscopic properties if the spectroscopic technique depends either on vibrational or nuclear properties.

As a minor curiosity, I'm given to understand this also means that heavy water is mildly hazardous to your health, since deuterated water and deuterated analogues of various vital chemicals have a nasty tendency to react mostly irreversibly or very slowly with vital enzymes that are "tuned" for hydrogen.