It will be much easier to handle this by defining your own isotopeData function. Something along the lines of

isotopeData[0] = {Entity["Isotope", "Neutron"]}
isotopeData[x___] := IsotopeData[x]

plus any other cases that you might need for the neutron.

This is easier than having many Ifs.

It is probably worth noting that the form

IsotopeData[_Integer]

is not a documented form (http://reference.wolfram.com/language/ref/IsotopeData.html) and that the defined and thus supported form is

IsotopeData[{Z,A}]

But I agree that if IsotopeData[1] yields the isotopes of Hydrogen then IsotopeData[0] should perhaps yield "neutronium", though sticklers for nomenclature may argue that an element should only be one with possible electrons in its configuration.

Oops, you're right. I stand corrected--actually I am sitting, but I am corrected nonetheless.