Neutrons don't act at distance with the electric force, because they don't have electric charge.
They act at distance with the gravitational force, because they have mass.
For example, you can fill a glass with heavy water that has Deuterium instead of Hydrogen. https://en.wikipedia.org/wiki/Heavy_water
The number of molecules is almost equal, because the "size" of the molecule does not depend on the number of neutrons (almost). But the mass and the weigh will increase like a 10% because now there are more neutrons in the glass. The neutrons are acting at distance with the Earth, so the weight measure with an usual balance is greater.