What scientific instrument was used to dicover that not all atoms of neon have the same mass?

Mass Spectrometer - is an analytical technique that ionizes chemical species and sorts the ions based on their mass-to-charge ratio. It measures the masses within a sample.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.