What made scientists believe that atoms contain equal numbers of protons and neutrons?

I wasn't aware they ever did. All atoms of the same element have the same number of protons, but can easily have different numbers of neutrons. Those are called isotopes.What made scientists believe that atoms contain equal numbers of protons and neutrons?
Not all atoms contain an equal number of protons and neutrons. Atoms that have a different number of neutrons than protons are called isotopes. An elements various isotopes have different atomic masses. Follow the link below: each of these hydrogen isotopes has a different mass and half-life. Scientists probably looked at the various masses and theorized that various atoms of the same element could have differing nuclei configurations.What made scientists believe that atoms contain equal numbers of protons and neutrons?
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