The given assumption is not valid.
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The given Dalton's assumption is not valid because because atoms of the same element may have different properties. Such atoms are known as isotopes. Isotopes are the atoms of an element having same atomic number(i.e number of protons or electrons) but different atomic mass due to different number of neutrons. For examples hydrogen element has three isotopes Protium, Deuterium and Tritium. They all are the atoms of the same element of hydrogen but have different properties.