How can Protactinium have a smaller mass number than thorium?

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A thorium atom has an atomic number of 90 and a mass number of 232 while a Protactinium atom has an atomic number of 91 and a mass number of 231. What is unsual about this? How is this possible?

Not sure why Protactinium has a smaller mass number as compared to thorium but our modern periodic table is based on the increasing atomic number and not on the increasing mass number so if this question is based on this fact that thorium in spite of being placed after Protactinium in the periodic table then we should remember that our elements in our modern periodic table are arranged in the order of increasing atomic number and not mass number .