Which one of the following statements is correct?

Statement 2 is correct as the mass of an electron is approximately 1/1836 (often rounded to 1/2000 in introductory texts) of the mass of a proton. Statement 1 is incorrect; while early hypotheses suggested a neutron might be a proton-electron composite, it was discovered by James Chadwick in 1932 as a distinct fundamental subatomic particle [1]. Statement 3 is incorrect because an isotope of iodine (I-131), not cobalt, is used in the treatment of goiter and thyroid disorders [2]. Cobalt-60 is typically used in cancer radiotherapy. Statement 4 is incorrect because J.J. Thomson proposed the 'plum pudding' model where electrons were embedded in a positive sphere; the concept of a nucleus was later established by Ernest Rutherford, and neutrons were not discovered until much later.

Sources

1. [1] https://www.sciencedirect.com/science/article/pii/S1631070517300993
2. [2] https://www.osti.gov/opennet/servlets/purl/16005024.pdf