Which one of the following statements is correct?

Statement 1 is incorrect because a neutron is a fundamental subatomic particle, not a combination of an electron and a proton.

Statement 2 is correct because the mass of an electron is approximately $\dfrac{1}{1837}$ times the mass of a proton, which is roughly $\dfrac{1}{2000}$.

Statement 3 is incorrect because an isotope of iodine (I-131) is used in the treatment of goiter, whereas an isotope of cobalt (Co-60) is used in the treatment of cancer.

Statement 4 is incorrect because J. J. Thomson proposed the plum pudding model of the atom. The nucleus was discovered by Rutherford, and neutrons were discovered by James Chadwick.

Answer: The mass of an electron is about 1/2000 times that of a proton