A.A neutron is formed by the combination of an electron and a proton. Hence, it is neutral
B.The mass of an electron is approximately 1/2000 times the mass of a proton✓ Correct
C.An isotope of cobalt is used in the treatment of goiter
D.J.J. Thomson proposed that the nucleus of an atom contains only neutrons
Explanation
Statement (b) is correct. The mass of an electron is approximately 1/1836 (≈1/2000) times the mass of a proton. Statement (a) is incorrect — a neutron is a fundamental particle, not formed by combining electron and proton. Statement (c) is incorrect — an isotope of iodine (I-131), not cobalt, is used to treat goiter and thyroid disorders; cobalt-60 is used in cancer therapy. Statement (d) is incorrect — J.J. Thomson proposed the plum pudding model and did not propose the nucleus; Rutherford discovered the nucleus.
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