Which one of the following statements is correct?
- A.A neutron is formed by combination of an electron and a proton. Therefore, it is neutral
- B.The mass of an electron is about 1/2000 times that of a proton
- 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
Correct answer
Explanation
The correct answer is B. An electron's mass is about 1/1836 of a proton's, which is roughly 1/2000.
The electron has a mass of about 9.1 × 10⁻³¹ kg, while the proton has about 1.67 × 10⁻²⁷ kg, nearly 1,836 times heavier. That is why the mass of an atom is taken to sit almost wholly in its nucleus and electrons are left out when the mass number is counted. The neutron, found by James Chadwick in 1932, is slightly heavier than the proton and has no charge.
A is wrong because the neutron is a distinct particle made of quarks; it is not simply an electron joined to a proton, even though a free neutron decays into a proton and an electron.
C is wrong because goitre is treated with an isotope of iodine (iodine-131); cobalt-60 is used in cancer radiotherapy.
D is wrong because Thomson proposed the plum pudding model, with no nucleus; the nucleus came from Rutherford and the neutron from Chadwick.
Exam tip: proton ≈ 1836 × electron; iodine-131 – thyroid and goitre, cobalt-60 – cancer therapy.
Practice Questions
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- B.1 s
- C.1.5 s
- D.2 s
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