Free Composition of Atomic Nuclei MCQs with Answers

1. Isotopes of an element have the same

• A. number of neutrons but different numbers of protons
• B. number of protons but different numbers of neutrons
• C. mass number but different atomic numbers
• D. number of protons and neutrons

Explanation: Atomic number fixes the element, so isotopes share chemical behaviour but differ in mass, as carbon 12 and carbon 14 do. Different proton numbers would make them different elements entirely. The difference in mass is what allows isotopes to be separated in a mass spectrometer.

2. The number of neutrons in a nucleus of uranium 235, atomic number 92, is

• A. 92
• B. 143
• C. 235
• D. 327

Explanation: Neutron number is mass number minus atomic number, so 235 minus 92 gives 143. The mass number counts protons and neutrons together, which are collectively called nucleons. Adding the two numbers, as in the 327 distractor, has no physical meaning.

3. The force that holds protons and neutrons together in a nucleus is

• A. the electrostatic force
• B. the gravitational force
• C. the strong nuclear force, which is short ranged and attractive
• D. the magnetic force

Explanation: The strong force acts over only a few femtometres but is far stronger than the electrostatic repulsion between protons at that range, which is what keeps the nucleus bound. Because it is so short ranged, very large nuclei become unstable as the repulsion, which has unlimited range, begins to win. Gravity is utterly negligible at nuclear scales.

4. Nuclear fission is the process in which

• A. two light nuclei join to form a heavier one
• B. a heavy nucleus splits into two lighter nuclei, releasing energy and neutrons
• C. a nucleus emits an alpha particle
• D. an electron is captured by a nucleus

Explanation: Splitting a uranium 235 nucleus releases about 200 MeV together with two or three neutrons, and those neutrons can trigger further fissions in a chain reaction. Joining light nuclei is fusion, which powers the Sun and releases even more energy per unit mass. Both release energy because the products are more tightly bound than the reactants.

5. In a nuclear reactor, the moderator is used to

• A. absorb excess neutrons and shut the reactor down
• B. slow fast neutrons so that they are more likely to cause further fission
• C. cool the reactor core
• D. shield the operators from radiation

Explanation: Graphite or heavy water slows the neutrons through repeated collisions, because slow neutrons are far more readily captured by uranium 235 and sustain the chain reaction. Absorbing surplus neutrons to control the rate is the job of the control rods, usually boron or cadmium. Confusing the moderator with the control rods is the standard error here.

6. The energy released in a nuclear reaction comes from

• A. the chemical bonds between atoms
• B. the small loss of mass, converted according to E equals mc squared
• C. the kinetic energy of the electrons
• D. friction inside the nucleus

Explanation: The products of fission or fusion have slightly less total mass than the reactants, and that mass defect appears as energy, which is why nuclear reactions release millions of times more energy per event than chemical ones. The binding energy per nucleon curve, peaking near iron, explains why light nuclei release energy by fusing and heavy ones by splitting. Chemical bonds involve only the outer electrons.

7. The binding energy per nucleon is greatest for nuclei near

• A. hydrogen
• B. iron
• C. uranium
• D. helium

Explanation: The curve of binding energy per nucleon rises steeply from hydrogen, peaks around iron and nickel at about 8.8 MeV, then falls slowly towards uranium. Nuclei on the left release energy by fusing towards the peak and those on the right by splitting towards it. This is why iron is the end point of fusion in a massive star.

8. Nuclear fusion in the Sun releases energy by converting

• A. uranium into lead
• B. hydrogen into helium
• C. helium into hydrogen
• D. carbon into oxygen

Explanation: Four hydrogen nuclei combine through the proton proton chain to give one helium nucleus, and the small mass difference appears as energy. Enormous temperature and pressure are needed to overcome the electrostatic repulsion between the positively charged nuclei, which is why sustained fusion is so hard to achieve on Earth. Fusion releases more energy per kilogram of fuel than fission and leaves far less long lived waste.

9. The protons in the nucleus of an atom DO NOT exit due to

• A. Electromagnetic force
• B. Gravitational force
• C. Weak nuclear force
• D. Strong nuclear force

Explanation: The strong nuclear force acts over only a few femtometres but far exceeds the electrostatic repulsion between protons at that range, so it is what holds the nucleus together. The electromagnetic force pushes the protons apart and would blow the nucleus open on its own. Gravity is utterly negligible at nuclear scale and the weak force governs beta decay rather than binding.