Question

which statement is always true when nuclear fusion occurs? (1 point)
the number of protons in the resulting nucleus is less than in each
starting nucleus.
the number of protons in the resulting nucleus is double that of a
starting nucleus.
the combined number of protons and neutrons remains constant.
the total number of protons and neutrons in each nucleus remains
constant.

Explanation:

Nuclear fusion is a process where two or more atomic nuclei combine to form a new nucleus. According to the law of conservation of mass - energy (in the context of nuclear reactions, mass - number conservation is a key aspect), the combined number of protons and neutrons (mass number) in the reactant nuclei is equal to the combined number of protons and neutrons in the product nucleus.

Let's analyze each option:

• Option 1: In nuclear fusion, the resulting nucleus has a higher atomic number (more protons) than at least one of the starting nuclei (e.g., \(_{1}^{2}\text{H}+_{1}^{3}\text{H}\to_{2}^{4}\text{He}+_{0}^{1}\text{n}\)), so this option is wrong.
• Option 2: There is no rule that the number of protons in the resulting nucleus is double that of a starting nucleus. For example, in the fusion of hydrogen isotopes to form helium (\(_{1}^{2}\text{H}+_{1}^{3}\text{H}\to_{2}^{4}\text{He}+_{0}^{1}\text{n}\)), it is not a double - proton relationship, so this option is wrong.
• Option 3: According to the conservation of mass number (protons + neutrons) in nuclear reactions, the combined number of protons and neutrons (mass number) of the reactants is equal to that of the products.
• Option 4: The total number of protons and neutrons in each nucleus does not remain constant. For example, in the reaction \(_{1}^{2}\text{H}+_{1}^{3}\text{H}\to_{2}^{4}\text{He}+_{0}^{1}\text{n}\), the individual nuclei (e.g., \(_{1}^{2}\text{H}\) has 2 nucleons (\(1\) proton + \(1\) neutron), \(_{1}^{3}\text{H}\) has 3 nucleons (\(1\) proton+\(2\) neutrons), \(_{2}^{4}\text{He}\) has 4 nucleons (\(2\) protons+\(2\) neutrons), \(_{0}^{1}\text{n}\) has 1 nucleon (\(0\) protons+\(1\) neutron)). The number of nucleons per nucleus changes, but the total number of nucleons (protons + neutrons) of all reactants and products is conserved.

Answer:

The combined number of protons and neutrons remains constant.