Question 1

Natural uranium composition: approximately what percentage of uranium-238 (U-238) is present in natural uranium (the remainder being mostly U-235 with trace U-234)?

Accepted Answer

Introduction / Context: Understanding the isotopic composition of natural uranium is foundational for nuclear engineering, fuel-cycle calculations, and reactor physics. Natural uranium is not a single nuclide; rather, it is a mixture dominated by uranium-238 (U-238), with a small fraction of uranium-235 (U-235) and a trace amount of uranium-234 (U-234). Knowing these proportions helps explain why enrichment is required for many thermal reactors and why fertile U-238 plays a major role in breeding reactions. Given Data / Assumptions: Natural uranium isotopes: mainly U-238 and U-235; U-234 present in trace amounts. We assume standard terrestrial isotopic abundances. Percentages are by atom (or close to by mass for quick estimates). Concept / Approach: The accepted textbook composition of natural uranium is approximately U-238 ≈ 99.274–99.284%, U-235 ≈ 0.711–0.720%, and U-234 ≈ 0.005–0.006%. Rounded values often used in entrance exams are 99.29% for U-238 and 0.71% for U-235. These small differences in atomic mass and abundance drive the need for enrichment processes (e.g., gaseous diffusion, centrifugation) to raise the fissile U-235 fraction for most light-water reactors. Step-by-Step Solution: Identify typical abundances: U-238 ~ 99.29%, U-235 ~ 0.71%, U-234 ~ 0.005%.Match to options: 99.29% corresponds to U-238's share.Therefore, the correct percentage of U-238 in natural uranium is about 99.29%. Verification / Alternative check: Chemical handbooks and reactor physics texts consistently report U-238 at roughly 99.28–99.29% in natural uranium, validating the rounded exam figure of 99.29% as accurate for problem solving and quick recall. Why Other Options Are Wrong: 0.71%: that is the common figure for U-235, not U-238.0.015%: closer to the trace U-234 content, not U-238.29.71% or 97.00%: inconsistent with well-established natural abundances. Common Pitfalls: Swapping the U-235 and U-238 percentages.Assuming mass percentages differ dramatically from atomic percentages; for these isotopes the difference is small. Final Answer: 99.29%

Question 2

Nuclear terminology: atoms that contain the same number of neutrons but a different total number of nucleons (i.e., different Z and/or A) are called what?

Accepted Answer

Introduction / Context: Nuclear classification terms—such as isotopes, isobars, isotones, and isomers—appear frequently in chemistry and nuclear engineering exams. Each term captures a different way nuclides can be “the same” or “different” with respect to proton number, neutron number, or overall nucleon number. Knowing the exact definitions prevents common mix-ups when interpreting nuclear reaction equations or decay series. Given Data / Assumptions: We compare nuclides by Z (protons), N (neutrons), and A (nucleons = Z + N). We assume ground-state identity unless otherwise specified (isomers differ by nuclear energy state). Concept / Approach: Isotopes: same Z, different N (therefore different A).Isobars: same A, different Z (and thus different N).Isotones: same N, different Z (and usually different A).Isomers: same Z and A but different nuclear energy states (metastable vs. ground state). Given the stem—“same number of neutrons, but different number of nucleons”—we are clearly describing nuclides that share N yet do not share A, which is the hallmark of isotones. Step-by-Step Solution: Translate the statement “same neutrons” → same N.“Different nucleons” → different A.Same N with different A implies different Z as well.By definition, such nuclides are isotones. Verification / Alternative check: Examples: 14C (Z = 6, N = 8) and 15N (Z = 7, N = 8) are isotones (same N = 8), not isotopes or isobars. This aligns with the formal definitions used in nuclear data tables and textbooks. Why Other Options Are Wrong: Isobars: same A, not applicable here.Isotopes: same Z, not “same N.”Isoters: not a standard nuclear classification term in this context.Isomers: same Z and A, differing by nuclear energy state; not governed by N sameness. Common Pitfalls: Confusing “bar” (A same) with “tone” (N same).Assuming any two nuclides with the same mass number must be isotopes; they are isobars if Z differs. Final Answer: Isotones

Question 3

Nuclides with the same atomic number Z (i.e., the same element) but different mass numbers are termed what in nuclear chemistry?

Accepted Answer

Introduction / Context: Precise nuclear vocabulary enables clear communication about nuclides. When two atoms share the same atomic number Z, they are the same chemical element; if their mass numbers A differ, we call them isotopes. This is central to explaining fractional atomic weights and why natural samples of an element often contain multiple isotopes in fixed proportions. Given Data / Assumptions: Z is identical for the nuclides being compared. Mass numbers differ due to different neutron counts. Chemical behavior is largely the same because electron configuration depends on Z. Concept / Approach: Definitions: isotopes (same Z, different N and thus different A); isobars (same A, different Z); isotones (same N, different Z); isomers (same Z and A, different nuclear energy states). The stem specifies “same atomic number,” which immediately maps to isotopes. Step-by-Step Solution: Recognize that Z determines the element.Different A means neutron counts differ.Same Z + different A → isotopes by definition. Verification / Alternative check: Classic examples include chlorine-35 and chlorine-37, carbon-12 and carbon-13, and uranium-235 and uranium-238. Each pair shares Z but has a different A, matching the concept of isotopes. Why Other Options Are Wrong: Isobars: same A, not same Z.Isotones: same N, not same Z.Isomers: nuclear excited vs. ground state, same Z and A.Allotropes: refer to different structural forms of the same element in chemistry (e.g., graphite vs. diamond), not nuclide identity. Common Pitfalls: Interchanging the meanings of isobars and isotopes.Confusing nuclear terms with chemical terms like allotropy. Final Answer: Isotopes

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