Isotopes and Isobars – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Define isotopes. Explain why isotopes have similar chemical properties but different physical properties. Use hydrogen as an example.
Answer:
- Isotopes are atoms of the same element with the same atomic number but different mass numbers.
- They have the same number of protons and electrons, so their chemical properties are similar.
- Chemical behavior depends on electrons, which are controlled by the atomic number.
- They differ in neutrons, so their mass changes. This changes physical properties like density and boiling point.
- Example: Hydrogen isotopes — Protium (¹H), Deuterium (²H), and Tritium (³H).
- Heavy water (D₂O) has different physical properties than normal water (H₂O), but both undergo similar chemical reactions.
Q2. Chlorine exists as 35Cl and 37Cl in a 3:1 ratio. Calculate its average atomic mass and explain what this value means.
Answer:
- Chlorine has two isotopes: Chlorine-35 and Chlorine-37.
- Their natural abundance is 75% for 35Cl and 25% for 37Cl.
- Average atomic mass = [(35 × 75) + (37 × 25)] / 100 = 35.5 u.
- This value shows the weighted average mass of chlorine atoms found in nature.
- It is not a whole number because it depends on the relative abundance of the isotopes.
- In chemical reactions, chlorine behaves the same, but the mass affects some physical properties.
Q3. Describe the three isotopes of hydrogen and list one property or application for each.
Answer:
- Protium (¹H) has 1 proton and 0 neutrons; it is the most abundant hydrogen isotope.
- It shows normal chemical behavior in forming water and acids.
- Deuterium (²H) has 1 proton and 1 neutron; it forms heavy water (D₂O).
- Heavy water has different boiling/melting points and is used in some research.
- Tritium (³H) is radioactive and has 1 proton and 2 neutrons.
- Tritium is used in nuclear fusion research and as a tracer in studies.
Q4. What are isobars? Explain with two examples why their chemical properties are different.
Answer:
- Isobars are atoms of different elements with the same mass number but different atomic numbers.
- Example: Calcium-40 (²⁰Ca⁴⁰) and Argon-40 (¹⁸Ar⁴⁰) are isobars.
- Example: Nitrogen-14 (⁷N¹⁴) and Carbon-14 (⁶C¹⁴) are also isobars.
- Their proton numbers differ, so their electron arrangements are different.
- Chemical properties depend on atomic number, not on mass number.
- Hence, isobars show different chemical behavior even though their total nucleons are the same.
Q5. Differentiate between isotopes and isobars with suitable examples and key points.
Answer:
- Isotopes: Same atomic number, different mass number; example: ¹²C and ¹⁴C.
- Isobars: Same mass number, different atomic numbers; example: ⁴⁰Ca and ⁴⁰Ar.
- Isotopes have similar chemical properties but different physical properties.
- Isobars have different chemical properties because they are different elements.
- Isotopes help in average atomic mass and have uses like medicine and research.
- Isobars are important in nuclear reactions and discussions of stability.
High Complexity (Analysis & Scenario-Based)
Q6. A lab has two chlorine samples. Sample A has 75% 35Cl and 25% 37Cl. Sample B has 50% 35Cl and 50% 37Cl. Predict and explain the differences in their average masses and any physical effects.
Answer:
- Sample A average mass = [(35 × 75) + (37 × 25)]/100 = 35.5 u.
- Sample B average mass = [(35 × 50) + (37 × 50)]/100 = 36.0 u.
- Sample B is heavier on average due to more 37Cl.
- Both samples have the same chemical properties because atomic number is the same.
- Some physical properties may shift slightly, like density and diffusion rate.
- Heavier average mass can make gases diffuse slower and may change boiling point a little.
Q7. You must trace a chemical reaction pathway without changing how the substance reacts. Would you use an isotope or an isobar? Justify your choice.
Answer:
- Choose an isotope of the same element, not an isobar.
- Isotopes have the same atomic number, so chemical properties stay the same.
- This keeps the reaction mechanism unchanged.
- You can detect the isotope by its mass or radioactivity (like tritium as a tracer).
- Isobars are different elements, so they react differently.
- Using isobars would change the chemistry, which is not desired in tracing.
Q8. Carbon-14 decays to Nitrogen-14 in nuclear processes. Explain how this shows the idea of isobars and discuss stability.
Answer:
- Carbon-14 (⁶C¹⁴) and Nitrogen-14 (⁷N¹⁴) have the same mass number (14).
- They are isobars because their atomic numbers differ (6 vs 7).
- In decay, the number of protons changes, but total nucleons stay the same.
- This shows why isobars appear in nuclear reactions and beta decay.
- Stability depends on the balance of protons and neutrons.
- The nucleus tends to move toward a more stable arrangement, creating an isobar.
Q9. A climate study uses hydrogen and oxygen isotope ratios in ice cores. Explain how isotopes help infer past temperatures.
Answer:
- Isotopes of hydrogen and oxygen have different masses.
- During evaporation and condensation, lighter isotopes move more easily.
- Cold periods trap different ratios of heavy to light isotopes in ice.
- Scientists measure these ratios to estimate past temperatures.
- The chemical properties stay the same, but the physical behavior changes due to mass.
- So isotope data becomes a natural thermometer for climate history.
Q10. In cancer therapy, Cobalt-60 is used. Explain why an isotope is chosen and how its properties are useful and controlled.
Answer:
- Cobalt-60 is an isotope that is radioactive.
- It emits energy that can destroy cancer cells.
- Its chemical nature as cobalt is not the main factor; its nuclear property is key.
- Isotopes can be selected for the right radiation and half-life.
- Safety needs shielding, time limits, and proper handling.
- This shows a practical use of isotopes beyond everyday chemistry.