📝 Isotopes definition and working (10 MCQs)

📖 From Campbell Biology • 2. The Chemistry of Life • 10 questions available

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What is Isotopes definition and working?

Definition:
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, giving them different atomic masses, but the same chemical properties, and they can be stable (non-radioactive) or unstable (radioactive), and they are used extensively in biological research, medicine, and geology for tracing, imaging, and dating.

Working:
Isotopes work by having the same number of electrons (and therefore same chemical behavior) but different mass, which affects physical properties like density and diffusion; stable isotopes are used in metabolic tracing (e.g., $^{13}C$ ), and radioactive isotopes decay at predictable rates, emitting radiation, with the decay described by the equation $N(t) = N_0 e^{-\lambda t}$ , where $\lambda$ is the decay constant, and this property is used in radiometric dating and as tracers in medicine to track biochemical pathways or image organs.

Example:
A simple example is carbon-12 ( $^{12}C$ ) and carbon-14 ( $^{14}C$ ); both have 6 protons, but $^{12}C$ has 6 neutrons and is stable, while $^{14}C$ has 8 neutrons and is radioactive, used in carbon dating; another example is iodine-131, a radioactive isotope used to treat thyroid cancer, demonstrating the practical use of isotopes.

Reason:
Isotopes are essential in biology and medicine for diagnostics, therapy, and research, as they allow tracking of molecules, imaging of organs, and dating of fossils, making them invaluable tools in scientific and clinical practice.

Easy

Medium

Hard

📝 All Isotopes definition and working MCQs

Q1. An atom of element X contains 17 protons and 18 neutrons. Another atom of the same element contains 17 protons and 20 neutrons. Which conclusion is most accurate?

A.The atoms are different elements because their neutron numbers differ.

B.The atoms are isotopes because they have the same proton number but different neutron numbers. ✅

C.The atoms have different atomic numbers but identical mass numbers.

D.The atoms must have different chemical symbols because their masses differ.

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Both atoms contain 17 protons, so they have the same atomic number and therefore belong to the same element. Their neutron numbers differ, producing different mass numbers. Atoms of the same element with different neutron numbers are isotopes.

Q2. A student claims, "If two atoms have different mass numbers, they must be different elements. Which example most directly disproves this reasoning?

^{12}C

and

^{14}C

✅

^{12}C

and

^{12}B

^{14}N

and

^{14}C

^{16}O

and

^{17}F

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: $^{12}C$ and $^{14}C$ have different mass numbers but both contain six protons. Because the proton number determines the element, these atoms are isotopes of carbon rather than different elements.

Q3. A sample contains two isotopes of an element: isotope A has mass $10\,u$ , and isotope B has mass $12\,u$ . If isotope A is much more abundant, which statement best predicts the sample's average atomic mass?

A.It must be exactly

12\,u

B.It must be exactly

11\,u

C.It should be closer to

10\,u

than to

12\,u

. ✅

D.It should be greater than

12\,u

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: The average atomic mass is a weighted average, so the isotope present in greater abundance contributes more strongly to the result. Since isotope A has mass $10\,u$ and dominates the sample, the average must lie closer to $10\,u$ .

Q4. A laboratory needs to identify whether two particles belong to the same element. Particle P has 8 protons and 8 neutrons, while particle Q has 8 protons and 10 neutrons. A researcher compares their identities using proton number rather than total mass. Why is this approach appropriate?

A.Protons determine elemental identity, while neutrons can vary among isotopes. ✅

B.Neutrons determine elemental identity, while protons mainly determine mass.

C.Mass number is always identical for atoms of one element.

D.Electrons determine the element regardless of proton number.

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Elemental identity is determined by the number of protons in the nucleus. P and Q both contain eight protons, so they represent the same element. Their different neutron numbers change their isotopic forms rather than their elemental identity.

Q5. Two isotopes of element Y occur naturally. Isotope Y-20 makes up 75% of the sample and isotope Y-22 makes up 25%. Ignoring other isotopes, which average atomic mass is most reasonable?

20.25\,u

20.50\,u

✅

21.00\,u

21.50\,u

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The weighted average is calculated as $(0.75)(20)+(0.25)(22)=15+5.5=20.5\,u$ . Because the lighter isotope is three times as abundant as the heavier isotope, the average is pulled substantially toward $20\,u$ .

Q6. A researcher mistakenly calculates the average atomic mass of isotopes with masses $30\,u$ and $32\,u$ by simply adding them and dividing by two, even though their abundances are 90% and 10%. What is the main error?

A.The researcher ignored the effect of isotopic abundance. ✅

B.The researcher used mass numbers instead of atomic numbers.

C.The researcher assumed both isotopes have different proton numbers.

D.The researcher should have subtracted the neutron numbers first.

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: The simple average is valid only when the isotopes have equal abundances. Here, the $30\,u$ isotope represents 90% of the sample, so it must contribute much more to the average than the $32\,u$ isotope. A weighted average is required.

Q7. A graph plots isotopic abundance against isotope mass. The tallest peak occurs at $24\,u$ , a smaller peak at $25\,u$ , and a very small peak at $26\,u$ . Which inference is best supported by the graph?

A.The

26\,u

isotope must have the greatest effect on average atomic mass.

B.The

24\,u

isotope is likely the most abundant and will strongly influence the average atomic mass. ✅

C.All three isotopes must have identical abundances.

D.The element's atomic number must equal

24

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The height of an abundance peak represents the relative amount of that isotope in the sample. Since the $24\,u$ peak is tallest, that isotope is most abundant and therefore has the greatest influence on the weighted average atomic mass.

Q8. A sample initially contains 80% isotope A with mass $40\,u$ and 20% isotope B with mass $42\,u$ . After purification, the sample contains 30% A and 70% B. What happens to the average atomic mass?

A.It decreases because isotope A is still present.

B.It remains exactly

40\,u

because the element has not changed.

C.It increases because the heavier isotope becomes more abundant. ✅

D.It becomes

82\,u

because the isotope masses are added.

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: Initially, the lighter $40\,u$ isotope dominates, giving an average near $40.4\,u$ . After purification, the heavier $42\,u$ isotope becomes dominant, giving an average near $41.4\,u$ . Thus the average atomic mass increases.

Q9. Two samples contain the same element. Sample 1 has isotopes with masses $50\,u$ and $52\,u$ in nearly equal proportions. Sample 2 contains the same isotopes, but the $52\,u$ isotope is much more abundant. Which comparison is most defensible?

A.Sample 1 has an average closer to

52\,u

than Sample 2.

B.Sample 2 has an average closer to

50\,u

than Sample 1.

C.Sample 2 has a higher average atomic mass than Sample 1. ✅

D.Both samples must have exactly the same average atomic mass because their isotope masses match.

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: Sample 2 contains a greater proportion of the heavier $52\,u$ isotope, so its weighted average must shift upward toward $52\,u$ . Sample 1, with nearly equal proportions, has an average near $51\,u$ , making Sample 2's average higher.

Q10. An unknown element has two stable isotopes, X and Y, with masses $100\,u$ and $104\,u$ . The measured average atomic mass is $103.6\,u$ . Which abundance pattern is most consistent with this result?

A.X is about 90% and Y about 10%.

B.X is about 50% and Y about 50%.

C.X is about 10% and Y about 90%. ✅

D.X and Y must have equal abundances because only two isotopes exist.

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: Let the fraction of X be $f$ . The weighted average satisfies $100f+104(1-f)=103.6$ . Solving gives $f=0.10$ , meaning X is about 10% and Y about 90%. The average lies close to the heavier isotope because Y dominates.

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