Questions: Isotopes and Nuclear Composition

Question 1

An atom has 6 protons and 8 neutrons. A student claims this is a different element from an atom with 6 protons and 6 neutrons. Is the student correct?

• A. Yes — the extra neutrons change the atomic number, making it a different element
• B. Yes — the different mass means the two atoms belong to different periods on the periodic table
• C. No — both atoms are carbon; they are isotopes with different mass numbers (C-14 and C-12)
• D. No — atoms with the same total of protons plus neutrons are always the same element

Answer: (C)

Element identity is determined solely by proton count (the atomic number Z). Any atom with 6 protons is carbon, regardless of its neutron count. The atom with 6 protons and 8 neutrons (mass number 14) is carbon-14; the one with 6 protons and 6 neutrons (mass number 12) is carbon-12. Both are carbon. Options A and B contain the core misconception: that neutrons affect element identity. They do not. Neutrons affect mass and nuclear stability, but not which element an atom is or where it sits on the periodic table.

Question 2

Chlorine exists as Cl-35 (75.8% natural abundance) and Cl-37 (24.2% natural abundance). Why does the periodic table list chlorine's atomic mass as approximately 35.45 rather than 35 or 37?

• A. 35.45 is the mass of the most common chlorine ion found in seawater solutions
• B. The atomic mass is a weighted average of all naturally occurring isotope masses, reflecting their relative abundances
• C. The decimal results from protons and neutrons having slightly different masses, pulling the value below 36
• D. The atomic mass is the simple average of 35 and 37, rounded to two decimal places

Answer: (B)

Atomic mass on the periodic table is a weighted average: (0.758 × 35) + (0.242 × 37) ≈ 26.53 + 8.95 = 35.48 amu, close to 35.45. Because neither isotope has 100% abundance, the average is pulled between the two masses by their proportional contributions. Option D would give 36, which is wrong. Option C captures a real but minor effect. The key insight is that atomic mass describes a natural sample of the element — a mixture of isotopes — not any single atom, which is why atomic masses are almost never whole numbers.

Question 3

Two isotopes of the same element — for example, carbon-12 and carbon-14 — have essentially identical chemical reactivity because they have the same number of protons and therefore the same electron configuration.

• T. True
• F. False

Answer: True

Chemical behavior is determined by how electrons are arranged around the nucleus, which depends on the number of protons (and thus electrons in a neutral atom). Both C-12 and C-14 have 6 protons and 6 electrons, so they have identical electron configurations, form the same bonds, and participate in the same reactions. The extra two neutrons in C-14 are confined to the nucleus and do not affect electron behavior. Their only differences are physical: different masses and different nuclear stability. C-14 happens to be radioactively unstable, but chemically it is indistinguishable from C-12.

Question 4

The atomic mass listed on the periodic table for any element represents the mass of its most abundant naturally occurring isotope.

• T. True
• F. False

Answer: False

The atomic mass is a weighted average of all naturally occurring isotopes, weighted by their fractional abundance in nature. If it represented only the most abundant isotope, chlorine's atomic mass would be exactly 35 (Cl-35 is 75.8% abundant). Instead it is ~35.45 because the 24.2% of Cl-37 pulls the average upward. For elements where one isotope dominates at ~99% (like carbon, where C-12 is 98.9%), the atomic mass is close to a whole number, but this is a consequence of abundance — the weighted average still accounts for all isotopes.

Question 5

Why do isotopes of the same element behave identically in chemical reactions but can differ significantly in nuclear stability? What determines each property?

Model answer: Chemical behavior is governed by electron configuration, which depends on proton count. Since all isotopes of an element share the same proton number, they share the same electron configuration and therefore the same chemical reactivity — same bond-forming ability, same electronegativity, same reaction pathways. Nuclear stability, by contrast, depends on the ratio of protons to neutrons within the nucleus. If the neutron-to-proton ratio deviates too far from the stable range for that element, the nucleus is unstable and undergoes radioactive decay. These are entirely separate phenomena: the electron shell is determined by protons alone, while nuclear stability depends on the proton-neutron balance.

This separation is what makes isotopic tracers so useful: carbon-14 behaves chemically just like carbon-12 and is incorporated into organic molecules in predictable proportions (used in radiocarbon dating), but its nucleus is radioactively unstable and decays at a known rate. The chemistry doesn't distinguish the isotopes, but the nuclear physics does. Similarly, radioactive medical tracers work because the body's metabolic chemistry doesn't distinguish isotope variants, allowing a radioactive tracer to follow a metabolic pathway while emitting detectable radiation.