Which of the following pairs must represent the same element?

Question

Examoogle · Accepted Answer

The correct answer is Option D: Same atomic number, different mass numbers. Atoms belonging to the same element are defined exclusively by their atomic number (number of protons). If two atoms have different mass numbers but the same atomic number, they are called isotopes of the same element.

Step-by-Step Solution & Concepts:
Atoms belonging to the same element are defined exclusively by their atomic number (number of protons). If two atoms have different mass numbers but the same atomic number, they are called isotopes of the same element.

Working Principle: The chemical identity of an element is determined by the number of protons in its nucleus, which dictates the electron configuration and, consequently, its chemical properties. Isotopes occur when atoms possess the same number of protons ($Z$) but a different number of neutrons ($N$), which changes the total mass number ($A = Z + N$). Since $Z$ remains constant, they occupy the same position in the Periodic Table.

Key Formulas: $A = Z + N$ — where $A$ is the mass number, $Z$ is the atomic number (protons), and $N$ is the number of neutrons.; $_Z^AX$ — standard notation for an element $X$ where $Z$ is atomic number and $A$ is mass number.

Key Points: Atomic number ($Z$) equals the number of protons.; Isotopes exhibit nearly identical chemical properties due to the same electronic structure.; Physical properties like density and diffusion rate may differ between isotopes due to mass differences.

Advantages: Isotopes are widely used in medical imaging (e.g., $^{131}I$).; Carbon dating ($^{14}C$) relies on the existence of isotopes.

Disadvantages: Radioactive isotopes can be harmful if not handled with appropriate radiation shielding.

Applications: Nuclear power generation using Uranium-235.; Cancer radiotherapy using Cobalt-60.

Additional Info: Option A describes isobars (different elements, same mass).. Option B describes isotones (different elements, same number of neutrons).. Option C does not describe a standard categorical pair in chemistry; it implies different elements entirely.

Answer: D is correct — Atoms with the same atomic number are isotopes, which by definition must be the same element.

Exam Tip: Always remember that protons define the element; electrons define the chemical behavior (valency), and neutrons define the isotope/stability.

Examoogle · Answer

Option A: Same mass number, different atomic numbers

Examoogle · Answer

Option B: Same number of neutrons, different atomic numbers

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Option C: Different number of protons, same number of neutrons

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