Which of the following pairs of atoms must belong to the same element?

Question

Examoogle · Accepted Answer

Option B: Atoms with same atomic number but different number of neutrons

Explanation & Concepts:
Atoms belonging to the same element are defined by having an identical atomic number ($Z$), which represents the number of protons in the nucleus. Isotopes are variants of the same element that have the same number of protons but differ in the number of neutrons, thereby changing their mass number ($A$).

Working Principle: The identity of a chemical element is determined solely by its proton count ($Z$), as this dictates the electronic configuration and chemical reactivity of the atom. Variations in the neutron count ($N$) change the nuclear mass and stability (radioactivity) without altering the element's fundamental chemical nature, resulting in isotopes.

Key Formulas: $Z = 	ext{Number of protons}$; $A = Z + N$ — where $A$ is mass number, $Z$ is atomic number, and $N$ is neutron count

Key Points: Elements are uniquely identified by their atomic number ($Z$).; Chemical properties are determined by electrons, which are fixed for a neutral atom of a given atomic number.; Isotopes exhibit identical chemical behavior but varying physical properties like density and nuclear stability.; Atomic number remains constant for all atoms of a specific element, such as Carbon-12 and Carbon-14.

Advantages: Isotopic labeling allows for tracking chemical reactions in biological systems.; Radioisotopes provide essential diagnostic and therapeutic tools in medicine.

Disadvantages: Radioactive isotopes can be harmful to biological tissues if not handled correctly.

Applications: Carbon dating using Carbon-14 isotopes.; Use of Iodine-131 in thyroid treatment.

Additional Info: Atoms with the same mass number but different atomic number are called Isobars.. Atoms with the same number of neutrons but different atomic number are called Isotones.. A) refers to Isobars (different elements).. C) refers to Isotones (different elements).. D) refers to ions or different elements depending on the counts.

Answer: B is correct — Atoms must have the same number of protons ($Z$) to be categorized as the same element, even if their neutron counts differ.

Exam Tip: Always remember that the 'Proton count' is the 'Fingerprint' of an element; if the proton count changes, the element changes entirely.

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Option A: Atoms with same mass number but different atomic number

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Option C: Atoms with same number of neutrons but different atomic number

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Option D: Atoms with different number of protons and electrons

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