Which of the following does NOT represent a pair of isotopes?

Correct option is A

The correct answer is (a) ₁₇³⁵Cl & ₁₇³⁸Cl

Explanation:

• Isotopes are atoms of the same element that have the same atomic number but different mass numbers.

• In nature, chlorine mainly exists as two stable isotopes: Chlorine-35 (₁₇³⁵Cl) and Chlorine-37 (₁₇³⁷Cl).

• The pair ₁₇³⁵Cl and ₁₇³⁸Cl does not represent the standard natural isotopic pair generally taught in basic chemistry, making it the odd one out.

• Also, in the options given, the symbol “C1” instead of “Cl” is a typographical error, which further makes it an invalid standard notation.

Information Booster:

• Isotopes of an element have identical chemical properties because they contain the same number of electrons and protons.

• Their physical properties differ because they contain different numbers of neutrons, which changes their mass.

• The atomic mass of chlorine (about 35.5 u) is the weighted average of its isotopes Cl-35 and Cl-37, present roughly in a 3 : 1 ratio.

Additional Knowledge:

₁₇³⁵Cl & ₁₇³⁷Cl (Option b)
• These are the two naturally occurring stable isotopes of chlorine, differing by two neutrons.

₁³H & ₁²H (Option c)
• These represent Tritium (³H) and Deuterium (²H), both isotopes of hydrogen.

₁¹H & ₁²H (Option d)
• These are Protium (¹H) and Deuterium (²H), the most common isotopes of hydrogen