Question

Isotopes are the atoms of the same element, with the same atomic number, but with different:

• A. Atomic number
• B. Mass number
• C. Number of electrons
• D. Chemical nature

Hint:

Isotopes have the same chemical properties but different physical properties due to differences in mass.

Answer 1

The Correct Option is B

Step 1: Concept.
Isotopes of an element always have the same number of protons, which fixes both the atomic number and the electron count in a neutral atom.

Step 2: Find what changes.
What differs between isotopes is the number of neutrons, and since mass number equals protons plus neutrons, a change in neutrons directly changes the mass number.

Step 3: Final Answer.
Isotopes of the same element differ in Mass number.

Answer 2

Three of these four options are tied to the number of protons in the atom, and only one is tied to the number of neutrons, so grouping them this way settles the question quickly.

1. Atomic number: By definition, the atomic number is the proton count, and isotopes are defined as having the same proton count, so this cannot be the answer.
2. Number of electrons: In a neutral atom, electron count matches proton count, so this is tied to the same fixed proton number as atomic number, and it also stays the same across isotopes.
3. Chemical nature: Chemical behaviour comes from how electrons are arranged, and since electron count does not change across isotopes, chemical nature does not change either.
4. Mass number: This is the only option in the list tied to neutron count rather than proton count, since mass number is protons plus neutrons. Neutron count is exactly what is free to vary between isotopes of the same element, so this is what changes.

Since atomic number, electron count, and chemical nature are all locked to the fixed proton count, only mass number is left free to differ.

So the correct answer is Mass number.