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

• Option 1: Atomic number - This is incorrect. Isotopes have the same atomic number, as they are atoms of the same element. The atomic number refers to the number of protons, which remains the same in isotopes. 
• Option 2: Mass number - This is the correct answer. Isotopes of the same element have the same atomic number but different mass numbers, as they contain different numbers of neutrons.
• Option 3: Number of electrons - This is incorrect. The number of electrons in isotopes remains the same if the atoms are neutral. The number of electrons does not affect the classification of isotopes.
• Option 4: Chemical nature - This is incorrect. Isotopes of the same element have the same chemical nature because they have the same number of electrons and the same atomic structure.

Explanation:

Isotopes are atoms of the same element that have the same atomic number (same number of protons) but different mass numbers. This difference in mass number occurs due to varying numbers of neutrons in the nucleus, which gives isotopes of an element different atomic masses but identical chemical properties.

Answer 2

Working through a real example, carbon-12 and carbon-14, shows directly which property changes between isotopes and which stays fixed.

1. Atomic number: Both carbon-12 and carbon-14 have 6 protons, so their atomic number is 6 in both cases. Since this number is identical for isotopes by definition, it cannot be the thing that differs.
2. Mass number: Carbon-12 has 6 protons plus 6 neutrons, giving a mass number of 12, while carbon-14 has 6 protons plus 8 neutrons, giving a mass number of 14. The proton count stayed the same but the neutron count changed, and that changed the mass number. This is exactly the property that separates one isotope from another.
3. Number of electrons: A neutral carbon-12 atom and a neutral carbon-14 atom both carry 6 electrons, since electron count matches proton count in a neutral atom. This stays the same, just like the atomic number.
4. Chemical nature: Both isotopes react the same way in chemical reactions because chemistry is governed by the electron arrangement, which is identical for both. So their chemical nature does not change either.

Comparing carbon-12 and carbon-14 shows that only the neutron count, and therefore the mass number, changes between isotopes.

So the correct answer is Mass number.