Question

Isotones are the atoms of different elements having:

• A. Same mass number
• B. Same atomic number
• C. Same number of neutrons
• D. Same number of electrons

Hint:

Isotones are characterized by having the same number of neutrons, which differentiates them from isotopes and isobars.

Answer 1

The Correct Option is C

• Option 1: Same mass number - This is incorrect. Isotones do not have the same mass number; they have the same number of neutrons but different mass numbers, because the number of protons (and thus the atomic number) is different.
• Option 2: Same atomic number - This is incorrect. Isotones are atoms of different elements, so they must have different atomic numbers (the number of protons). Having the same atomic number refers to isotopes, not isotones.
• Option 3: Same number of neutrons - This is the correct answer. Isotones are atoms of different elements that have the same number of neutrons but different atomic numbers (and thus different mass numbers).
• Option 4: Same number of electrons - This is incorrect. The number of electrons in isotones can vary depending on the ionization state, and it is not a defining characteristic of isotones. 

Explanation:

Isotones are atoms of different elements that have the same number of neutrons but different atomic numbers and mass numbers. For example, carbon-14 (C-14) and nitrogen-15 (N-15) are isotones because they both have 8 neutrons, but they differ in the number of protons (atomic number) and thus belong to different elements.

Answer 2

Isotones are identified by asking what stays fixed while the atomic number changes between two atoms of different elements. Testing each option against this idea shows which property is the one that must remain the same.

1. Same mass number: If the mass number stayed fixed while the atomic number changed, that would describe isobars, not isotones, so this is not the matching property for isotones.
2. Same atomic number: Atoms of different elements, by definition, cannot share the same atomic number, since the atomic number is what identifies which element an atom belongs to. This option contradicts the very premise of comparing different elements.
3. Same number of neutrons: Take oxygen-17 (8 protons, 9 neutrons) and fluorine-18 (9 protons, 9 neutrons). Both atoms carry 9 neutrons, but they are different elements with different atomic numbers and different mass numbers, which is exactly the isotone relationship.
4. Same number of electrons: The electron count of a neutral atom simply equals its atomic number, and it can also change further if the atom is ionized. Neither case gives a fixed nuclear property that defines isotones.

Working through the oxygen-17 and fluorine-18 example confirms that a shared neutron count, not mass number, atomic number, or electron count, is what makes two atoms of different elements isotones.

Therefore, the correct answer is same number of neutrons.