Isotope Definition and Explanation
The term 'Isotope' refers to atoms of the same chemical element that have the same number of protons but different numbers of neutrons.
Key Characteristics of Isotopes
- Same Atomic Number ($Z$): Isotopes belong to the same element because they have the identical number of protons, which defines the atomic number ($Z$).
- Different Mass Number ($A$): Isotopes differ in the number of neutrons. Since the mass number ($A$) is the sum of protons and neutrons ($A = Z + N$), a different number of neutrons results in a different mass number ($A$).
Analysis of Options
Let's analyze the given options based on the definition:
- Option 1 (elements having same bond structure): Bond structure is related to electron configuration and chemical bonding, not the core definition of isotopes.
- Option 2 (elements having different atomic number but same mass number): This is incorrect. Different atomic numbers mean different elements, and isotopes have the same atomic number.
- Option 3 (elements having same atomic number but different mass number): This accurately describes isotopes. For example, Hydrogen has isotopes Deuterium and Tritium, all having $Z=1$ but different mass numbers ($A=2$ and $A=3$ respectively).
- Option 4 (elements having same physical and chemical properties): While isotopes of an element share very similar chemical properties (due to the same number of electrons), their physical properties (like density and melting/boiling points) can differ slightly due to the mass difference. This is a consequence, not the definition.
Therefore, the correct definition hinges on the atomic number and mass number.
