The correct option is (a)
Explanation
Isotopes are atoms of the same chemical element that possess the same number of protons (atomic number) but a different number of neutrons (mass number). This difference in nuclear composition leads to variations in mass but not in electronic configuration.
Statement-wise Analysis
- Statement 1 is Correct: Isotopes are defined as atoms of the same element having the same atomic number ($Z$) but different mass numbers ($A$). For example, Carbon-12 and Carbon-14 both have atomic number 6, but mass numbers 12 and 14 respectively.
- Statement 2 is Incorrect: The chemical properties of an element are determined by its electronic configuration (specifically valence electrons). Since isotopes have the same number of electrons, their chemical properties are similar. However, their physical properties (such as density, boiling point, and rate of diffusion) depend on mass; therefore, their physical properties are different. The statement incorrectly reverses this relationship.
- Statement 3 is Incorrect: A pure substance is defined as matter consisting of a single type of particle. Since isotopes of an element have different masses and physical properties, they are distinct particles. A mixture of isotopes can be separated by physical means (e.g., centrifugation or fractional distillation). Therefore, a mixture of isotopes is technically considered a mixture, not a pure substance in the strict sense of particle uniformity.
Key Takeaway
Isotopes have the
same chemical properties (due to identical electronic configuration) but
different physical properties (due to different atomic masses). They are separable by physical methods.