Isobars MCQs Quiz | Class 9

This quiz covers Unit I: Matter – Its Nature and Behaviour for Class IX Science (Code 086). The questions focus on the definition of isobars, examples like Argon and Calcium, and the key differences between isobars and isotopes. Submit your answers to view your score and download the PDF answer key.

Overview of Isobars

In the study of atomic structure, isobars are atoms belonging to different chemical elements that possess the same mass number but different atomic numbers. This means that while the total number of nucleons (protons + neutrons) is identical, the specific count of protons and neutrons differs between the two atoms.

Key Characteristics

  • Different Elements: Isobars always belong to different elements (e.g., Argon and Calcium).
  • Atomic Number (Z): The atomic number is different because the number of protons differs.
  • Mass Number (A): The mass number is the same.
  • Chemical Properties: Since chemical properties depend on the electronic configuration (which is determined by the number of electrons/protons), isobars exhibit completely different chemical properties.
  • Physical Properties: Physical properties may be similar in terms of mass, but density and boiling points usually differ.

Common Examples

The most cited example in Class 9 chemistry involves Argon and Calcium:

  • Argon (Ar): Atomic Number 18, Mass Number 40 (18 protons + 22 neutrons).
  • Calcium (Ca): Atomic Number 20, Mass Number 40 (20 protons + 20 neutrons).

Both have a mass number of 40, making them isobars.

Isotopes vs. Isobars

Feature Isotopes Isobars
Definition Same element, different mass numbers. Different elements, same mass number.
Atomic Number Same Different
Chemical Properties Identical Different
Electronic Config. Same Different

Quick Revision List

  1. Isobars have the same sum of protons and neutrons.
  2. They occupy different positions in the periodic table.
  3. Unlike isotopes, isobars cannot be separated easily by chemical means alone, but their physical distinctness helps.
  4. Radioactive Carbon-14 and Nitrogen-14 are isobars (Mass number 14).

Extra Practice Questions

  • 1. Why do isobars have different chemical properties?
  • 2. Calculate the number of neutrons in Potassium-40 and Calcium-40.
  • 3. Are isobars placed in the same slot in the modern periodic table?
  • 4. Give an example of a pair of isobars other than Ar and Ca.
  • 5. Does the number of electrons match in a neutral pair of isobars?