Question 8
Explain with examples
(i) Atomic number,
(ii) Mass number,
(iii) Isotopes and
(iv) Isobars.
Give any two uses of isotopes.
(i) Atomic number — The number of protons present in the nucleus of an atom is defined as the atomic number and is denoted by Z.
Example: Atomic number of hydrogen (Z) = 1 as only one proton is present in the nucleus of hydrogen atom.
(ii) Mass number — The total number of protons and neutrons present in the nucleus of an atom is known as the mass number and is denoted by A.
Example: Mass number of Carbon (A) = 12 as it has 6 protons and 6 neutrons.
(iii) Isotopes — The atoms of the same element having the same atomic number but different mass number are known as isotopes.
Example: Carbon has two isotopes 6C12 and 6C14
(iv) Isobars — Atoms of different elements with different atomic numbers, which have same mass number are known as isobars.
Example: Calcium [20Ca40] atomic number 20 and argon [18Ar40] atomic number 18 are isobars as both have same mass number i.e., 40 and different atomic numbers.
Isotopes have special properties which makes them useful in various fields like:
- An isotope of uranium is used as a fuel in nuclear reactors.
- An isotope of cobalt is used in the treatment of cancer.
Chapter 4: Structure of the Atom — Quick Reference
Quick Revision Points
- Thomson: positive sphere with embedded electrons (plum pudding)
- Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
- Bohr: electrons in fixed energy levels (shells K, L, M, N)
- Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
- Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
- Isotopes: same Z, different A. Isobars: different Z, same A
- Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻
- Read the detailed chapter notes for complete coverage of all NCERT topics.
- Practice all NCERT in-text and back exercise questions — they are frequently asked in exams.
- Focus on comparison tables, diagrams, and definitions — these are high-scoring areas.
- For numericals (if applicable), practice at least 20 problems of varying difficulty.
- Refer to the practice question bank (200+ questions) for thorough preparation.
- Detailed Notes: ch04-structure-of-the-atom.html
- Practice Questions: 100+ questions with answers in 05-practice-questions/
- Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
- Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html