C
CraftExam
high importancemedium8 min

Isotopes and Isobars

Isotopes are atoms of the same element with the same atomic number but different mass numbers. Isobars are atoms of different elements with the same mass number but different atomic numbers.

Practice This Concept

Learn the concept

Student-friendly explanation

Isotopes have the same number of protons but different numbers of neutrons. Therefore, they are the same element but differ in mass number. Isobars have the same mass number but different proton numbers, so they are different elements.

How to write this in exams

  1. 1

    Start with the exact idea

    Isotopes are atoms of the same element with the same atomic number but different mass numbers. Isobars are atoms of different elements with the same mass number but different atomic numbers.

  2. 2

    Then show how to use it

    Write Z and A for both atoms, compare Z first, compare A next, then decide isotope or isobar.

  3. 3

    Add one concrete example

    Carbon-12 and carbon-14 are isotopes. Argon-40 and calcium-40 are isobars.

  4. 4

    Avoid this incomplete answer

    Calling calcium-40 and argon-40 isotopes is wrong because their atomic numbers are different, so they are different elements with same mass number.

Definition

Isotopes are atoms of the same element with the same atomic number but different mass numbers. Isobars are atoms of different elements with the same mass number but different atomic numbers.

Example

Carbon-12 and carbon-14 are isotopes. Argon-40 and calcium-40 are isobars.

Rule to remember

Isotopes: same Z, different A. Isobars: same A, different Z.

Memory hook

IsoTOPES: same proton number; isoBARS: same big mass number.

Examples and method

Worked example

Carbon-12 has Z = 6, A = 12 and carbon-14 has Z = 6, A = 14. Same Z but different A means they are isotopes.

Method to apply

Write Z and A for both atoms, compare Z first, compare A next, then decide isotope or isobar.

Diagram support

Use a two-column comparison table showing Z and A for isotope and isobar examples.

How CBSE asks it

Asked as pair identification, comparison table, or reason why two given atoms are isotopes or isobars.

Avoid common mistakes

Common confusion

Students often say isotopes have different atomic numbers, but isotopes have the same atomic number.

Common wrong answer

Calling calcium-40 and argon-40 isotopes is wrong because their atomic numbers are different, so they are different elements with same mass number.

Exam tip

For isotopes, check same Z. For isobars, check same A.

Quick check

How can you quickly distinguish isotopes from isobars?

Check atomic number and mass number: isotopes have the same atomic number but different mass numbers, while isobars have the same mass number but different atomic numbers.

Answer writing and exam use

1-mark answer

Isotopes are atoms of the same element with the same atomic number but different mass numbers. Isobars are atoms of different elements with the same mass number but different atomic numbers.

2-mark answer

Isotopes are atoms of the same element with the same atomic number but different mass numbers. Isobars are atoms of different elements with the same mass number but different atomic numbers. Isotopes: same Z, different A. Isobars: same A, different Z. Carbon-12 and carbon-14 are isotopes. Argon-40 and calcium-40 are isobars.

3-mark answer

Isotopes have the same number of protons but different numbers of neutrons. Therefore, they are the same element but differ in mass number. Isobars have the same mass number but different proton numbers, so they are different elements. Isotopes: same Z, different A. Isobars: same A, different Z. Carbon-12 has Z = 6, A = 12 and carbon-14 has Z = 6, A = 14. Same Z but different A means they are isotopes. Asked as pair identification, comparison table, or reason why two given atoms are isotopes or isobars. Calling calcium-40 and argon-40 isotopes is wrong because their atomic numbers are different, so they are different elements with same mass number.
MCQ Quiz

Practice this concept with focused MCQs

Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.

10 MCQs5 MinutesInstant Results
Practice This Concept

Help improve this page

Found something confusing, incorrect, or missing?

Practice This Concept