medium difficulty4 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.

In one line: IsoTOPES: same proton number; isoBARS: same big mass number.

Practice This Concept

Understand it

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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1Carbon-12 has Z = 6, A = 12 and carbon-14 has Z = 6, A = 14.
2Same Z but different A means they are isotopes.

Method you can reuse

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

Sketch it in your answer

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

How can you quickly distinguish isotopes from isobars?

Think first, then show answer

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.

Common wrong answer

Calling calcium-40 and argon-40 isotopes.

Why it loses marks: Their atomic numbers are different, so they are different elements with same mass number.

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

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

1
Enough for 1 mark
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
Add for 2 marks
Example: Carbon-12 and carbon-14 are isotopes. Argon-40 and calcium-40 are isobars.
3
Add for 3 marks
Therefore, they are the same element but differ in mass number.

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