Isobars

Two numbers describe an atom. The atomic number is the count of protons, and it decides which element the atom is. The mass number is protons plus neutrons. Isobars share the mass number, so they have about the same mass. But they are different elements.

They are easy to mix up with isotopes. Isotopes are atoms of the same element with different mass numbers. Carbon-12 and carbon-14 are isotopes. Both have 6 protons, but one has 6 neutrons and the other 8.

A small example

Calcium-40 and argon-40 both have mass number 40. Calcium has atomic number 20 and argon has 18. Find the neutrons in each.

• Calcium: 40 - 20 = 20 neutrons.
• Argon: 40 - 18 = 22 neutrons.
• Same mass number, different atomic numbers. So they are isobars.

Because they are different elements, isobars behave quite differently. Calcium is a reactive metal. Argon is a gas that hardly reacts at all. Their electrons, not their mass, decide how they react.

A quick way to remember: isotopes share the proton count, isobars share the total count.

Where it fits

This goes beyond the Class 10 board syllabus. HBCSE's Junior Science syllabus lists it under the structure of atoms. That syllabus is for NSEJS, which the Indian Association of Physics Teachers (IAPT) conducts.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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