Different atoms, same mass number

The idea

Atoms of different elements with the same mass number but different atomic numbers are called isobars; their atoms have the same total number of nucleons.

A surprising match

Argon, potassium and calcium are three different elements. Their atomic numbers are 18, 19 and 20.

How can their atoms be alike in mass number?

• The total of protons and neutrons can be the same
• They must have the same number of protons
• They must be the same element

Three atoms, one mass number

Open each tab.

Isobars

Argon, potassium and calcium each have a mass number of 40, but they have different atomic numbers. Atoms of different elements with the same mass number are called isobars.

Isobars are atoms of different elements with the same mass number but different atomic numbers. Their atoms have the same total number of nucleons.

Notes

Isobars: atoms of different elements with the same mass number but different atomic numbers, so the same total number of nucleons.

Check yourself

What are isobars?

Argon (Z = 18) and calcium (Z = 20) both have mass number 40. What is true of them?

Potassium has atomic number 19 and mass number 40. How many neutrons does a potassium atom of this kind have?

Answer: 21

40 − 19 = 21 neutrons.

Which pair of atoms are isobars?

• Atoms of the same element with different mass numbers. Those are isotopes.
• Atoms of different elements with the same mass number — correct. Yes!
• Atoms of different elements with the same number of protons. Atoms with the same number of protons are of the same element.
• Same atomic number, different mass numbers. Their atomic numbers 18 and 20 are different, and their mass numbers are both 40.
• Same atomic number and same mass number. Their atomic numbers are different, so they are different elements.
• Same mass number, different atomic numbers — correct. Yes!
• One with Z = 18 and A = 40, and one with Z = 20 and A = 40 — correct. Yes!
• One with Z = 6 and A = 12, and one with Z = 6 and A = 14. Same atomic number, different mass numbers: these are isotopes.
• One with Z = 17 and A = 35, and one with Z = 17 and A = 37. Same atomic number, different mass numbers: these are isotopes.