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# Help Zone

### Student Question

Secondary IV • 1yr.

How do I solve this question? Step by step please.

Science

## Explanations (1)

• Explanation from Alloprof

Explanation from Alloprof

This Explanation was submitted by a member of the Alloprof team.

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Team Alloprof • 1yr. edited May 15

Good morning UnusualKamino2636.

Welcome in the Zone!

The mass number of an atom (A) is equal to the sum of the number of protons (Z) and neutrons in the nucleus. So, if an atom contains 20 protons and 40 neutrons, his mass number is then 60 (20+40).

When it comes to writing the isotope, you have to use this way ...

Since the atomic number (or number of protons) is 20, your only choice is Ca, because Mg has an atomic number of 12.

I hope theses explanations helped you to understand how to to write isotopes.

This Alloprof webpage explains atomic mass and number:

Thank you for your confidence in Alloprof. Have a nice day.