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Student Question

Secondary IV • 1yr.


I was reviewing and trying to remember how to find the number of protons, electrons and neutrons in an atom? (I will put an example below) Thank you !!!

$$ _{92}^{235}U $$


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Explanations (1)

  • Explanation from Alloprof

    Explanation from Alloprof

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

    Team Alloprof • 1yr.


    First let's start by remembering the components of the Berzelius notation, which is the notation used in the example you put in your question. The notation always has the following form:

    $$ _Z^AX $$


    $$ Z\ :\ atomic\ number $$

    $$ A\ :\ mass\ number $$

    $$ X\ :\ symbol\ of\ the\ element $$

    You can find the number of protons and electrons thanks to the atomic number (Z).

    To find the number of neutrons, we can use the mass number. This number represents the mass of the atom, in other words, the sum of the protons’ and the neutrons’ mass. Both have a mass of 1 u. The electrons’ mass is negligible in this calculation since it is much smaller than that of the protons and neutrons.

    To find the number of neutrons, you subtract the number of protons from the mass number. This can be done by subtracting the atomic number from the mass number.

    In summary:

    $$ Number\ of\ electrons\ =\ Z $$

    $$Number\ of\ protons\ =\ Z $$

    $$ Number\ of\  neutrons\ =\ A\ -\ Z $$

    In your example there would therefore be 92 protons and 92 electrons. There will also be (235 - 92) = 143 neutrons.

    Hope that answers your question! Keep up the good work!😊