Answer
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Procedure
By looking at the periodic table, you will notice that the elements have an atomic number and the atomic number is the same as the number of protons and electrons in a neutrally charged element.
In the case of potassium, the number of protons is 19. The number of neutrons is caluculated as mass-protons, therefore in the first case; 39-19=20 neutrons.
Chlorine is the name of the second element. The electrons are 18 because we have a -1 negative charge (meaning the represented is not a neutral element). The number of neutrons is 35-17=18.