(This is for science, not physics.)
A neutral atom of potassium (K) has an average mass of 39 amu and 19 electrons. How many neutrons does it have?

A) 10
B) 19
C) 20
D) 58

Respuesta :

The answer would be B, 19 protons. Hope this helps!
  The average mass of potassium is 39 amu and 19 electrons. 19 electrons mean there have to be 19 protons. 39-19=20. Therefore potassium has 20 neutrons. The answer is C, 20.