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
The average mass of an atom is determined by the sum of its protons and neutrons. Therefore, if a neutral atom of potassium has an average mass of 39 amu, it must have 39 - 19 = <<39-19=20>>20 neutrons.
Therefore, the correct answer is c) 20.