To determine the number of neutrons in an atom, you can use the formula:
\[ \text{Number of Neutrons} = \text{Atomic Mass} - \text{Atomic Number} \]
For potassium (K):
- Atomic mass ≈ 39
- Atomic number = 19
Now plugging in the values:
\[ \text{Number of Neutrons} = 39 - 19 = 20 \]
Therefore, potassium has 20 neutrons.