Question

all atoms of the element potassium have 19 protons. one of the most stable types of potassium atoms has the mass number 39. how many neutrons make up one of these potassium atoms? 19 20 38 58

Explanation:

Step1: Recall the formula for mass number

The mass number ($A$) of an atom is the sum of the number of protons ($Z$) and neutrons ($N$) in its nucleus, so the formula is $A = Z + N$.

Step2: Identify the given values

We know that for potassium, the number of protons $Z = 19$ and the mass number $A = 39$.

Step3: Solve for the number of neutrons

Rearrange the formula to solve for $N$: $N = A - Z$. Substitute the values: $N = 39 - 19 = 20$.

Answer:

20 (corresponding to the option "20")