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 mass number formula

The mass number of an atom is the sum of protons and neutrons, so:
$$\text{Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}$$
Rearranged to solve for neutrons:
$$\text{Number of Neutrons} = \text{Mass Number} - \text{Number of Protons}$$

Step2: Substitute given values

Substitute mass number = 39, protons = 19:
$$\text{Number of Neutrons} = 39 - 19$$

Step3: Calculate the result

$$\text{Number of Neutrons} = 20$$

Answer:

20 (B. 20)