Question

if potassiums atomic mass is 39.10 amu and its atomic number is 19, how many neutrons does the most common isotope contain?
39
19
21
20

Explanation:

Step1: Recall the formula

Atomic mass ≈ number of protons + number of neutrons. Atomic number = number of protons.

Step2: Identify values

Atomic number of potassium is 19, so number of protons = 19. Atomic mass is 39.10 (we can use the whole - number value 39 for the most common isotope).

Step3: Calculate neutrons

Number of neutrons=Atomic mass - number of protons. So, 39 - 19 = 20.

Answer:

D. 20