How many neutrons are in the nucleus of a potassium atom with a mass number of 39?

• A. 21
• B. 20
• C. 19
• D. 18

Answer: (B)

Verified step by step guidance

1. Identify the atomic number of potassium (K) from the periodic table, which is 19. This represents the number of protons in the nucleus.
2. Understand that the mass number of an atom is the sum of its protons and neutrons. For this potassium atom, the mass number is given as 39.
3. Use the formula: \( \text{Number of neutrons} = \text{Mass number} - \text{Atomic number} \).
4. Substitute the known values into the formula: \( \text{Number of neutrons} = 39 - 19 \).
5. Calculate the result to find the number of neutrons in the nucleus of the potassium atom.