What is the mass number of potassium?

Step 1. Formula for mass number:

$\text{Mass number}\left(\text{A}\right)=\text{number of protons}\left(\text{Z}\right)+\text{number of neutrons}\left(\text{n}\right)$

Step 2. Calculation of mass number of potassium$\left(\mathrm{K}\right)$:

• The atomic number of potassium is $19$ and the atomic number of an element is equal to the number of protons.
• So, the number of protons in potassium is $19$.
• The number of neutrons in potassium is $20$.
• Using the above formula, the mass number of potassium $\text{=19+20}=39$.

Hence, the mass number of potassium is $39$.