Potassium’s atomic number is 19, and its atomic mass is approximately 39. How many neutrons does potassium have? 19

58

20

39

20

Explanation:

Atomic number is same as the number of protons of an atom and atomic mass is equal to the sum of number of protons and number of neutrons of an atom.

Atomic number = 19

So, number of protons (p) = 19

Atomic mass = 39

So, sum of number of neutrons and number of protons (n + p) = 39

Number of neutons (n)

= (n + p) - p

= 39 - 19

= 20

20

Explanation:

The atomic mass is the sum of the number of protons and the number of neutrons, and also, the atomic number is the number of protons, so:

19 + number of neutrons = 39

number of neutrons = 39 – 19

number of neutrons = 20