Question

What is the ratio of the number of neutrons present in potassium (Z=19) and magnesium (Z=12) atoms with mass numbers 39 and 24, respectively? (Z = atomic number)​

• A. 5 : 3
• B. 9 : 12
• C. 5 : 6
• D. 4 : 3

Answer 1

The correct option is A 5 : 3

The number of protons gives the atomic number. The number of protons and neutrons together gives the mass number. ​

Thus,​

Number of neutrons (n) = Mass number - Atomic number ​

For potassium, n = 39 - 19 = 20 ​

For magnesium, n= 24 - 12 = 12. ​

Hence, ratio of neutrons is 20:12 or 5:3.​