Question

What is the ratio of the number of neutrons present in potassium and magnesium atoms with mass number 39 and 24 respectively?
A. 19:12
B. 5:3
C. 5:6
D. 4:3

Answer 1

Hint: Mass number is the sum of number of neutrons and number of protons in an atom. The number of protons in an atom is equal to its atomic number.

Complete step by step answer:
We know that, atomic number of Potassium (K) is 19 and the atomic number of Magnesium (Mg) is 12. Also, given that mass number of Potassium = 39 and mass number of Magnesium = 24.

Now, for Potassium,
Mass number = no. of neutrons + no. of protons
39 = no. of neutrons + 19
no. of neutrons = 39-19
No. of neutrons = 20

For magnesium,
Mass number = no. of neutrons + no. of protons
24 = no. of neutrons + 12
no. of neutrons = 24-12
no. of neutrons = 12

Now, we know that the number of neutrons in potassium equal to 20 and the number of neutrons in magnesium equal to 12.
Therefore, the ratio of the number of neutrons present in potassium to that of magnesium = \[\dfrac{{20}}{{12}}\] = \[\dfrac{5}{3}\]

Hence, the correct answer is (B) \[\dfrac{5}{3}\].

Note: A student must be careful between the number of neutrons and number of protons. He might be deceived in the language of question sometimes. Remember that the mass number depicts the sum of the number of neutrons and protons while the atomic number depicts the number of protons in an atom.