Question

Which element has a mass number of 20 and contains 11 neutrons?
(A) Copper
(B) Sodium
(C) Fluorine
(D) Calcium

Answer 1

Hint: Think about the difference between atomic number and atomic mass number. Write the formula used to calculate the atomic mass number and calculate the mass of protons. From the mass of protons, identify the element to get the answer for this question.

Complete step by step solution:
- Atomic number, Z is the total number of protons or electrons present in an atom.
- Therefore, Z = No. of protons/electrons (1)
- Atomic mass number, A is the sum of the total number of protons and neutrons present in an atom.
- Therefore, A = No. of protons + No. of neutrons (2)
- Substituting equation (1) in equation (2) we get,
A = Z + No. of neutrons (3)
- Now, in the question we have been given, the mass number is 20 so, A= 20 and the number of neutrons is 11.
- Substituting the above values in equation (3) we get,
A = Z + No. of neutrons
20 = Z + 11
Z = 20 – 11 = 9
- Therefore, the atomic number of the element is 9. The element having atomic number 9 is fluorine.
- Therefore, the element having a mass number of 20 and containing 11 neutrons is fluorine.

- Therefore, the answer is option (C).

Note: Remember atomic mass number is the sum of both neutrons and protons whereas atomic number is the total number of protons or electrons. For hydrogen, there is no neutron and so the atomic number is equal to atomic mass number.