Question

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

Answer 1

The mass number and number of neutrons are given in the question. We know the formula used to calculate the atomic mass, so we can calculate the number of protons. And from the number of protons, we can identify the name of the element.

Complete step by step answer:
Atomic Number (Z) is the total number of electrons or protons present in an element.
So, we can say that Z = number of electrons / protons
We can define mass number as the sum of the total number of protons and neutrons present in an atom. It is represented as A
So, we can say that A = No of protons + no. of neutrons
Therefore, A = Z + no. of neutrons
In the question, it is given that the mass number is 20. $\left( {A = 20} \right)$
Also, number of neutrons = 11 (given in the question)
A = Z + no. of neutrons
$\Rightarrow$ $20 = Z + 11$
$\Rightarrow Z = 20-11 = 9$
Therefore, the atomic number (Z) of the element is 9 and the element having atomic number 9 is Fluorine (F).
So, the element having a mass number of 20 and containing 11 neutrons is Fluorine.

Therefore, the correct answer is option (E).

Note: We should never get confused with the two term, atomic mass and atomic number remember that the atomic mass of an element is the sum of the number of protons and number of neutrons whereas the atomic number of an element is the number of electrons or protons. In the case of Hydrogen, there are no neutrons. So, its atomic number is its mass number.