Question

Calculate the number of electrons in 4.2 g of ${N^{3 - }}$(nitride ion)?
A) 3
B) 2
C) 1.5
D) 4.2

Answer 1

An atom comprises a nucleus having neutrons and protons which are surrounded by electrons in orbits. Number of protons (atomic number) can be considered to be equal to the total number of electrons in a neutral atom (denoted as Z). Whereas, the summation of the number of neutrons and number of protons equals the mass number of an atom (denoted as M).

Complete step by step answer:
To calculate the number of moles, use the following formula:
$Number\;of\;moles = {\dfrac{Given\;mass}{ Molecular\;mass\;of\;the\;given\; species}}$
We know the charge on nitride ion is i.e. ${N^{3 - }}$. Molecular mass of nitride is similar to that of nitrogen and that is 14.
Now, we will calculate the number of moles of nitride ion in 4.2 g by using the above formula:
$No.{\text{ }}of{\text{ }}moles{\text{ }}nitride{\text{ }}ion = \dfrac{{4.2}}{{14}} = 0.3$
So, No. of moles of nitride = 0.3 moles
One atom of neutral nitrogen element contains 7 electrons because its atomic number is 7. But here nitrogen gains three extra electrons which provide it with three negative charges. So one nitride atom contains a total of 10 electrons.
Number of electrons contained by one atom is 10.
So, the total no. of electrons carried by one mole is 10 electrons.
Now, total number of electrons carried by 0.3 moles of nitride ion is:
$10 \times 0.3 = 3{\text{ }}moles$

Hence, the correct answer is Option A i.e. 3 moles of electrons.

Note: Electrons, negatively charged particles found in the electronic shells surrounding the nucleus, possess no mass but are significant in chemical reactions. Thus, always remember that by the addition or removal of electrons from any species, there is no change in molecular mass of that species.