Question

Total no. of electrons present in $48{\text{g}}$ of $M{g^{2 + }}$ are:
A.$24{{\text{N}}_{\text{A}}}$
B.$2{{\text{N}}_{\text{A}}}$
C.$20{{\text{N}}_{\text{A}}}$
D.None of these

Answer 1

In this question, we have to calculate the number of electrons in $M{g^{2 + }}$ . We shall first calculate the moles of magnesium ions present and thus the total ions present. Then, we can find the number of electrons in each ion and multiply it with the total ions present.
The formula used:
$\,{\text{No}}{\text{. of moles = }}\dfrac{{{\text{Mass of the Substance in grams}}}}{{{\text{Molar mass of a Substance}}}} = \dfrac{{{\text{Number of Atoms or Molecules}}}}{{6.022 \times {{10}^{23}}}}$

Complete step by step answer:
The atomic number of magnesium is 12 which means that in a non-ionized state the number of electrons in magnesium is 12. In $M{g^{2 + }}$ the number of electrons will be 10.
Calculating the number of moles of $48{\text{g}}$ $M{g^{2 + }}$ will be $\dfrac{{48}}{{24}} = 2$ .
Now for calculating the no. of electrons, we multiply the no. of moles with Avogadro's number $= 2 \times {{\text{N}}_{\text{A}}}$
The total number of electrons in 48g of $M{g^{2 + }}$ will $= 2 \times {{\text{N}}_{\text{A}}} \times 10 = 20{{\text{N}}_{\text{A}}}$

Hence, the correct answer to this question is option C.

Note:
Make sure you remember the difference between isotopes, isotone, isobars and isoelectronic species. Isotopes refer to the atoms of the same element having the same atomic number but different mass numbers. According to Moseley’s law, the chemical properties of an element depend on the number of electrons present and their configuration within an atom and as the isotopes have the same number of electrons it results in the fact that they exhibit similar chemical properties.
Isotones refers to two nuclides which have the same number of neutrons, but different proton number.
Isoelectronic species refers to the elements or ions that have an equal number of electrons.
Isobars refer to the atoms of different elements with different atomic numbers but the same mass numbers. They have different chemical properties because they have different atomic numbers.