   Question Answers

# Number of $Na$ atoms present in the unit cell of $NaCl$ crystal is : A. $1$B. $2$C. $3$D. $4$  Hint: Firstly, A unit cell is referred to as the smallest repeating unit in a crystal lattice. Unit cells occur in many different varieties. And the cubic crystal system is one of them which has three types of unit cells and one of them is face-centered unit cell and $NaCl$ is an example of fcc lattice.

In $NaCl$ the $C{l^ - }$ ions are present at the corners of the unit cell and at the face-centers of the unit cell and the Na+ ions are present at the octahedral voids of the unit cell. Imagining a unit cell as a cube there are $8$ corners and $6$ face-centers. The octahedral voids present in a unit cell are “$n$”, where ”$n$” is the number of atoms present in a unit cell.
Before calculating the number of carbon atoms in the unit cell one must know about the contribution of the atoms towards a unit cell in the lattice. An atom present at the corner contributes $\dfrac{1}{8}$to a unit cell as it is shared to $8$ different unit cells attached together, whereas the one present at the face-center contributes $\dfrac{1}{2}$to a unit cell, and the contribution of atoms present at octahedral voids is $1$.
Now, let’s calculate the number of $Na$ atoms present in $NaCl$:
As there are $4$ atoms present at octahedral voids therefore,
$1 \times 4 = 4$,where $n = 4$ for fcc lattice.
So, the total number of $Na$ atoms is $4$.
So, the correct answer is Option D.

Note:
A student can be confused about the number of octahedral (“ $n$ ”) and tetrahedral voids (“ $2n$ ”) present in a lattice so a student should keep the values in mind. And when it comes to the contribution of the octahedral and the tetrahedral voids, they have a contribution of $1$, as they are totally a part of the lattice.
Unit of Velocity  CBSE Class 12 Physics Atoms Formula  The Solid State  Unit of Electricity  Significance of Genetics in the Process of Evolution  Derivation of Reynolds Number  Number of Moles Formula  The Idea of Time  What is the Cell Envelope?  Measuring the Rate of Change of Motion  Important Questions for CBSE Class 11 Biology Chapter 8 - Cell The Unit of Life  Important Questions for CBSE Class 12 Chemistry Chapter 1 - The Solid State  NCERT Books Free Download for Class 11 Biology Chapter 8 - Cell The Unit of Life  Important Questions for CBSE Class 9 Science Chapter 5 - The Fundamental Unit of Life  NCERT Books Free Download for Class 12 Chemistry Chapter 1 - The Solid State  Important Questions for CBSE Class 8 English Honeydew Chapter 1 - The Best Christmas Present in the World  Important Questions for CBSE Class 12 Chemistry Chapter 6 - General Principles and Processes of Isolation of Elements  Important Questions for CBSE Class 12 Physics Chapter 12 - Atoms  Important Questions for CBSE Class 12 Chemistry Chapter 7 - The p-Block Elements  Important Questions for CBSE Class 12 Chemistry Chapter 16 - Chemistry in Everyday life  Chemistry Question Paper for CBSE Class 12  CBSE Class 12 Chemistry Question Paper 2020  Chemistry Question Paper for CBSE Class 12 - 2013  Chemistry Question Paper for CBSE Class 12 - 2015  CBSE Class 12 Chemistry Question Paper 2017 - Free PDF  CBSE Class 12 Chemistry Question Paper 2019 - Free PDF  CBSE Class 12 Chemistry Question Paper 2018 - Free PDF  Previous Year Question Paper for CBSE Class 12 Chemistry - 2014  Chemistry Question Paper for CBSE Class 12 - 2016 Set 1 E  Chemistry Question Paper for CBSE Class 12 - 2016 Set 1 S  NCERT Solutions for Class 11 Biology Chapter 8 Cell: The Unit of Life in Hindi  NCERT Solutions for Class 12 Chemistry Chapter 1 The Solid State in Hindi  NCERT Solutions for Class 11 Biology Chapter 8  NCERT Solutions for Class 12 Chemistry Chapter 1  NCERT Solutions for Class 9 Science Chapter 5 The Fundamental Unit of Life in Hindi  NCERT Solutions for Class 12 Physics Chapter 12  Lakhmir Singh Chemistry Class 9 Solutions Chapter 4 - Structure Of The Atom  NCERT Exemplar for Class 9 Science Chapter 5 - The Fundamental Unit of Life (Book Solutions)  NCERT Solutions for Class 12 Chemistry Chapter 6 General Principles and Processes of Isolation of Elements in Hindi  NCERT Solutions for Class 12 Chemistry Chapter 7 The p-Block Elements In Hindi  