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STRUCTURE OF SODIUM CHLORIDE

Sodium chloride, also known as Rock salt or halite is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions with a molar mass of 58.4 g/mol. With molar masses of 22.99 and 35.45 g/mol respectively, 100 g of NaCl contain 39.34 g Na and 60.66 g Cl.
It occurs naturally as white crystals. It is extracted from the mineral form halite or evaporation of seawater. The structure of NaCl is formed by repeating the face centered cubic unit cell.
Compounds with the sodium chloride structure include alikali halides and metal oxides and transition-metal compounds.

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Sodium Chloride (NaCl) Structure

Structure of NaCl (Rock salt ) is of type AX , where A is the cation

And X is the anion.

The space lattice of NaCl is of fcc type. The smaller ions are the Na⁺ with has an atomic radius of 102 pm, and the larger ions are the Cl^-with an atomic radium of 181 pm.

The crystal structure of sodium chloride, NaClNaCl, a typical ionic compound. The smaller purple spheres represent sodium cations, Na+Na+, and the larger green spheres represent chloride anions, Cl−Cl−

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Sodium Chloride (NaCl) Structure

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Chloride ions are fcc type of arrangement, i.e., it contains chloride ions at the corners and at the center of each face of the cube.
Sodium ions are so located that there are six chloride ions around it. This equivalent to saying that sodium ions occupy all the octahedral sites.
As there is only one octahedral site for every chloride ion, the stoichiometry is 1 : 1.
It is obvious from the diagram that each chloride ion is surrounded by six sodium ions which are disposed towards the corners of a regular octahedron. We may say that cations and anions are present in equivalent positions and the structure has 6 : 6 coordination.
The structure of sodium chloride consists of eight ions a unit cell, four are Na⁺ ions and the other four are Cl^– ions.
However, the structure of this molecule allows their positions to be switched since The coordinate number of each Na and Cl ion in this structure is 6.

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Each corner ion is shared between eight unit cells, each ion a face of the cell by two cells, each ion on a edge by four cells and the ion inside the cell belongs entirely to that unit cell.�So the positions of the ions are the followings (with Na at the center of axis) :

The position of Cl and Na ions are:

Cl : (0, 0, 0); ( a/2,a/2, 0); (a/2, 0, a/2); (0, a/2, a/2);

Na : (a/2, a/2, a/2); (0,0, a/2); (0,a/2,0), (a/2,0,0)

There are four types of site: unique central position, face site, edge sites and corner site, which are used to determine the number of Na⁺ ions and Cl^- ions in the unit cell of NaCl.

When counting the number of ions, a corner site would be shared by 7 other unit cells. Therefore, 1 corner would be 1/8 of an ion.

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A similar occurrence happens with the face site and the edge sites. For a face site, it is shared by 1 other unit cell and for an edge site, the ion is shared by 3 other unit cells. NaClNaCl is a face centered cubic unit cell which has four cations and four anions. This can be shown by counting the number of ions and multiplying them in relation to their position.

Na+Na+:

1center+12edge×1/4=4sodium ions total per cell1center+12edge×1/4=4sodium ions total per cell

Cl−Cl−:

4face×12+8corner×18=4chloride ions total per cell4face×12+8corner×18=4chloride ions total per cell

Each ion in this lattice has six of the other kind of ion as its nearest neighbors, and twelve of the same kind of ions as its second nearest neighbors. There are many ionic compounds that assume this structure including all other halides of Na, Li, K and Rb. CsF, AgF, AgCl, BaO, CoO, and SrS are also among many that will form similar structures to NaCl.

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Method to draw NaCl Structure.

To draw the structure of a sodium chloride lattice first of all draw a square.

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Step 2: Draw an identical square, offset from the first one.

Getting the offset right is important. If you get it wrong, when you draw the ions, they will all end up in a muddle! The only way you are going to find this out is by trial and error. If you take the diagram above as a guide, you won't go far wrong.

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Step3: Join the two squares together to make a cube.

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Step 4: Divide the cube into 8 smaller cubes

Start by drawing lines from the mid-points of each edge to the edge opposite on the same face of the cube. When you have done each face, it will look like this:

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Step 5: Draw three lines through the centre of the cube from the centre of each face to the centre of the opposite face.

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Step 6: Put the ions on.

Add a key showing which ion is which.

Chloride ions have a diameter which is roughly twice the size of the sodium ions so show the chloride ions as bigger and sodium ion as smaller ones.

In the top layer of ions, it doesn't matter whether you start with sodium ions or chloride ions in the corners. All that matters is that you alternate the ions so that you never get two ions the same joined directly by one of the lattice lines.

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Structure of Diamond

Diamond is a metastable allotrope of carbon where the

each carbon atom is bonded covalently with other

surrounding four carbon atoms and are arranged in a

variation of the face centered cubic crystal structure

called a diamond lattice

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The diamond structure is face centered cubic with four additional atoms in the body diagonals. Out of four additional atoms, two atoms are placed at ¾ length of first and second diagonal, remaining two atoms are placed at ¼ lengths of 3rd and 4th body diagonals. Total number of C atoms is 8.

The diamond cubic structure is a combination of two interpenetrating FCC sub lattices displaced along the body diagonal of the cubic cell by 1/4th length of that diagonal. Thus the origins of two FCC sub lattices lie at (0, 0, 0) and (1/4, 1/4,1/4)

The whole appearance seems complicated thus for simplicity, we draw the projection of atoms on a plane (say XY plane) as shown in second figure. In XY pane, only five atoms are at the face (base of cube) ABCD in actual and other atoms are projections of atom near to face.

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Figure shows four atoms (green) bonded to four others within the volume of the cell. Six atoms fall on the middle of each of the six cube faces, showing two bonds. Out of eight cube corners, four atoms bond to an atom within the cube. The other four bond to adjacent cubes of the crystal.

Number of Atoms per Unit Cell:-

Eight carbon atoms are at eight corners.

Thus, contribution of corner atoms = 8 x 1/8 = 1

There are six carbon atoms at the centers of each face.

Thus, contribution of face centered atoms = 1/2 x 6 = 3

Four atoms are placed completely inside the unit cell along the diagonal in such a way that each of them has tetrahedral link with one corner atom.

∴ Number of atom per unit cell, n = 1 + 3 + 4 = 8

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To draw structure of diamond

Step 1: Draw a carbon atom, and then arrange 4 others around it

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The arrangement around the central carbon atom is described as tetrahedral. A tetrahedron is a pyramid with a triangular base.

Step 2: Draw three more carbon atoms directly underneath the bottom ones in the last diagram.

�

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