Close Packing
The constituent particles in crystalline solids are arranged in a regular and repeating pattern. The crystal lattice is a diagrammatic representation of three-dimensional arrangements of constituent particles in a crystal, where each particle is represented as a point in space. The atoms in a crystal lattice are packed so closely together that there is very little space between them. As a result, the cubic form of a lattice's unit cell. There will still be some empty spaces in the cell as we stack the spheres. The arrangement of these spheres must be very effective in order to eliminate these empty spaces. To avoid empty spaces, the spheres should be placed as close together as possible.
The definition of Coordination Number is also related. In a crystal lattice structure, the coordination number is the number of atoms that surround a central atom. As a consequence, there are three directions in which the constituent particles are tightly packed. Let's take a look at each of them individually.
The following sections discuss close packing in crystals in solids in various dimensions:
Close Packing in One Dimension
Packing in one dimension involves arranging spheres in a row such that neighbouring atoms are in contact. The number of nearest neighbour particles is known as the coordination number. The coordination number is two in the case of one-dimensional close packing.
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Close Packing in Two Dimension
A row of closed packed spheres is stacked to create a two-dimensional pattern in two-dimensional close packing. There are two methods for stacking:
Square Close Packing:
In a close packing, the second row can be put exactly below the first row. As a result, if we call the first row a "A" type row, the second row, which is organized identically to the first, is also an "A" type row. Each sphere is in contact with four other spheres in this configuration. As a result, it has a four-coordination number. We note that when the centres of the four closest spheres are joined, a square is formed. In crystalline solids, this type of packing is known as square close packing in two dimensions.
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Hexagonal Close-Packing (ABAB Type Arrangement):
The second row can be staggered below the first row, with its spheres fitting into the depressions of the first row. As a result, if we refer to the first row as an "A" type row, the second row, which is organized differently, can be referred to as a "B" type row. The third-row displays as “A” form once more. The “ABAB” sort of packing is what it's called. Each sphere is in contact with six other spheres in this configuration. As a result, it has a teamwork number of six. When the centres of the six closest neighbouring spheres are connected, a hexagon is formed. Hexagonal close packing in two dimensions is the name given to this type of solid packing. In contrast to square close packing, it has less free space and thus higher packing efficiency.
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Three Dimensional Close Packing
True lattices and structures are formed by packing three-dimensionally close together. They're created by piling two-dimensional sphere layers on top of each other. This can be accomplished in two ways:
Close packing in three dimensions derived from two-dimensional square close-packed layers
Close packing in three dimensions derived from two-dimensional hexagonal close packing layers
Three-Dimensional Close Packing from Two-Dimensional Close-Packing in Crystals:
By putting the second square closed packing exactly above the first, three-dimensional close packing in solids can be created. The spheres are properly positioned horizontally and vertically in this tight packing. Similarly, we can make a simple cubic lattice by stacking more layers one on top of the other. The primitive cubic unit cell is the unit cell of the basic cubic lattice.
Three-dimensional Close Packing from Two-Dimensional Hexagonal Close-Packing in Crystals:
Close packing in three dimensions derived from two-dimensional hexagonal close packing layers
With the support of two-dimensional hexagonal packed layers, three-dimensional close packing can be shaped in two ways:
Stacking the second layer on top of the first
Stacking the third layer on top of the second
Stacking the Second Layer Over the First Layer
Assume that we take two hexagonal tight-packed layers ‘A' and position them over the second layer B (because the spheres in both layers are aligned differently) such that the spheres of the second layer are positioned in the depressions of the first layer. When a sphere from the second layer is positioned directly above the void (space) from the first layer, a tetrahedral void is created. We also note octahedral voids at the points where the second layer's triangular voids are placed next to the first layer's triangular voids in such a way that triangular space does not overlap. Six spheres surround the octahedral voids.
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By Stacking the Third Layer Over the Second Layer
By covering tetrahedral voids
By covering octahedral voids
Did You Know?
The spheres of the third layer are aligned directly above the spheres of the first layer in three-dimensional packing. If we give the first layer the letter A and the second layer the letter B, the pattern would be ABAB... and so on. The resulting structure is known as a hexagonal close-packed structure, or HCP.
Spheres are not put with either the second layer or the first layer in this form of packaging. The pattern would be ABCABC if we call the first layer A, the second layer B, and the third layer C (as it is now a separate layer). The resulting structure is also known as a cubic close packed (ccp) structure or a face-centred packed cubic structure (fcc). Metals such as copper and iron, for example, crystallize in the structure. Since each sphere in the system is in direct contact with 12 other spheres, the coordination number in both cases would be 12. The packing is extremely effective, with approximately 74% of the crystal being fully occupied.
FAQs on Solid Packing
1. What is meant by the packing of constituent particles in solids?
In solid-state chemistry, packing refers to the arrangement of constituent particles (atoms, ions, or molecules) within a crystal lattice. To achieve maximum stability, these particles arrange themselves to minimise empty space. This efficient arrangement, known as close packing, aims to maximise the packing efficiency, which is the percentage of total space occupied by the particles.
2. What are the two main types of close packing in two dimensions?
In two dimensions, there are two primary ways to pack spheres (particles):
- Square Close Packing (AAA type): In this arrangement, the spheres of each row are placed directly above the spheres of the row below it, creating a square pattern. Each sphere is in contact with four others, resulting in a coordination number of 4.
- Hexagonal Close Packing (ABAB type): Here, the spheres of the second row are placed in the depressions of the first row. This staggered arrangement is more efficient. Each sphere touches six others, giving it a coordination number of 6.
3. How is a three-dimensional crystal structure built from two-dimensional layers?
Three-dimensional close-packed structures are formed by stacking two-dimensional layers on top of one another. The process typically starts with a hexagonal close-packed layer (layer A). The second layer (layer B) is placed over the first, with its spheres sitting in the depressions of layer A. The arrangement of the third layer then determines the final structure:
- If the third layer is placed directly over the first (an ABAB... pattern), it forms a Hexagonal Close Packed (HCP) structure.
- If the third layer is placed in the remaining depressions, creating a new position C (an ABCABC... pattern), it forms a Cubic Close Packed (CCP) or Face-Centred Cubic (FCC) structure.
4. What are tetrahedral and octahedral voids, and how do they form?
Voids are the empty spaces left between the constituent particles in a close-packed structure. The two main types are:
- Tetrahedral Voids: A tetrahedral void is a small empty space created when a sphere of the second layer is placed directly over the depression formed by three touching spheres in the first layer. This void is surrounded by four spheres.
- Octahedral Voids: An octahedral void is a larger empty space formed by the combination of two triangular voids from adjacent layers (one from layer A and one from layer B) that are pointing in opposite directions. This void is surrounded by six spheres.
5. What is the key difference between Hexagonal Close Packing (HCP) and Cubic Close Packing (CCP)?
The primary difference between HCP and CCP structures lies in their layer stacking sequence. While both originate from stacking hexagonal 2D layers and have the same packing efficiency (74%) and coordination number (12), their patterns differ:
- HCP Structure: Follows an ABABAB... stacking pattern. The spheres in the third layer are vertically aligned with the spheres in the first layer.
- CCP (or FCC) Structure: Follows an ABCABC... stacking pattern. The spheres in the third layer are placed in a new position, not aligned with either the first or second layer. The fourth layer then aligns with the first.
6. Why is hexagonal close packing in two dimensions more efficient than square close packing?
Hexagonal close packing is more efficient because its staggered arrangement allows the spheres to fit more snugly together, minimising the empty space (voids) between them. In square packing, the spheres are aligned directly on top of each other, leaving larger, squarish gaps. The superior arrangement in hexagonal packing allows each sphere to be in contact with six neighbours instead of just four, leading to a higher density and greater stability in the layer.
7. How does the choice of covering tetrahedral versus octahedral voids influence the final 3D structure?
The choice of which voids to cover with the third layer directly determines the resulting 3D crystal structure. Starting with two layers (A and B):
- If you place the third layer's spheres over the tetrahedral voids of the second layer, these spheres will align perfectly with the first layer (A). This creates the repeating ABAB... pattern characteristic of the HCP structure.
- If you place the third layer's spheres over the octahedral voids of the second layer, these spheres will not align with either layer A or B, creating a new layer C. This results in the ABCABC... pattern of the CCP (or FCC) structure.
8. What are some real-world examples of metals that exhibit CCP and HCP packing?
The type of packing significantly influences a metal's physical properties. Common examples include:
- HCP (Hexagonal Close Packed): Metals like Magnesium (Mg), Zinc (Zn), Cobalt (Co), and Titanium (Ti) adopt this structure.
- CCP (Cubic Close Packed) / FCC: This structure is very common in metals such as Copper (Cu), Silver (Ag), Gold (Au), Aluminium (Al), and Nickel (Ni).
