The atomic structure of any element is made up of a nucleus with positively charged, neutral particles and negatively charged electrons that surround the nucleus. Chemical bonds involve electrons sharing or transferring between atoms. There are some typical bonds, such as ionic bonds, covalent bonds, metallic bonds and hydrogen bonds. The solid structures depend on the way the atoms or molecules arrange themselves. There exists 14 different types of crystal unit cell structures or lattice in nature (NDT-Education, nd.a). The unit cell is the smallest component. The whole crystal can be generated by repeating the unit cell (Lister and Renshaw, 2000). This essay will discuss the different types of chemical bonds and crystal structures.
Ionic bonding occurs when electrons are transferred from atoms (usually metals) to other atoms (usually non-metal). As a result, each new charged particle can achieve a full outer shell (Octet Rule). Figure 1 below shows the Lewis Structure (dot and cross) of sodium chloride molecule.
Figure1. The Lewis Structure of NaCl (NDT- Education, nd.a)
Since each electron has a negative charge, the sodium atom that loses electrons will result in being positively charged (cation). Alternately, the chloride atom that gains extra electrons becomes a negatively charged ion (anion).
Sodium chloride is considered a typical ionic solid. It has a giant ionic structure since there maybe billions of sodium ions and chloride ions held together by this electrostatic attraction (lane, 2010). The small part of sodium chloride structure can be shown in Figure 2(a). The cation (sodium) in the centre is surrounded by six chloride ions. And each chloride is also surrounded by six sodium ions. With this reason, sodium chloride is described as being 6:6 coordination (Lister and Renshaw, 2000).
(a). Space-filling (b). Expanded
Figure 2. The Sodium Chloride Structure (Chemguide, 2008)
The stronger attraction between cations and anions can lead to the more energy released. The more energy released causes the structure to become more stable (Chemguide, 2008). Before the anions touch each other, the maximum number of chloride ions around the central sodium ion is six, which is the most stable. When the anions start touching, the crystal will become less stable due to the repulsions between negative atoms (Chemguide, 2008). Owing to a large size of the caesium ions, the caesium chloride structure is 8:8 coordination (Lister and Renshaw, 2000).
Compounds with ionic bonds have high melting temperatures (Lister and Renshaw, 2000). They will not conduct electricity when solid. However, when melted or dissolved in water, they are conductors of electricity, because the charged ions are free to move and can carry a current (Lister and Renshaw, 2000).
Covalent Molecules
A covalent bond is usually formed by non-metal atoms moving close together. They can share pairs of electrons. Each atom becomes stable by obtaining a full outer shell (octet). Molecular covalent compounds may be solids, liquids or gases.
Although the covalent bonds between atoms (interatomic) are strong, the bonds between molecules (intermolecular) are weak, so the substances with covalent bonds have low melting and boiling temperatures (Lister and Renshaw, 2000). With no charged particles, these compounds are not good at conducting electricity in any state (Lister and Renshaw, 2000). However, there are some differences in giant covalent molecules. The covalent bonds within giant molecules become a network extending throughout the compound.
Giant covalent structures are variable depending on the arrangement of atoms. Allotropes are different forms of the same element (Lane, 2009). Although diamond and graphite is allotropes of carbon, they have different structures and properties.
Diamond has a three- dimensional (3D) structure (Lane, 2009). As Figure 3 shows, each carbon forms four single covalent bonds with adjacent carbon atoms.
Figure 3. The Structure of Diamond (Chemguide, 2000a)
The strong carbon-carbon covalent bonds in 3D structure cause diamond to become the hardest natural substances and have a very high melting point (around 4000K) (Lister and Renshaw, 2000). With such hardness, diamonds can be used industrially to tip drills (Lister and Renshaw, 2000).
Compared with diamond, graphite has a two- dimensional structure (Figure 4). The carbon atoms use three of its electrons to form single bonds. The fourth electrons become delocalized and a negative system over the whole layer (Lister and Renshaw, 2000).
Figure 4. The Graphite (University of Oxford, 1999)
Since the delocalized electrons freely move throughout the sheets, graphite also conducts electricity (Lister and Renshaw, 2000). In this structure, the distance (2.5d) between layers is about 2.5 times the distance (d) between two atoms in the same layer (Chemguide, 2000a). The additional bonding caused by the delocalized system make the covalent bonds within graphite strong, while the bonds between the layers are weak (Chemguide, 2000a). As a result, the layers can slide over each other easily. When used as in pencils, the layers can be rubbed off to stick to the paper (Chemguide, 2000a).
Metal Structures
Metallic bonds are formed in metals or alloys of metals. The electrons from the outer shell freely move around the atoms and form a pool called a 'sea of electrons' (Lister and Renshaw, 2000). Metals are able to conduct heat and electricity easily since there are considerable mobile electrons in the 'sea' (Lane, 2009). The sea structures also make metals have high melting point. Ductility and malleability are other properties of metals (Lister and Renshaw, 2000).
Some metals have unit cell structures referred to hexagonal close packing (HCP) (Figure 5). As shown in Figure 5(b), the HCP structure has three layers of atoms. There are 12 atoms in the corners of the top and bottom layers. Each atom contribute 1/6 atom to the unit cell (12 -1/6=2). Two atoms in the center of the top and bottom layers each contributes 1/2 atom (2 -1/2=1). In the middle layer, three atoms each contributes 1 atom (3-1=3). Therefore, there are six (2+1+3) atoms in each unit cell.
(a). Expanded (b). The Unit Cell
Figure 5. The Structure HCP (NDT -Education, nd.b)
As Figure 6 shows, the HCP has a way of arranging layers called ABA, in which the third layer is placed directly above the first one (Lister and Renshaw, 2000). It has a coordination number of 12 since there are 12 adjacent atoms around the atom in the center. The volume of atoms in a unit cell occupying the total volume of the cell is called the packing factor (NDT -Education, nd.c). It can be calculated by (NatomsVatom)/Vunit cell, where Natoms is the total number of atoms in each unit cell, Vatom is the volume of one atom, and Vunit cell is the volume of the unit cell (NDT -Education, nd.c). The HCP has a packing factor of 0.74 (Lister and Renshaw, 2000).
Figure 6. The Structure HCP (NDT -Education, nd.b)
Another common packing is body- centered cubic (BCC). As Figure 7 shows, there are 8 atoms in the corners of the top and bottom layers. Each atom contribute 1/8 atom to the unit cell (8 -1/8=1). One atom in the center contributes 1 atom. Therefore, each BCC unit cell consists of two atoms (Lane, 2009). The coordination number is 8 and the packing factor is 0.68 (Lister and Renshaw, 2000).
(a). Expanded (b). The Unit Cell
Figure 7. The Structure BCC (NDT -Education, nd.b)
Simple Molecule
Like water molecules, simple molecules are another type of solid structure. The water molecule has covalent bonds between an oxygen atom and two hydrogen atoms. Water molecules can be arranged in a diamond structure (Figure 8).
Figure 8. The Structure of Water (Chemguide, 2000b)
Electronegativity is a relatively measure of a specified atom's ability to attract electrons (Lewis, 2000). Since hydrogen atoms are bonded to electronegative atoms (oxygen) and lone pairs of electrons from oxygen, there exists hydrogen bonds to hold one water molecule to other molecules (Lister and Renshaw, 2000). Due to the hydrogen bonds, the molecules are less closely packed than in liquid state. Therefore, ice is less dense than water. This prevents water freezing from deep to surface. It allows oceans to remain liquid and fish to survive (Lane, 2009). As the temperature increases (from 0℃ to 4℃), some hydrogen bonds are broken, which causes the density of water to increase (Chemguide, 2000b). However, after 4℃, the density falls again because the thermal motion makes molecules move apart(Chemguide, 2000b).
Conclusion
In conclusion, different chemical bonds have different features. Compared with covalent bonds, ionic bonds have transferred electrons rather than shared electrons. Metallic bonds form 'sea of electrons'. Different structures cause different properties. For example, the giant structure makes diamond and graphite have high melting points. The freely moving or delocalized electrons cause ionic molecules (liquid state and aqueous solution), metals to conduct electricity. Metals have different unit structures, such as simple cubic, HCP, BCC. Some materials seem to have mixed character. For instance, with covalent, ionic and metallic electron sea, graphite can conduct electricity well.
