Structure of Solid State of Substances and Physical Properties

Lattice.A lattice is an ordered set of points that define the structure of a crystal or crystal-forming particles. Its points identify the unit cell of a crystal. A crystal lattice is a symmetrical three-dimensional structural arrangement of atoms, ions, or molecules, as points. Ions in the crystal lattice need less energy to remain together. That is due to their systematic arrangement which make them stable. What more do you know about its features?.

Common features of a crystal lattice. In a crystal lattice, each atom, ion or molecule is represented by single point. These points are known as lattice points. They which are joined by straight-lines. By connecting these lines we get a three dimensional structure of a crystal lattice also known as Bravais lattice.Unit cells.The unit cell is the smallest part of the crystal lattice. It is the simplest repeating unit by which the entire lattice is generated. There are several types of unit cells.

Primitive unit cell.A Primitive unit cell is seen when only corner positions are taken up by particles.

Centered unit cell.A Centered unit cell has three types.body-centered, when particles are situated at the center of the body. Face centered, when particles are situated at the center of the body. Base-centered, when the particle is situated at the center of two opposite faces.

Sodium chloride.NaCl has a cubic unit cell. It is best thought of as a face-centered cubic array of anions with an interpenetrating face-centered cation lattice. NaCl is an alkali halide with an FCC crystal structure.Potassium permanganate.Potassium permanganate has an orthorhombic unit cell that contains molecules with dimensions a=9.09Å, b=5.72Å and c=7.41Å.

Classifications.The crystal lattice is classified into seven different lattice systems.Triclinic System.In a Triclinic system, all three axes have an inclination towards each other which are of the same lengths. All angles α, β and γ are all not equal to 90 degrees.

This image is a picture of Microcline, a crystal belonging to the Triclinic system. It is important to note that the shape of the unit cell contribute to the final shape. However, the shape seen here does not represent the shape of the unit cell.

Monoclinic System.In this two axes are at a 90 degrees with each other, while the third axis has an inclination. All of these have different lengths. Here you can see a monoclinic crystal.

Orthorhombic system.All three axes are at the 90 degrees with each other. These have different lengths. There are four types. primitive orthorhombic shown on the top left. Base-centered orthorhombic shown on the top-right side. Body-centered orthorhombic shown on the bottom left. Face-centered orthorhombic shown on the bottom-right side.

Here is an example of a crystal belonging to an Orthorhombic system. It is from the Salsigne Mine in France. It is an Aragonite crystal.

Trigonal System.In Trigonal systems, there are three sides, shaped into a pyramid.Hexagonal system.It has four axes among which three are of the same length and also are on one plane. They intersect each other at an angle of 60. The fourth axes intersect other axes at the 90 degrees.Tetragonal system.In Tetragonal systems, two axes are of the same length and are in the same plane while the third axes vary in length, and can be short or long.Cubic system.In a Cubic system, all three axes intersect at a 90 degrees with each other and are of the same length.

Bonding formed during lattice formation.Atoms arrange themselves in a lattice because of the net attractive force between their electrons and nuclei. Crystals formed by lattice formation can be one of three categories. They are classified into different types due to type of bonding in them. This bonding can be of covalent, ionic and metallic nature. In ionic bonding atoms exchange electrons during lattice formation when they come into contact. In covalent bonding, molecules share paired electrons during lattice formation. In metallic bonding, there is a force of attraction between free moving electrons and positive metal ions during lattice formation.

The magnitude of the forces which holds an ionic substance together is very high. It has a dramatic effect on many of its properties. So melting points vary with lattice energies of substances having similar structure. Because the magnitude of force holding substances together is high, the melting point is also high. The distance between ions is also affected in such a way that the less distance is, the stronger the bonding is.

Homoatomic Lattices.What do you know about these latties?.Molecules that are made up of atoms of the same element are called homoatomic. One type of atom comes together and forms molecules by arranging themselves in a three-dimensional arrangement, forming a lattice. So lattices formed by homogeneous atoms are called homoatomic lattices.

Diamond.The crystal structure of a diamond is a face-centered cubic lattice. This cubic crystal structure is a repeating pattern of atoms. In these arrangements, atoms can form four very strong covalent bonds. Therefore it is evident that atoms tend to form three-dimensional structures.

Graphite.Graphites consist of sheets of strongly bonded hexagonal rings. Because sheets are far from each other and also weakly bound to the next, these sheets can move parallel to one another which makes graphite a soft lubricant. There are no covalent bonds between the layers. Each carbon form three covalent bonds with other carbon atoms.

Hetroatomic Lattices.Atoms of different elements form a covalent bonds and are arranged in a three-dimensional structure called heteroatomic lattices. These molecules are made by covalent bonding between atoms of different elements. These covalently bonded molecules arrange themselves in the lattice structure.Because heteroatomic lattices are formed by the covalent bonds between an atom of different elements, they have a very high magnitude of force holding these molecules together. This is why they have high hardness and high melting and boiling points.

Usually, the individual melting point, boiling point, and solubility depend upon the polarity of the molecule. Ionic compounds are more polar and soluble in water. But those heteroatomic lattices which have less polarity and more non-polar characteristics are insoluble in water.Heteroatomic lattices have formed the molecules which have covalently bonded atoms of different elements. Because these molecules are covalently bonded and the lattice magnitude of the force is high. Such lattices cannot conduct electricity. This is also because these do not have free electrons.

Silica is an oxide of silicon. This is found in nature as quartz and is also seen in various living organisms. This has a linear structure like CO₃²-. Silicon atoms exhibit tetrahedral coordination with four oxygen atoms that surrounded a central Si atom. Therefore silicon dioxide from a three dimensional network solids. In this, each atom is covalently bonded with the four other oxygen atoms in a tetrahedral manner. Its structure is also referred to as a giant covalent structure.

Non Polar Molecular Lattice.Non-polar molecules are formed by atoms like noble gasses or formed by non-polar covalent bonds. In molecular lattice, lattice points are taken up by molecules. In non-polar molecules lattices, the lattice points are taken up by non-polar molecules or noble gasses.In non-polar molecules lattices, atoms or molecules are held by weak dispersion forces. They are called Van der wall forces. There are no strong covalent bonds between lattice points or layers. Therefore, van der wall forces held them together in a lattice.

The melting and boiling points of non-polar molecular lattices are very low and are usually in liquid or a gaseous state at room temperature and pressure. There is strong covalent bonding between the molecules which keeps the atom together. Due to these intermolecular covalent bonds electrons are localized. That’s why molecular lattices are soft and bad conductors of electricity.

Non-polar molecules are likely to dissolve in non-polar solvents. Hexane is a non-polar solvent so non-polar molecules are more easily dissolved in it. It is also because the two are attracted to one another and bonds binding the molecules of non-polar solute are broken. Some examples of non-polar solvents are chloroform, toluene, hexane and dichloromethane.

Iodine crystal.Iodine is a non-metallic, nearly black solid, at room temperature. It has a glittering crystalline appearance. Its molecular lattice contains discrete diatomic molecules, which are also seen in the molten and gaseous states. An Iodine non-polar molecular lattice is dissolved in chloroform and hexane, a non-polar solvent. It does not dissolve in water. It has a high hardness, melting point and boiling point.

Polar Molecular Lattices.These types of molecules are formed by polar covalent bonds. These molecules are held together by relatively stronger dipole-dipole interactions. The polar molecules lattices contain polar molecules at lattice points.These polar molecules are more likely to dissolve in the polar solvent. This is because polar solvents have a negative and positive charge at different places in their substances. These help in dissolving other polar molecules. The dipole-dipole interaction are helpful at this point. Some polar solvents are water, acetone, acetonitrile, isopropanol, and methanol.

Because these solids are soft and do not have free electrons, they do not conduct electricity. These molecules are polar. Since these molecules have dipole-dipole interaction, there are partial positive and partial negative charges in them. Each molecule has a permanent dipole moment. Because of stronger dipole-dipole interaction, these polar molecular lattices have high hardness, high melting points and and high boiling points.

Ice.Ice is a crystalline solid that consists of ions held together by the electrical attraction of opposite charges. In this, molecules are connected by hydrogen bonds which are permanent due to their frozen state. It results in an interconnected hexagonally shaped framework of molecules.

Ion Lattices.An ionic compound is a giant structure of ions. Because ions have a regular, repeating arrangement it is called an ion lattice. This is formed because the ions attract each other. They form a regular pattern with oppositely charged ions next to each other. This three dimensional lattice is held together by ionic bonds. These are shown as straight-lines between ions. Ionic bonds are strong electrostatic forces between oppositely charged ions.

A giant ionic lattice contains a large number of ions and ionic bonds. Therefore, a lot of energy is required to break the attraction between these oppositely charged ions. As a result, ionic compounds have high melting points and high boiling points.Ionic substances are generally most soluble in the polar solvent because the lattice energy is high. So more polar solvent is required to overcome the lattice energy of the ionic compound in order to dissolve it. Therefore, water is the most common solvent for ionic compounds. This occurs because of the following reason. The positive cation from the ionic solid is attracted to the negative end of water. Also, the negative anion of the ionic compound is attracted to the positive end of the water molecule.

Ionic solids have no free electrons. Any charged particle can carry out current but in a solid lattice, all the ions are trapped. So they cannot move from their fixed positions. Hence solid ionic compounds do not conduct electricity.Ionic compounds when in a molten state have free ions from plane to plane. So in molten state ionic compounds can conduct electricity. When ionic compounds are in a solution, they have free ions which move freely. They conduct electricity because free ions move from one place to another.

Sodium Chloride.Ionic bonds occur usually between non-metal and metal ions. As sodium is a metal and chloride is a non-metal, they form ionic compound NaCl using an ionic bond. This is salt. In salt, both sodium and chlorine complete their octet by sodium donating its valance electrons to chlorine. Its molecules arrange in a 3-dimensional structure as a FCC array of the anion with an interpenetrating FCC cation lattice. Each ion is six coordinates and contains local octahedral geometry. It has a cubic unit cell.

Metallic Lattices.It is the type of bond that is formed to create the structure of metals. In these lattices, the positively charged metal ions are arranged in regular rows. Their delocalized electrons are shared among all of them. These electrons circulate the rows in a de-localized way. In metallic lattice, metals are composed of atoms in ordered layers which form a three dimensional crystalline structure. It usually shows a body-centered cubic lattice in which each atom is surrounded by eight nearest neighbors.

It is also shown in the face-centered cubic lattice in which a given atom has twelve nearest neighbors. It also shows a closed packed or hexagonal array in which each metal is connected to six adjacent ions in one plane.Magnesium is a metallic solid. In its lattice units that take up lattice sites are mg ions which are surrounded by de-localized electrons. The arrangement of ions is in one plane, which is a hexagonal array or closed metal packed layer. Therefore, each metal has six adjacent ions in one plane.

When atoms of Sodium come together they form the body-centered cubic lattice. Every sodium atom is surrounded by eight others neighboring sodium atoms which are organized in a cubic array. In illustration, as we can see central sodium is surrounded bu 8 neighbouring sodium atoms. Aluminum has a face-centered cubic crystal structure at room temperature. It is a metal of great importance because it has high electric and thermal conductivity, high corrosive resistance, and had good reflectivity.