Chemistry

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Classification of Crystalline Solids

The structural units of an ionic crystal are cations and anions. They are bound together by strong electrostatic attractive forces. To maximize the attractive force, cations are surrounded by as many anions as possible and vice versa. Ionic crystals possess definite crystal structure; many solids are cubic close packed. Example: The arrangement of Na⁺ and Cl^– ions in NaCl crystal.

Characteristics:

1. Ionic solids have high melting points.
2. These solids do not conduct electricity, because the ions are fixed in their lattice positions.
3. They do conduct electricity in molten state (or) when dissolved in water because, the ions are free to move in the molten state or solution.
4. They are hard as only strong external force can change the relative positions of ions.

Covalent Solids:

In covalent solids, the constituents (atoms) are bound together in a three dimensional network entirely by covalent bonds. Examples: Diamond, silicon carbide etc. Such covalent network crystals are very hard, and have high melting point. They are usually poor thermal and electrical conductors.

Molecular Solids:

In molecular solids, the constituents are neutral molecules. They are held together by weak vander Waals forces. Generally molecular solids are soft and they do not conduct electricity. These molecular solids are further classified into three types.

(i) Non-Polar Molecular Solids:

In non polar molecular solids constituent molecules are held together by weak dispersion forces or London forces. They have low melting points and are usually in liquids or gaseous state at room temperature. Examples: naphthalene, anthracene etc.,

(ii) Polar Molecular Solids

The constituents are molecules formed by polar covalent bonds. They are held together by relatively strong dipole-dipole interactions. They have higher melting points than the nonpolar molecular solids. Examples are solid CO₂, solid NH₃ etc.

(iii) Hydrogen Bonded Molecular Solids

The constituents are held together by hydrogen bonds. They are generally soft solids under room temperature. Examples: solid ice (H₂O), glucose, urea etc.,

Metallic Solids:

You have already studied in XI STD about the nature of metallic bonding. In metallic solids, the lattice points are occupied by positive metal ions and a cloud of electrons pervades the space. They are hard, and have high melting point. Metallic solids possess excellent electrical and thermal conductivity. They possess bright lustre. Examples: Metals and metal alloys belong to this type of solids, for example Cu, Fe, Zn, Ag, Au, CuZn etc.

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Crystal Lattice and Unit Cell

Crystalline solid is characterised by a definite orientation of atoms, ions or molecules, relative to one another in a three dimensional pattern. The regular arrangement of these species throughout the crystal is called a crystal lattice. A basic repeating structural unit of a crystalline solid is called a unit cell. The following figure illustrates the lattice point and the unit cell.

A crystal may be considered to consist of large number of unit cells, each one in direct contact with its nearer neighbour and all similarly oriented in space. The number of nearest neighbours that surrounding a particle in a crystal is called the coordination number of that particle.

A unit cell is characterised by the three edge lengths or lattice constants a, b and c and the angle between the edges α, β and γ

The crystal lattice is the arrangement of the constituent particles like atoms, molecules, or ions in a three dimensional surface. On the other hand, the unit cell is known to be the building blocks of the crystal lattice, as they get repeated in three-dimensional space to yield shape to the crystal.

A unit cell is the smallest portion of a crystal lattice that shows the three-dimensional pattern of the entire crystal. A crystal can be thought of as the same unit cell repeated over and over in three dimensions.

The total three-dimensional arrangement of particles of a crystal is called the crystal structure. The actual arrangement of particles in the crystal is a lattice. The smallest part of a crystal that has the three dimensional pattern of the whole lattice is called a unit cell.

What is Crystal Lattice? The crystal lattice is the symmetrical three-dimensional structural arrangements of atoms, ions or molecules (constituent particle) inside a crystalline solid as points. It can be defined as the geometrical arrangement of the atoms, ions or molecules of the crystalline solid as points in space.

In total there are seven crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic.

A lattice system is a class of lattices with the same set of lattice point groups, which are subgroups of the arithmetic crystal classes. The 14 Bravais lattices are grouped into seven lattice systems: triclinic, monoclinic, orthorhombic, tetragonal, rhombohedral, hexagonal, and cubic.

A lattice is a hypothetical regular and periodic arrangement of points in space. It is used to describe the structure of a crystal. A basis is a collection of atoms in particular fixed arrangement in space.

Lattice Points:

Point in a crystal with specific arrangement of atoms, reproduced many times in a macroscopic crystal. The choice of the lattice point within the unit cell is arbitrary.

Crystal Basis:

Arrangement of atoms within the unit cell.

There are four types of crystals:

1. Ionic
2. Metallic
3. Covalent network, and
4. Molecular

A lattice is an ordered array of points describing the arrangement of particles that form a crystal. The unit cell of a crystal is defined by the lattice points. For example, the image shown here is the unit cell of a primitive cubic structure. In the structure drawn, all of the particles (yellow) are the same.

The arrangement of atoms in a crystal. Each point represents one or more atoms in the actual crystal, and if the points are connected by lines, a crystal lattice is formed; the lattice is divided into a number of identical blocks, or unit cells, characteristic of the Bravais lattices.

Diamond is composed of carbon atoms stacked tightly together in a cubic crystal structure, making it a very strong material. This shows us that it is not only important to know what elements are in the mineral, but it is also very important to know how those elements are stacked together.

They are cubic, tetragonal, hexagonal (trigonal), orthorhombic, monoclinic, and triclinic. Seven-crystal system under their respective names, Bravias lattice.

A lattice is an abstract structure studied in the mathematical subdisciplines of order theory and abstract algebra. It consists of a partially ordered set in which every two elements have a unique supremum (also called a least upper bound or join) and a unique infinitum (also called a greatest lower bound or meet).

Crystals are composed of three-dimensional patterns. These patterns consist of atoms or groups of atoms in ordered and symmetrical arrangements which are repeated at regular intervals keeping the same orientation to one another.

A lattice is made by crisscrossing pieces of lath, thin strips of wood, at right angles. The small squares left open between the strips of wood create a gridlike, ornamental pattern. Panels of lattice often enclose other structures, such as a garden bench or gazebo.

The crystal structure is formed by associating every lattice point with an assembly of atoms or molecules or ions, which are identical in composition, arrangement and orientation, is called as the basis. The atomic arrangement in a crystal is called crystal structure.

A lattice point is a point at the intersection of two or more grid lines in a regularly spaced array of points, which is a point lattice. In a plane, point lattices can be constructed having unit cells in the shape of a square, rectangle, hexagon, and other shapes.

The definition of lattice is a structure made from wood or metal pieces arranged in a criss-cross or diamond pattern with spaces in between. A metal fence that is made up of pieces of metal arranged in criss-cross patterns with open air in between the pieces of metal is an example of lattice.

The most common and important are face-centred cubic (FCC) and hexagonal close-packed (HCP) structures. To get a clear picture of arrangements of atoms in these two crystal structures, it is necessary to examine the geometry of possible close-packing of atoms.

There are four types of crystals: covalent, ionic, metallic, and molecular. Each type has a different type of connection, or bond, between its atoms. The type of atoms and the arrangement of bonds dictate what type of crystal is formed.

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Classification of Solids

We can classify solids into the following two major types based on the arrangement of their constituents.

• Crystalline solids
• Amorphous solids

The term crystal comes from the Greek word “krystallos” which means clear ice. This term was first applied to the transparent quartz stones, and then the name is used for solids bounded by many flat, symmetrically arranged faces.

A crystalline solid is one in which its constituents (atoms, ions or molecules), have an orderly arrangement extending over a long range. The arrangement of such constituents in a crystalline solid is such that the potential energy of the system is at minimum. In contrast, in amorphous solids (In Greek, amorphous means no form) the constituents are randomly arranged.

The following table shows the differences between crystalline and amorphous solids.

Isotropy

Isotropy means uniformity in all directions. In solid state isotropy means having identical values of physical properties such as refractive index, electrical conductance etc., in all directions, whereas anisotropy is the property which depends on the direction of measurement.

Crystalline solids are anisotropic and they show different values of physical properties when measured along different directions. The following figure illustrates the anisotropy in crystals due to different arrangement of their constituents along different directions.

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General Characteristics of Solids

We have already learnt in XI STD that gas molecules move randomly without exerting reasonable forces on one another. Unlike gases, in solids the atoms, ions or molecules are held together by strong force of attraction. The general characteristics of solids are as follows,

• Solids have definite volume and shape.
• Solids are rigid and incompressible
• Solids have strong cohesive forces.
• Solids have short inter atomic, ionic or molecular distances.
• Their constituents (atoms, ions or molecules) have fixed positions and can only oscillate about their mean positions.

solid have a fixed shape and a fixed volume, solid cannot be compressed solids have high density force of attraction between the particles is very strong. The space between the particles of solids is negligible.

Definite shape, definite volume, definite melting point, high density, incompressibility, and low rate of diffusion.

Solids have a definite mass, volume, and shape because strong intermolecular forces hold the constituent particles of matter together. The intermolecular force tends to dominate the thermal energy at low temperature and the solids stay in the fixed state. In a solid and liquid, the mass and volume remain the same.

Solids have many different properties, including conductivity, malleability, density, hardness, and optical transmission, to name a few.

A solid is a sample of matter that retains its shape and density when not confined. Examples of solids are common table salt, table sugar, water ice, frozen carbon dioxide (dry ice), glass, rock, most metals, and wood. When a solid is heated, the atoms or molecules gain kinetic energy.

Solids can be classified into two types: crystalline and amorphous. Crystalline solids are the most common type of solid. They are characterized by a regular crystalline organization of atoms that confer a long-range order. Amorphous, or non-crystalline, solids lack this long-range order.

The major types of solids are ionic, molecular, covalent, and metallic. Ionic solids consist of positively and negatively charged ions held together by electrostatic forces; the strength of the bonding is reflected in the lattice energy. Ionic solids tend to have high melting points and are rather hard.

The most obvious physical properties of a liquid are its retention of volume and its conformation to the shape of its container. When a liquid substance is poured into a vessel, it takes the shape of the vessel, and, as long as the substance stays in the liquid state, it will remain inside the vessel.

Some substances form crystalline solids consisting of particles in a very organized structure; others form amorphous (noncrystalline) solids with an internal structure that is not ordered. The main types of crystalline solids are ionic solids, metallic solids, covalent network solids, and molecular solids.

Mechanical Properties of solids describe characteristics such as their strength and resistance to deformation. Examples of mechanical properties are elasticity, plasticity, strength, abrasion, hardness, ductility, brittleness, malleability and toughness.

Solids like to hold their shape. In the same way that a large solid holds its shape, the atoms inside of a solid are not allowed to move around too much. Atoms and molecules in liquids and gases are bouncing and floating around, free to move where they want.

A solid is characterized by structural rigidity and resistance to a force applied to the surface. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire available volume like a gas.

Solids have definite shapes and definite volumes and are not compressible to any extent. There are two main categories of solids-crystalline solids and amorphous solids. Crystalline solids are those in which the atoms, ions, or molecules that make up the solid exist in a regular, well-defined arrangement.

In a solid, atoms and molecules are arranged in such a way that each molecule is acted upon by the forces due to the neighbouring molecules. These forces are known as inter molecular forces.

In crystalline solids, the atoms, ions or molecules are arranged in an ordered and symmetrical pattern that is repeated over the entire crystal. The smallest repeating structure of a solid is called a unit cell, which is like a brick in a wall. Unit cells combine to form a network called a crystal lattice.

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Importance and Applications of Coordination Complexes

The coordination complexes are of great importance. These compounds are present in many plants, animals and in minerals. Some Important applications of coordination complexes are described below.

1. Phthalo blue – a bright blue pigment is a complex of Copper (II) ion and it is used in printing ink and in the packaging industry.

2. Purification of Nickel by Mond’s process involves formation [Ni(CO)₄], which Yields 99.5% pure Nickel on decomposition.

3. EDTA is used as a chelating ligand for the separation of lanthanides,in softning of hard water and also in removing lead poisoning.

4. Coordination complexes are used in the extraction of silver and gold from their ores by forming soluble cyano complex. These cyano complexes are reduced by zinc to yield metals. This process is called as Mac-Arthur-Forrest cyanide process.

5. Some metal ions are estimated more accurately by complex formation. For example, Ni²⁺ ions present in Nickel chloride solution is estimated accurately for forming an insoluble complex called [Ni(DMG)₂].

6. Many of the complexes are used as catalysts in organic and inorganic reactions. For example,

• Wilkinson’s catalyst – [(PPh₃)₃RhCl] is used for hydrogenation of alkenes.
• Ziegler-Natta catalyst – [TiCl₄] + Al(C₂H₅)₃ is used in the polymerization of ethene.

7. In order to get a fine and uniform deposit of superior metals (Ag, Au, Pt etc.,) over base metals, Coordination complexes [Ag(CN)₂]^– and [Au(CN)₂]^– etc., are used in electrolytic bath.

8. Many complexes are used as medicines for the treatment of various diseases. For example,

• Ca-EDTA chelate, is used in the treatment of lead and radioactive poisoning. That is for removing lead and radioactive metal ions from the body.
• Cis-platin is used as an antitumor drug in cancer treatment.

9. In photography, when the developed film is washed with sodium thio sulphatesolution (hypo), the negative film gets filed. Undecomposed AgBr forms a soluble complex called sodiumdithiosulphatoargentate (I) which can be easily removed by washing the film with water.

AgBr + 2 Na₂S₂O₃ → Na₃[Ag(S₂O₃)₂] + 2 NaBr

10. Many biological systems contain metal complexes. For example,

• A red blood corpuscles (RBC) is composed of heme group, which is Fe²⁺ – Porphyrin complex it plays an important role in carrying oxygen from lungs to tissues and carbon dioxide from tissues to lungs.
• Chlorophyll, a green pigment present in green plants and algae, is a coordination complex containing Mg²⁺ as central metal ion surrounded by a modified Porphyrin ligand called corrin ring.
• It plays an important role in photosynthesis, by which plants converts CO₂ and water into carbohydrates and oxygen.
• Vitamin B₁₂(cyanocobalamine) is the only vitamin consist of metal ion. it is a coordination complex in which the central metal ion is Co⁺ surrounded by Porphyrin like ligand.
• Many enzymes are known to be metal complexes, they regulate biological processes.
• For example, Carboxypeptidase is a protease enzyme that hydrolytic enzyme important in digestion, contains a zinc ion coordinated to the protein.