Chemical bonding 1.pdf

CHEMICAL BONDING  Basic Concept The electrons present in the outermost shell of an atom determine the valency of an atom and hence called 'valency electrons'. The electronic theory of valency was originated by Kossel and Lewis. Octet rule/Duplet rule- The octet rule refers t...

CHEMICAL BONDING  Basic Concept The electrons present in the outermost shell of an atom determine the valency of an atom and hence called 'valency electrons'. The electronic theory of valency was originated by Kossel and Lewis. Octet rule/Duplet rule- The octet rule refers to the tendency of atoms to have eight /two electrons in the valence shell. Stable Configuration a A stable electron configuration refers to an atom in which the outermost (valence) shell is complete(8 or 2 electrons are present) Valency- Valency is the measure of the combining capacity of atoms or molecules. 1.lonic Bond /electrovalent bond lonic bonding is the complete transfer of valence electron(s) between atoms. It is a type of chemical bond that generates two oppositely charged ions. In ionic bonds, the metal loses electrons to become a positively charged cation, whereas the non-metal accepts those electrons to become a negatively charged anion. 2.Covalent Bond a The coming together and sharing of electron pairs leads to the formation of a chemical bond known as a covalent bond. Example-Two chlorine atoms come together and share their electrons to form a molecule of chlorine. In this way, each atom will have eight electrons in its valence shell. Single Bonds A single bond is formed when only one pair of the electron is shared between the two participating atoms. It is represented by one dash (-). For Example, HCL molecule has oneHydrogen atom with one valence electron and one Chlorine atom with seven valence electrons. In this case, a single bond is formed between hydrogen and chlorine by sharing one electrons. Double Bonds a double bond is formed when two pairs of electrons are shared between the two participating atoms. It is represented by two dashes (=). Example: Carbon dioxide molecule has one carbon atom with six  valence electrons and two oxygen atom with four valence electrons. Triple Bond A triple bond is formed when three pairs of electrons are shared between the two participating atoms. Triple covalent bonds are represented by three dashes (=) and are the least stable types of covalent bonds. For Example: In the formation of a nitrogen molecule, each nitrogen atoms having five valence electrons provides three electrons to form three electron pairs for sharing. Thus, a triple bond is formed between the two nitrogen atoms.  1.2 Types of Chemical Bond 1.2.1 Ionic or Electrovalent Bond or Kernel Bond Ionic bond is called kernel bond because during formation of cation outer-most orbit is destroyed and theremaining part in called core or kernel. "The number of electrons that atom of an element gains or losses to complete its last orbit (octet) is called electrovalency". Nature of ionic bond is electrostatic force of attraction and it is non directional bond. Ionic bond was introduced by Kossel. ◦ Types of Electrovalency or Ionic Valency 1. Positive electrovalency (Ionic valency) The valency obtained by loss of valency electrons from the metallic atom of an element so as to complete its last orbit (octet) is knows as positive electrovalency. For example, Sodium atom losses 'one valency electron' to complete its octet (last shell). Therefore, positive electrovalency of sodium atom is + 1. 2. Negative electrovalency The valency obtained by the 'gain of valency electrons' by the non- metallic atom of element, so as to complete their last orbit (octet) is knows as negative electrovalency. For Example, Chlorine atom gains 'one valency electron' so as to complete its last orbit. Therefore, the negative electrovalency of chlorine is - 1. In HCl formation chlorine show valency -1 with hydrogen. 1.2.1(B) Formation of Ionic or Electrovalent Compound (i) Ionization potential: The lower the value of ionization potential, the greater will be the ease of formation of cations. (i) Electron affinity: The higher the electron affinity the greater is the ease of formation of anions. 1. Formation of Sodium Chloride (NaCl) A molecule of sodium chloride (NaCl) consists of one atom of sodium and one atom of chlorine. It is formed by electrovalent linkage. Sodium atom (At. No. 11) has an electronic configuration (2, 8, 1) and chlorine atom (At. No. 17) has an electronic configuration (2, 8, 7). In the formation of sodium chloride, sodium atom loses its one valency electron and acquires a unit positive charge (+ charge) and attains a stable configuration of nearest inert gas element Neon (2, 8). The electron lossed by sodium atom is gained by chlorine atom it acquires a unit negative charge (- charge) and attains a stable configuration of nearest inert gas element Argon (2, 8, 8). These two equal and oppositely charged ions (Na* and Clions) which are produced, unite together by the electrostatic forces of attraction to form apparently neutral molecule of sodium chloride (NaCl). Such a combination of atoms by loss and gain of electrons is known as "electrovalent linkage". 2. Formation of Magnesium oxide (MgO) In a molecule of magnesium oxide one atom of magnesium combines with one atom of oxygen. Magnesium atom (At. No. 12) has an electronic configuration (2, 8, 2) and oxygen atom (At. No. 8) has an electronic configuration (2, 6). During combination of two atoms, 2 electrons are transferred from magnesium atom to oxygen atom.Hence by loss of 2 electrons the magnesium atom acquires + 2 charge and it attains the stable configuration of nearest inert gas element neon.The 2 electrons lossed by magnesium atom are gained by oxygen atom which acquires - 2 charge and attains a stable configuration of neon gas (2, 8).These two equal and oppositely charged ions (Mg** and 0**) are bound together with electrostatic forces of attraction and produce apparently neutral molecule of magnesium oxide (MgO).Such a union of atoms take places by loss and gain of electrons is called 'electrovalent linkage' and the compound formed is called electrovalent compound or ionic compound. 3. Formation of Calcium Chloride (Cacl) It is formed by electrovalent linkage. In CaCl, one atom of calcium combines with the two atoms of chlorine. Calcium atom has electronic configuration (2, 8, 8, 2) while chlorine atom (At. No. 17) has electronic configuration (2, 8, 7). In the formation of calcium chloride, calcium atom (2, 8, 8, 2) losses two valency electrons and acquires + 2 charge and attains a stable configuration of nearest inert gas element Argon and it forms Ca** ions. These two electrons are transferred from calcium atom to two chlorine atoms. Each chlorine atom gains one electron and acquires unit negative charge (-1) and it attains stable configuration of Argon to form 2CT. These equal and oppositely charged ion (Ca+2 and 2C| ) combine themselves by electrostatic force of attraction. 1.2.2 Covalent Bond Valency obtained by mutual sharing of electrons between either similar or dissimilar atoms of an element so as to complete their last shell (octet) is known as co-valency and the bond between combining atoms is covalent bond. Covalent bond is, Formed by mutual sharing of electron pairs between the combining atoms of the same or different elements. 1.2.2(A) Types of Covalent Bonds 1. Non-polar covalent bonds: When electrons are shared equally H2 or Cl. 2. Polar covalent bonds: When electrons are shared unequally, i.e. H20.  1.2.2(B) Formation of Covalent Compound 1. Formation of water molecule (H) A water molecule is formed by combining two atoms of hydrogen with one atom of oxygen. Each hydrogen atom (1) is in short of 1 electron to complete its duplet and oxygen atom is in short of 2 electrons to complete its octet. During combination, hydrogen atoms complete their duplet by contributing one electron each with oxygen atom and oxygen atom completes its octet by contributing two electrons with two hydrogen atoms. Therefore, in water molecule, two separate single covalent bonds are formed between hydrogen and oxygen atoms. 3. Formation of nitrogen molecule (N2) Nitrogen molecule is formed by combining two atoms of nitrogen hence it is diatomic. Each nitrogen atom (2, 5) is in need of 3 electrons to complete its last shell (octet). Therefore, each nitrogen atom contributes 3 electrons for sharing. A molecule of nitrogen is formed by sharing three pairs of electron between two nitrogen atoms. Thus, three shared pairs constitute a triple covalent bond. 4. Formation of chlorine molecule (Cl) In the formation of chlorine molecule one atom of chlorine combines with other atom of chlorine. Each chlorine atom (2, 8, 7) contains 7 valency electrons. So it is in short of one electron to complete theoctet. Therefore, each chlorine atom contributes one electron with other chlorine to constitute 'single covalent bond' between two chlorine atoms. 1.2.3 Coordinate Bond A coordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom. It is formed by two atoms sharing a pair of electrons. The atoms are held together because the electron pair is attracted by both of the nuclei. Formation Ammonium ion (NH4+) : An example of coordinate covalent bond is provided by the ammonium ion, NH*. The bonds in the ammonia molecule itself are of the covalent type. One unshared pair of electrons of the nitrogen atom is available for use in bond formation, as indicated by the readiness with which ammonia will combine with a hydrogen ion to form the ammonium ion. 1.2.4 Metallic bond and its characteristics: The bond formed by positive metal ion (i.e. kernel) and negative delocalized valence electrons held together by strong attractive force between them. In metallic bond kernel occupy stationary position but valency electrons are free to move in attice. Hence electrons in metal behave in same manner like the molecules of gas. These delocalized valence electrons are known by different names like cloud of electrons, sea of electrons, pool of electrons and mobile electrons. Explanation of few metallic properties : 1.. High electrical conductivity :It is due to the presence of the mobile valency electrons. They move readily in an electric field and thus conduct electricity throughout the metal from one end to the other. 2. High thermal conductivity :It is due to the presence of mobile electrons. If one part of a metal is heated, electrons of that part acquire kinetic energy. Being free, these electrons move rapidly throughout the crystal and conduct heat to other part of the metal. 3. Bright metallic lusture :As a beam of light falls on the surface of a metal, electrons start to and fro oscillations. Since, a moving charge always emits electromagnetic energy hence oscillating electrons emit electromagnetic energy in the form of light and metallic lusture appears. 4. High tensile strength :Strong electrostatic attraction between the positively charged metal ions and the 'sea' of electrons provide good tensile strength to metal and they can resist stretching without breaking. 5. Softness, malleability and ductility : The force of attraction between the metal ions and the valency electrons is uniform in all directions. There are no localized bonds. Also the bonds holding the crystal lattice in metals are not rigid as in covalent solids such as ice. Hence metal ions can be moved very easily from one lattice site to another. 1.2.5 Hydrogen bond : An electrostatic attractive force between covalent bonded H- atom of one molecule and electronegative atom (such as Fluorine, Oxygen, Nitrogen) of the other molecule. a) Intramolecular hydrogen bonding : If positive and negative end develop within the same molecule, electrostatic forces of attraction results intra molecular hydrogen bonding. In this bond hydrogen and electronegative element both present in the same molecule.  For example, in ortho-nitrophenol and ortho-hydroxy benzaldehyde. : b) Intermolecular hydrogen bonding In intermolecular hydrogen bonding hydrogen and other electronegative atoms are present in different molecules of the same substance. For example in water, HF, ammonia, alcohols and acids. On account of large distance between the two groups in p-nitrophenol and p-hydroxy benzaldehyde, they do not show any intra molecular hydrogen bonding. 1.2.5(B) Effects of Hydrogen Bonding on Properties 1. Boiling point: Substances having intermolecular Hydrogen bonding show higher melting and boiling points; (because of large association of molecules) in comparison to the substances do not having Hydrogen bonding. For example, B.P of CH, CH, OH is 78.5° C while that of CH, OCH, is - 23° C. 2. Solubility in water :Substances making hydrogen bonds with water show higher solubility than those who are not making hydrogen bonding.For instance lower alcohols are highly soluble in water but alkanes are insoluble, because alkanes cannot make H-bonding with water. 3. Physical state of matter : Because of hydrogen bonding in water molecule, it is found in liquid state but H,S is gas. A molecule of HaS remains free in absence of any hydrogen bonding. : 1.3 Molecular Arrangement in Solid, Liquid and Gases The molecular arrangement in solid-liquid and gases if different with regards to packing of the atoms/molecules, shape, volume etc. In a solid, the atoms are closely packed and held in place due to strong chemical forces. It has definite shape* & volume. In a liquid, the atoms are held together by weak chemical forces, and can have movement around themselves. It has definite volume, but acquire shape of container. In a gas, the atoms are not held together. The atoms can move around freely. It has no definite shape or volume. It can expand and be compressed too. Comparison between Gas-Liquid-Solid 1.4. Structure of Solids: Property Solid Liquid Gas Space Atoms have large. Atoms have Atoms are close space. moderate space. packed. Shape Molecule does Molecules have Possess definite not have definite no shape, but shape. shape. acquire shape of container. Forces No Chemical Weak chemical Strong chemical forces. forces. forces. Volume Molecules Molecules Molecules expand to fill remain in same remain in same container and volume in volume and also get container. shape. compressed under pressure. Characteristics of Crystalline solid :  The ionic crystals are formed by joining the appositively charged ions. These ions are joined together by strong electrostatic forces.These are hard substances. These have high melting and boiling point.These are generally freely soluble in water. These are good conductors of electricity in molten state and in the form of aqueous solution. These are brittle and non-malleable and non-ductile. 2. Amorphous solid : Amorphous solids are a type of solid that lacks definite shape, pattern and long-range order, and are being held by covalent bonds. Characteristics of Amorphous solids : Do not have a well-defined melting point. If an amorphous solid is maintained at a temperature just below its melting point for long periods of time, the component molecules, atoms, or ions can gradually rearrange into a more highly ordered crystalline form. Crystals have sharp, well-defined melting points; Its atomic structure is disordered like that of a liquid but it is rigid and holds its shape like a solid. For example, glass, rubber, gels and most plastics. An amorphous solid melt gradually over a range of temperatures, because the bonds do not break all at once. 1.4.1 Properties of Metallic Solids: Metallic solids Show many unusual properties: Exhibit lustre, a shiny surface that reflects light. Very High melting and boiling points. Very Good Conductors of heat and electricity. Malleable (can be made into different shapes without breaking) Ductile (can be moulded into wiring) Metallic lustre (shiny) Sometimes magnetic. Types of Ionic Crystals The basic simplest arrangement of spheres, which can represent / reproduce the entire crystal structure of the molecule when repeated is called a unit cell. E.g. Unit cell for simple cubic arrangement. Properties of metallic Solids-Unit Cell : Metallic solid are type of crystalline solid, so their structure is arranged in a crystal lattice. In metallic solids, the crystal lattice consists of positive ions and free- flowing electrons that are also known as a "sea" of delocalized electrons. Crystal lattice : "Crystal lattice is a highly ordered three-dimensional structure, formed by its constituent atom or molecules or ions." Unit Cell: "The unit cell is the smallest building unit in space of crystal, which when repeated over and over again in three-dimensions, results in a space lattice of the crystalline substance." The unit cell is the essential feature of the crystal structure The unit cell in a three-dimensional lattice is characterised by the lengths a, b and c and the angle α, β and γ. These are collectively known as the unit cell parameters. 1. Simple Cubic Crystal (SC) :In the unit cell of simple cubic structure, atoms are present only at the corners of the unit cell as show below. (a) Length and angle: In simple cubic structure, lengths of edges are same i. e. a = b = c and angle between is α = β = γ= 900 (b) Number of atoms in unit cell of simple cubic structure : Number of atoms in unit cell of SC = Total no. of atoms present x its contribution in unit cell Number of atoms in unit cell of SC = 1/8x8 = 1 atom per unit cell. (c) Coordination number of Simple Cubic Structure : The total number of nearest neighbour atoms of a particular atom in a crystal lattice is called coordination number. there are two nearest atoms, one along +X axis and other along -X or Similarly, there are two nearest neighbours along +Y axis and other along +Z axis. Thus in all, there are 2+2+2 =6 Hence, the coordination number is 6. (d) Example of Simple Cubic Structure : Very few examples of simple cubic lattices are known alpha - polonium is one of the few known simple cubic lattices. 2. Body Centerd Cubic Crystal (BCC): In the unit cell of Body Centred Cubic structure, atoms are present at the corners of the unit cell and also in the canter of body. (a) Length and angle: In Body Centred Cubic structure, lengths of edges are same i. e. a = b = c and angle between is a = B = y = 90° α = β = γ = 900 (b) Number of atoms in Unit cell of Body Centred Cubic Structure (BCC) : In the BCC structure, eight atoms are present at each corner of cube and one atom at the Centre of body. Hence, Number of atoms in unit cell of BCC = Total no. of atoms present at corner x its contribution in unit cell + one atom at the Centre of body Number of atoms in unit cell of BCC = 1/8x8+1 = 2 atoms per unit cell (C) Coordination Number of Body Centred Cubic Structure: In this, an atom in the Centre (or body) of the cell has all the eight corner atoms as its close neighbours.Hence, the coordination number is 8. (d) Example of Body Centred Cubic Structure : NaCl, Cr, Fe, Mo, W crystallize in BCC lattice. 3. Face Centred Cubic Crystal (FCC) : In the unit cell of Face Centred Cubic structure, atoms are present at the corners of the unit cell and also in the center of faces as shown below. (a) Length and angle: In Face Centred cubic structure, lengths of edges are same i. e. a = b = c and angle between is α = β = γ = 900 (b) Number of atoms in Unit cell of Face Centred Cubic Structure (FCC) : In the FCC structure, eight atoms are present at corners of cube and six atoms at the center of faces. Hence, Number of atoms in unit cell of FCC = Total no. of atoms present at the corners x its contribution in unit cell + No. of atoms present at the center of faces x its contribution in unit cell Number of atoms in unit cell of FCC = 8 + 1/8+ 6x 1/2 = 4 atoms per unit cell (c) Coordination number of Face Centred Cubic Structure : In this, each atom is in direct contain trom layer are neighbours -6 of which lies in one layer in the same plane, & from layer above and s from layer below. Hence, the coordination number is 12. (d) Example of Face Centred Cubic Structure : Cu, Au, Al Diamond crystallise in FCC lattice. 4. Hexagonal Close Packed Crystal (HCP) : In the unit cell of Hexagonal Close Packed structure, 6 atoms are present at the 6 corners of the each hexagons (total = 12), 1 at the each face of hexagons (total=02) and 3 at the center of body. Hexagonal Close packed (HCP) refers to layers of packed spheres such that spheres overlay each other in alternating layers. (a) Length and angle: In Hexagonal Close Packed structure, lengths is a = b # c and angle is α = β = γ = 900, γ = 120° (b) Number of atoms in Unit cell of Hexagonal Close Packed structure (HCP) : In the HCP structure, 12 atoms are present at corners of 2 hexagons and 2 atoms at the faces of 2 hexagons and 3 atoms at canter of body Hence, Number of atoms in unit cell of HCP = Total no. of atoms present at the corners of two hexagons x its contribution in unit cell + No, of atoms present at two faces of hexagon x its contribution in unit cell. + 3 atoms of center Number of atoms in unit cell of HCP = 12 + 1/6 + 2x1/2 + 3 = 6 atoms per unit cell (c) Coordination Number of Hexagonal Close Packed Structure : The Coordination Number of HCP is 12. (d) Example of Hexagonal Close Packed Structure : ZnO, SiO,, HgS, CdS, Mg and Zn.