Chemical Bonding PDF

Chemical compound formation and chemical bonding  Chemical bonding Ionic bonding Covalent bonding - general characteristics Sigma and pi bonds Bond length Formal charge Non-covalent interactions Hydrogen bond Metallic bond. Chemical bonding Chemical bonding involves the...

Chemical compound formation and chemical bonding  Chemical bonding Ionic bonding Covalent bonding - general characteristics Sigma and pi bonds Bond length Formal charge Non-covalent interactions Hydrogen bond Metallic bond. Chemical bonding Chemical bonding involves the union of atoms through the redistribution of electrons in their outermost electronic shell either through the transfer of electrons from one atom to another or through sharing of electrons among themselves so that all the atoms acquire the noble gas stable electronic configuration of minimum energy. Redistribution of electrons in atoms takes place through Ionic bond Covalent bond Co-ordination bond Metallic bond Hydrogen bond Chemical bonding Ionic bond Ionic bond results from the complete transfer of electrons from the outermost shell of one atom to the outermost shell of another atom. In this way both the atoms attain inert gas electronic configuration. The atom which looses electron attains a positive charge (cation) The atom which gains electron attains a negative charge (anion) Electrostatic attraction between the positively and negatively charged ions results in the formation of a bond. The number of electrons transferred from one atom of an element during ionic bond formation so that it attains stable gas configuration is called electrovalency of that element Chemical bonding Ionic bond Factors influencing the ionic bond 1. Ionisation energy Lesser the ionization energy, the greater the ease with which cations are formed. Alkali and alkaline earth metals have low ionisation energy. Hence they form cations easily. 2. Electron affinity Greater the energy released during the addition of an electron( high electron affinity) greater is the ease with which anions are formed. 3. Strong ionic bonds are formed by cations with low ionization energy and anioins with high electron affinities. Thus low ionization energy of an atom(metal)and high electron affinity of another atom (non-metal) favors the formation of ionic bond Chemical bonding Covalent Bond Atoms combine together by sharing the electrons in the outermost shell so that the combining atom attains noble gas electronic configurations. This type of bond is called a covalent bond. The number of electrons that an atom contributes for sharing in a covalent bond is called covalency of that atom Chemical bonding Covalent Bond Sharing of electrons- Orbital overlap An orbital can accommodate maximum 2 electrons with opposite spin. As long as such electron pairs are present in an orbital they are not available for chemical bond formation. If a single electron is present in an orbital of the outer shell, it will have a tendency to combine with a similar electron present in the outermost shell of another atom. According to orbital overlap concept formation of covalent bond between two atom results from the coupling of electrons with opposite spin belonging to the outermost shells of two atoms. This coupling is accomplished by the overlap of orbital containing these electrons. Chemical bonding Covalent Bond Sharing of electrons- Orbital overlap –different type of orbital overlap Sigma bond (σ Bond) This type of bond is formed by the overlapping of orbitals along the same axis. This is also called end on overlap The extent of orbital overlapping is very high in σ Bond and bond formed is very strong. Chemical bonding Covalent Bond Chemical bonding Covalent Bond Pi bond (π Bond) This type of bond is formed by the sideways overlapping (lateral overlapping) of atomic orbitals. The extent of orbital overlap is low in the π Bond and bond formed is weak. Π bonds are not formed by s orbitals Chemical bonding Co-ordinate covalent Bond- Dative bond This is a type of covalent bond Bond is formed by the sharing of electrons between two atoms where both the electrons are contributed by one atom. A coordinate bond is established between two atoms when one of which has a complete octet and has an unshared pair of electrons and the other atom has an incomplete octet. The atom contributing the electron pair is called the donor and the atom accepting the electron pair is called the acceptor atom. Donated pair of electrons is called lone pair of electrons. It is directional and denoted by an arrow Chemical bonding Co-ordinate covalent Bond- Dative bond Chemical bonding Metallic bond Characteristics of metals High melting and boiling point High electrical and thermal conductivity High tensile strength Malleability and ductility These properties are due to special types of bonding in metals. Bonding in metal is explained by the free electron model or electron sea model Chemical bonding Metallic bond The free electron model or The electron sea model In metals, electrons are very loosely bound. Valence electrons are essentially free and may be shared by a number of atoms. Metallic bond - Assembly of positively charged metal ions periodically (kernels) in a sea of mobile electrons. Thus each electron belongs to a number of positive ions and positive ions belong to so many electrons. The electrostatic force of attraction between the positively charged kernel to a number of electron within its influence called metallic bond Chemical bonding Metallic bond –explanation of metallic properties ✔ High MP and BP- it is due to the strong force of attraction between positively charged metal ions and a large no of electrons ✔ High thermal conductivity: Due to the presence of loosely held highly mobile valence electrons ✔ High electrical conductivity: Due to the presence of loosely held highly mobile valence electrons. They move readily in an electric field conducting electricity ✔ Metallic luster: Presence of a large no of loosely held electrons. When light falls on metals it causes the oscillation of free electrons. This moving charge always emits electromagnetic waves which gives luster to the metals. ✔ Malleability and ductility: Metallic bonds holding positive ions and electrons are non-directional. The force of attraction holding metals and electrons are uniform in all directions. And the bond-forming metal in the metallic lattice is not rigid like covalent bonds. ✔ High tensile strength- They can resist stretching without breaking. This is due to the strong electrostatic attraction between positively charged metals ions and free electrons surrounding them. Chemical bonding Covalent Bond- Polarity of covalent bonds Covalent bond formed between two identical atoms is said to be a non-polar covalent bond but if it is formed between two dissimilar atoms it is called a polar covalent bond. When a covalent bond is formed between two dissimilar atoms electron pair is attracted towards atom with more electronegativity. This atom attains a partial negative charge and the other atom attains a partial positive charge. Molecules formed by such atoms are called polar molecules. A polar covalent bond has a partial ionic character. Two opposite charges at the ends are called dipoles. Chemical bonding Covalent Bond- Polarity of covalent bonds Chemical bonding Covalent Bond- Polarity of covalent bonds Chemical bonding Covalent Bond- Polarity of covalent bonds Chemical bonding Hydrogen bond The strong electronegative atoms like F, O and N have very high electronegativity compared to hydrogen atoms. So when these atom form bonds with hydrogen, hydrogen acquires a partial positive charge and this atom acquires a partial negative charge. This is called a dipole because one end carries a positive charge and the other end negative charge. These dipoles will attract other molecular dipoles present in the system. One molecule's positive end is attracted to another's negative end. This is a strong electrostatic force of attraction. This bond is called hydrogen bond. Hydrogen bond is the intermolecular electrostatic force of attraction holding together molecules of hydrogen containing dipoles. It is denoted by dotted lines. Chemical bonding Hydrogen bond Chemical bonding Hydrogen bond Hydrogen bonds are usually formed in compounds having hydrogen and NOF atoms NOF – Nitrogen, oxygen and fluorine Due to the high electronegativity difference between hydrogen and NOF atoms (H-2.1, F-4.0, O-3.6, N-3.4) There is no H-bond exists in methane , ethane etc C- electronegativity -2.5 Why water is liquid and methane and ethane are gases ? Chemical bonding Hydrogen bond Hydrogen bond is not a chemical bond. It is the force of attraction between molecules. Covalent bonds are 20 times stronger than hydrogen bonding Why H2O is liquid and H2S is gas TYPES OF HYDROGEN BONDING Intermolecular hydrogen bonding -between separate molecules Intramolecular hydrogen bonding-within parts of the same molecule TYPES OF HYDROGEN BONDING 1. Intermolecular hydrogen bonding- When the hydrogen bonds are formed between two or more molecules of same or different molecules are called intermolecular hydrogen bonding Eg: hydrogen fluoride, ammonia, water, alcohols, alcohols and ammonia in water TYPES OF HYDROGEN BONDING 2. Intramolecular hydrogen bonding- Intramolecular hydrogen bonds are those which occur within one single molecule. Eg: hydrogen fluoride, ammonia, water, alcohols, alcohols and ammonia in water Salicylic acid Chemical bonding- some important characteristics of covalent bond Bond length Bond length is defined as the average distance between nuclei of two atoms bonded to each other. When two atoms approach each other at a particular point electrostatic attraction between 2 atoms will be maximum and repulsion will be minimum. At this point, energy of the system will be minimum and atoms get bonded together. Bond length is usually denoted in nanometers, picometer or angstrom unit. Bond lengths are experimentally elucidated using microwave spectroscopy and X-ray crystallography. Chemical bonding- some important characteristics of covalent bond Bond angle Bond angle refers to the angle between the two bonds i.e. the angle between two orbitals that contains a pair of bonding electrons around the central atom in a complex molecule or an ion. This angle is usually measured in degrees, further calculated using the spectroscopic. Shape of the molecule controls the bond angle. Chemical bonding- some important characteristics of covalent bond Bond strength and Bond energy Bond energy or bond strength is the energy required to break one mole of bonds of that type in gaseous state Strength of the bond indicates the stability of the bond. As the bond order increases bond strength also increases. Bond strength of CΞ C > C=C > C-C Bond strength is expressed in kJ/mole or kcalories/mol As the bond length decreases bond strength increases. Bond energy is inversely proportional to bond strength. Chemical bonding- some important characteristics of covalent bond Chemical bonding- some important characteristics of covalent bond Chemical bonding- some important characteristics of covalent bond