Tiêu đề 4.CHEMICAL BONDING AND MOLECULAR STRUCTURE.pdf ✅

ChemContent | Chemical Bonding and Molecular Structure ChemContent 1 Introduction: Structure and Bonding is the heart of chemistry. Chemical bond is very important to explain the properties and structure of compound. The important aspect of each type of force is its relative strength, how rapidly it decreases with increasing distance and whether it is directional in nature or not. Chemical Bond: It is the force of attraction between two atoms which hold them together in a compound or molecule. Nature loves stability and bond formation is associated with stability. Every element has a tendency to occupy inert electronic configuration which is considered as very stable. Noble gas electronic configuration can be achieved by 1. Transference of electrons 2. Mutual sharing of electrons 3. Donation of lone pair of electrons Types of Bond In order to explain the formation of a chemical bond in terms of electrons, Lewis postulated that atoms achieve stable octet when they are linked by a chemical bond. On the basis of this chemical bonds are following type: 1. Ionic bond 2. Covalent bond 3. Co-ordinate bond 4. Metallic bond 5. Hydrogen bond 6. van der Waal’s bond Lewis Dot Structures Valence Electrons: In the formation of a molecule only the outer shell electrons take part in chemical bond combination and they are known as valence electrons. In Lewis symbols, an element is shown with symbol and valence electrons. Octet Rule It is proposed by Kossel and Lewis and according to this, "Every atom has a tendency to attain Noble gas electronic configuration or to have 8 valence electrons". This is known as law of octet rule or if it has two valence electrons then this is known as law of duplet. According to Lewis, only those compounds will be stable which follow octet rule.
ChemContent | Chemical Bonding and Molecular Structure ChemContent 2 Formal Charge Formal charge on an atom is the difference between the number of valence electrons is an isolated atom and the number of electrons assigned to that atom in a Lewis structure. It is expressed as: Ionic Bond An ionic bond is formed by complete transference of one or more electrons from the valence shell of one atom to the valence shell of another atom. In this way both the atoms acquire stable electronic configurations of noble gases. The atom which loses electron becomes a positive ion and the atom which gains electron becomes negative ion. Note: Electrovalency is the number of electrons lost or gained during the formation of an ionic bond or electrovalent bond. Characteristics of Ionic Compounds: 1. They are hard, brittle and crystalline. 2. They have high melting and boiling points. 3. They are polar in nature.
ChemContent | Chemical Bonding and Molecular Structure ChemContent 3 4. The linkage between oppositely charged ions is non rigid and non directional. 5. They are soluble in polar solvents such as water and insoluble in non polar solvents such as CCl4, Benzene, ether etc. 6. They are good conductors of electricity in fused state and in solution due to mobility of the ions. They are bad conductors of electricity in solid state because ions are unable to move. Covalent Bond A force which binds atoms of same or different elements by mutual sharing of electrons is called a covalent bond. If the combining atoms are same the covalent molecule is known as homoatomic. If they are different, they are known as heteroatomic molecule. Valence Bond Theory (VBT) Valence bond theory was introduced by Heitler and London (1927) and developed by Pauling and others. It is based on the concept of atomic orbitals and the electronic configuration of the atoms. Let two hydrogen atoms A and B having their nuclei NA and NB and electrons present in them are eA and eB. As these two atoms come closer new attractive and repulsive forces begin to operate. 1. The nucleus of one atom is attracted towards its own electron and the electron of the other and vice versa. 2. Repulsive forces arise between the electrons of two atoms and nuclei of two atoms. Attractive forces tend to bring the two atoms closer whereas repulsive forces tend to push them apart.
ChemContent | Chemical Bonding and Molecular Structure ChemContent 4 Orbital overlap concept If we refer to the minimum energy state in the formation of hydrogen molecule the two H- atoms are enough near so as to allow their atomic orbitals to undergo partial interpenetration. This partial interpenetration of atomic orbitals is called overlapping of atomic orbitals. The overlap between the atomic orbitals can be positive, negative or zero depending upon the characteristics of the orbitals participating to overlap. Types of overlapping The covalent bonds can be classified into two different categories depending upon the type of overlapping. These are: Sigma (σ) bond: This type of covalent bond is formed by the axial overlapping of half-filled atomic orbitals. The atomic orbitals overlap along the internuclear axis and involve end to end or head on overlap. There can be three type of axial overlap among s and p-orbitals as discussed below: i. s-s overlap: In this case, there is overlap of two half-filled s-orbitals along the internuclear axis as shown below. ii. s-p overlapping: It involves the overlapping of half filled s-orbitals of one atom with the half filled p-orbitals of the other atom. The bond thus formed is called s-p sigma bond. iii. p-p overlapping: It involves the co-axial overlapping between half filled p-orbitals of one atom with half filled p-orbitals of the other atom. The bond as formed is called p-p sigma bond.