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BONDING in SOLIDS Engineering Physics Vignan BONDING IN SOLIDS Engineering Physics 1.1 Types of Bonding:- Different charge distributions in the atoms give rise to different types of bonding. They classified into five classes 1. Ionic bonding 2. Covalent bonding 3. Metallic bonding 4. Hydrogen bonding 5. Vander-Waals bonding Based on the bond strength atomic bonding can be grouped into “primary” and “secondary” bonding. Primary bonding:- Primary bonding have energies in the range of 1-10 eV. Ionic, covalent and metallic bonds are examples of primary bonding among these ionic and covalent bondings are generally stronger than the metallic bonding. Secondary bonding:- Secondary bonding have energies in the range of 0.01- 0.5 eV/ bond Hydrogen bonding and Vander Waals bonding are examples of secondary bonding. Generally, Vander Waals bonding is the weakest of all. 1 Vignan BONDING IN SOLIDS Engineering Physics 1.2 Ionic bonding (or) Hetro-Polar bond:- 1. Ionic bond is also called as “Hetro-Polar bond”. 2. Crystals in which ionic bond is present are known as “ionic crystals”. Ionic crystals are mostly insulating in character. Ex:-Nacl, NaBr, KBr, Mgo etc... 3. An Ionic bonding can only be formed between two different atoms, one electropositive and other electronegative. 4. Electropositive elements readily give up electrons and are usually group Ι (or) ΙΙ Elements e.g. Na, k, Ba. 5. Whereas electronegative elements readily take up electrons and are usually Group VΙ (or) VΙΙ elements e.g. Cl, Br and O. An example of the ionic bonding is NaCl. When neutral atom of a Na and Cl are brought close together it is easy for the valence electron of the sodium atom to be transferred to the chlorine atom so that both of them acquire a stable inert gas electronic configuration. 2 Vignan BONDING IN SOLIDS Engineering Physics There exits an electrostatic attraction between positively charged sodium cation and negatively charged chlorine anion. A negative charge attracts all positive charges in the neighbourhood, and vice versa. Consequently, in the crystalline solid Na+ ions will be surrounded by Cl- and Cl- ions by the Na+ ions. The attraction between the neighbouring unlike charges exceeds the repulsion due to like charges. Properties of ionic solids:- 1. Ionic bond is fairly strong 2. They have strong binding energy 3. They have high melting and boiling points 4. Soluble in water 5. Transparent to visible light 6. Conductivity is less 7. The bonding in ionic crystal is non-directional. 1.3 Covalent Bonding (or) Homo-Polar bonds:- 1. Covalent bond is also called “Homo-Polar bonds”. 2. In this bonding the valence electrons are not transferred from one atom to the other atom as ionic crystals. Because the energy required to removing all the valence electrons is too large. 3. The covalent bonds are formed by sharing of valence electrons in the incomplete outer shell of the atoms. 4. Such sharing results in a lowering of the potential energy of the system. The element of group ΙΙΙ, ΙV and V of the periodic table shows covalent bonding. 3 Vignan BONDING IN SOLIDS Engineering Physics 5. In silicon, germanium and diamond materials the bonds are covalent in nature. Organic compounds such as CH4 is also having covalent bonding. 6. Polar covalent bonds: A polar bond is a covalent bond in which there is a separation of charge between one end and the other - in other words in which one end is slightly positive and the other slightly negative. Example the hydrogen-chlorine bond in HCl or the hydrogen-oxygen bonds in water are typical. 7. Coordinate covalent bonds: A covalent bond in which both shared electrons are donated by the same atom; 8. The simplest case of covalent bonding is hydrogen molecule (H2), in which two hydrogen atom atoms contribute their 1s1 electron to form covalent bond. In the molecular orbital of H2 the two electrons are equally shared between the nuclei and cannot be 4 Vignan BONDING IN SOLIDS Engineering Physics specifically identified with either nucleus. A covalent bond is also known as “electron-pair bond” 9. Thus in this example, two atoms are involved in the bond formation process and they share a single pair of electrons. Hence the resulting bond is known as a single covalent bond. 10. When the two coordinating atoms share two (or) three pair of electrons it results in a double (or) a triple bond. In oxygen double covalent is formed while in nitrogen triple covalent bond is formed. A covalent bond may also be formed when two (or) more atom of different non-metals share one (or) more pairs of valence electron. Ex:- H2o ,Co2 , Hcl and NH3. Properties of covalent bonding:- 1. Covalent bond crystals are usually hard and brittle 2. Bonding energy is high so that their melting and boiling points are high but low compared to ionic crystals 5 Vignan BONDING IN SOLIDS Engineering Physics 3. Covalent bonds are highly directional in character 4. These materials are soluble in non-polar solvents like benzene 5. Pure covalent solids are good insulators of electricity at low temperature. Conductivity increases with increase in temperature. 6. Covalent substances are insoluble in water. 1.4 Metallic Bonding:- 1. The metallic bonding is an electrostatic attraction between positive metal ions which are fixed in a metal to the electron cloud which are moving freely in between the metal ions. 2. The bonding in a metal must be considered in terms of all the atoms of the solid taken together. No valence electron is associated with a specific atom. 3. The valence electrons from all the atoms belonging to the crystal are free to move throughout the crystal. 6 Vignan BONDING IN SOLIDS Engineering Physics 4. Materials bound in this manner are good conductors of electricity and heat. The metallic bonding can be considered as a limiting case of the ionic bonding in which the negative ions are just electrons. For example sodium chloride contains equal number of Na+ and Cl- , while metallic sodium contains equal number of Na+ and e-. 5. The crucial difference is that the mass of an electron is very small as compared to the mass of Cl- ions. As a result of this, its zero point motion is large so that it is not localized on a lattice. They are held together by the resulting electrostatic interaction between the positively charged metal ions and the cloud of negative electrons. 6. The unsaturated nature of metallic bonding accounts for the alloying properties of metals. If the atoms have more loosely held valence electrons, the more metallic is the bonding. 7. For example Sodium, Copper, Silver has high electrical and thermal conductivities because their valence electrons are very mobile. They are opaque because the free electrons absorb energy from light and they have high reflectivity, because these free electrons re-emit this energy as they fall back to lower energy. Properties of metallic bond materials:- 1. They have high electrical and thermal conductivities 2. The metals are opaque to all electromagnetic radiations 3. The metals have high optical reflection and absorption co-efficient 4. The metallic bond is comparatively weaker than the ionic and covalent bond 7 Vignan BONDING IN SOLIDS Engineering Physics 5. Metallic solids have crystalline structure 6. Metallic crystals posses a high degree of crystal symmetry due to symmetrical arrangement of positive ions 1.5 Hydrogen Bonding:- 1. A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, like nitrogen, oxygen or fluorine. The hydrogen must be covalently bonded to another electronegative atom to create the bond. 2. There exits two types of hydrogen bonds 1). Intermolecular hydrogen bond can occur between molecules Ex: HF 2). Intramolecular hydrogen bond within different parts of a same molecule. Ex: Nitro phenol Hydrogen bonds are common in covalently bonded molecules which contain hydrogen, such water Ho2 and hydrogen fluoride molecule (HF). Since the 8 Vignan BONDING IN SOLIDS Engineering Physics bonds are primarily covalent; the electrons are shared between the hydrogen and fluorine atom. However, the electrons tend to spend more time around the fluorine atom. This leads to a small positive charge around the hydrogen atoms, and a negative charge around the fluorine atom. When other molecules with this type of charge are came nearby, the negative charged end of one molecule will be Weakley attracted to the positively charged end of the other molecule. The hydrogen bond (5 to 30 kJ/mole) is stronger than a Vander Waals interaction, but weaker than covalent or ionic bonds. This type of bond occurs in both inorganic molecules such as water and organic molecules such as DNA. Properties of hydrogen bonding:- The hydrogen bonds are directional The bonding is relatively strong as compared to other dipole-diople interactions Hydrogen bonded solids have low melting points Since no valence electrons are available in such solids they are good insulators of electricity They are soluble in both polar and non-polar solvents They are transparent to the light 9 Vignan BONDING IN SOLIDS Engineering Physics 1.6 Vander Waals bonding:- 1. The weak attractive forces that lead to bonding of atoms and molecules can be formed as the result of dipole interaction. This type of bonding is called the Vander Walls bonding. 2. A dipole occurs if a certain separation of center of gravity of positive charge and negative charge exists in a body. 3. There are three types of Vander waals forces 1. Dipole- dipole forces 2. Dipole- induced dipole forces 3. Dispersion forces Dipole-dipole forces: It is an interaction between two molecules having permanent dipole moment. For example polar molecules such as HCl have permanent dipole moment and attract other polar molecule.
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