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INTERATOMIC BONDING

BONDING IN MATERIALS DICTATES PROPERTIES

drrabiulhussain
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INTERATOMIC BONDING

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  1. DR.RABIUL HUSSAIN/966882081 ATOMIC STRUCTRE AND INTERATOMIC BONDING: lecture-2 • DR. RABIUL HUSSAIN • SCHOOL OF MATERIAL SCIENCE AND ENGINEERING • JIMMA INSTITUTE OF TECHNOLOGY • JIMMA UNIVERSITY, ETHIOPIA

  2. DR.RABIUL HUSSAIN/966882081 Electronic configurations:

  3. DR.RABIUL HUSSAIN/966882081 Electronic configurations:

  4. DR.RABIUL HUSSAIN/966882081 Survey of electronic configurations of elements

  5. Elements in any given group in the periodic table have the same type of electron arrangements in their outermost shells. • The outer shell electrons those that lie outside the orbitals occupied in the next lowest noble gas element are called its valence electrons, whereas the electrons in the inner shells are called the core electrons. • Valence electrons are most available for bonding and tend to control the chemical properties. • The valence electrons participate in the bonding between atoms to form atomic and molecular aggregates. DR.RABIUL HUSSAIN/966882081 Electronic configurations:

  6. All the elements are classified according to their electronic configuration in the periodic table. • The elements are arranged with increasing atomic number in seven horizontal rows called periods. The properties change gradually and systematically, on moving horizontally across a period. • Same column, or group, have similar chemical and physical properties due to similar valence electron configurations. • The groups are numbered at the top by the numbers 1,2…18 and or by Roman numerals I, II, III …VIII with A and B. Sometimes group 18 is also designated as 0(zero). • A group elements- Representative or main group elements • B group elements- Transition elements DR.RABIUL HUSSAIN/966882081 The periodic table

  7. DR.RABIUL HUSSAIN/966882081 The periodic table of elements

  8. The elements positioned in group 0 or 18, are called inert gases, which have filled electron shells and stable electronic configurations. • Group IA (1) and Group IIA (2)are ALKALI and ALKALINE EARTH METALS respectively having one and two electrons in excess of stable structures. • The elements in the three long periods, Groups IIIB through IIB are TRANSITION METALS, which have partially filled d-electron states and in some cases one or two electrons in the next higher shell. • Group IIIA, IVA and VA elements have characteristics between metal and nonmetals because of their valence electron configurations. They are called metalloids. • Group VIIA (halogens) and VIA elements= one and two electrons deficient respectively from having stable configurations. DR.RABIUL HUSSAIN/966882081 The periodic table

  9. Electronegativity: It is the ability of an atom to attract electrons toward itself when in a molecule. • It ranges from 0.7 to 4.0. Fluorine is the most electronegative atom. • Electronegativity tells us whether a given bond will be nonpolar covalent, polar covalent or ionic. • Electropositive Elements: Elements capable of giving up their valence electrons to become positively charged ions (located on the left of periodic table). • Electronegative Elements: Elements which readily accept electrons to form negatively charged ions or sometimes share their electrons with other atoms. • As a general rule, electronegativity increases in moving from left to right and from bottom to top in the periodic table. DR.RABIUL HUSSAIN/966882081 The periodic table

  10. DR.RABIUL HUSSAIN/966882081 The electronegativity values for elements

  11. Two questions to be answered: • Why the atoms clustered together? • How are they arranged? DR.RABIUL HUSSAIN/966882081 Atomic bonding in solids

  12. DR.RABIUL HUSSAIN/966882081 Atomic bonding in solids

  13. Things are made of atoms—little particles that move around, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence ... there is an enormous amount of information about the world. — Richard P. Feynman DR.RABIUL HUSSAIN/966882081 LETS TALK ABOUT ATOMIC BONDING

  14. DR.RABIUL HUSSAIN/966882081 Bonding forces and energies • Interatomic forces that bind the atoms together are important to understand many properties of materials. • Atomic bonding can be explained by interaction of two isolated atoms as they are brought into close proximity from an infinite separation. • At large distances, interactions are negligible. • As the distance decreases, each atom exerts forces on each other. Two types of forces exists: attractive force (FA) and repulsive forces (FR). • The net force (FN) between the atoms: FN = FA + FR • The attractive component (FA) is due to type of bonding (minimising energy through electronic configuration). • The repulsive component (FR) is due to negatively charged electron clouds for two atoms and important only at small values of interatomicseparation,r.

  15. DR.RABIUL HUSSAIN/966882081 Atomic bonding and energies • The net force, FN, which is a function of interatomic distance (r) is plotted in fig. • When FA and FR balance, there is no net force; that is , the system is at equilibrium. • FA + FR =0 • At the equilibrium spacing (r0) , the two atoms will counteract any attempt to separate them by an attractive force, or to push them together by a repulsive action. • It is more convenient to explain interaction between atoms in terms of energy.

  16. DR.RABIUL HUSSAIN/966882081 Bonding forces and energies • Net Energy, EN=EA +ER • Energy : Minimum energy corresponds to most stable state • Then, Eo=Bonding energy (Energy at minimum point) • The bonding energy represents the energy required to seperate these two atoms to an infinite seperation. • Bonding energy (E0); Solids >Liquids > Gases

  17. DR.RABIUL HUSSAIN/966882081 Bonding energy and properties: • A number of material properties depend on E0, the curve shape, and bonding type. • Materials with strong bonding energies exhibit high melting points or temperatures. • Tm is larger if E0 is larger

  18. DR.RABIUL HUSSAIN/966882081 Bonding energy and melting temperatures

  19. Bonding arises from the tendency of the atoms to assume stable electronic structures. • Valence electrons are involved. • The nature or type of bond depends on the electronic structures of the constituent atoms. • In general, the bonding arises from the tendency of the atoms to gain stable electronic structures like those of the inert gases, i.e., completely filled outermost shell. DR.RABIUL HUSSAIN/966882081 Bonding between atoms

  20. DR.RABIUL HUSSAIN/966882081 Types of bondings

  21. These bonds invariably involves valence electrons. • Nature of bond depends on electron arrangement in respective atoms. • Atoms tend to acquire stable electron arrangement in their valence orbitals by transferring (ionic), sharing (covalent, and metallic) valence electrons. This leads to formation of bonds. • Bond energies are in order of 1000 KJ/mole. DR.RABIUL HUSSAIN/966882081 Primary inter-atomic bonds

  22. DR.RABIUL HUSSAIN/966882081 IONIC BONDING • . This primary bond exists between two atoms when transfer of electron(s) results in one of the atoms to become negative(has an extra electron) and another positive (has lost an electron). • This bond is a direct consequence of strong Columbic attraction between charged ions. • The ionic bonding is non-directional, that is the magnitude of the bond is equal in all directions. • Found in compounds formed by metallic and non-metallic elements with large difference in electronegativities.

  23. DR.RABIUL HUSSAIN/966882081 IONIC BONDING • The predominant bonding in ceramics is ionic. • Bonding energies are high which generally ranges from 600 and 1500 KJ/mole. • Ionic materials are characteristically hard and brittle and futhermore, electrically and thermally insulative.

  24. DR.RABIUL HUSSAIN/966882081 Covalent bonding • This bonding comes into existence if valence electrons are shared between a pair of atoms, thus acquire stability by saturating the valence configuration. • Covalent bonding is directional; that is, it forms between two specific atoms and may exist only in the direction between one aton and another. • Example: formation of Methane (CH4)

  25. DR.RABIUL HUSSAIN/966882081 COVALENT BONDING • Non-metallic elemental molecules (H2,Cl2,F2,..etc.). • Non-metallic molecules containing dissimilar atoms (CH4,H2O, HNO3,HF, ..etc.). • Elemental solids such as diamond, silicon, germanium and other solid compounds composed of elements located on RHS of the periodic table.

  26. DR.RABIUL HUSSAIN/966882081 Covalent bonding • Number of covalent bonds= 8-N[N=number of valence electrons] • Covalent bonds can be very strong, e.g., diamond, SiC, Si, etc, and also can be very weak, e.g., Bismuth. Polymeric materials are covalently bonded materials. • % Ionic Character: Some bonds are partially ionic and partially covalent. The degree of either bond is controlled by the electronegativities of the composing atoms. % ionic character= { 1- exp[-(0.25)(XA-XB)2]}× 100 where XA and XB are the electronegativities of the respective elements.

  27. DR.RABIUL HUSSAIN/966882081 Metallic bonding • This bond comes into existence if valence electrons are shared between number of atoms, i.e. arranged positive nucleuses are surrounded by electron pool. • Shared electrons are not specific to a pair of atoms, in contrast to Covalent bond, i.e. electrons are delocalized. • As shared electrons are delocalized, metallic bonds are non-directional. • Very characteristic properties of metals like high thermal and electrical conductivities are result of presence of delocalized electron pool.

  28. DR.RABIUL HUSSAIN/966882081 Metallic bonding • Metallic bonding may be strong or weak; energies ranges from 68kj/mol for mercury to 850KJ/mol for tungsten. • Metallic bonding is found in the periodic table for group IA and IIA elements and in fact, for all elemental metals.

  29. DR.RABIUL HUSSAIN/966882081 Secondary bonding or van der WAALS BONDING • These bonds involves atomic or molecular dipoles. • Bonds can exists between induced and permanent dipoles (polar molecules). • Bond comes into existence because of Columbic attraction between positive end of one dipole and negative end of another dipole. • Bond energies are in order of 10 kJ/mol. • Existence of these depends on three kinds of dipoles –fluctuating dipoles, Polar-molecule dipoles and Permanent dipoles • Hydrogen bonding , a special type of secondary bonding , is found to exist between some molecules that have hydrogen as one of the constituents. • Hydrogen bonds is responsible for water t exist in liquid state at room temperature

  30. DR.RABIUL HUSSAIN/966882081 Fluctuating induced dipole bonds • A dipole may be created or induced in an atom or molecule that is normally electrically symmetric; that is, the overall spatial distribution of the electrons is symmetric with respect to the positively charged nucleus, as shown in Figure. All atoms are experiencing constant vibrational motion that can cause instantaneous and short-lived distortions of this electrical symmetry for some of the atoms or molecules, and the creation of small electric dipoles. • One of these dipoles can in turn produce a displacement of the electron distribution of an adjacent molecule or atom, which induces the second one also to become a dipole that is then weakly attracted or bonded to the first. • These attractive forces may exist between large numbers of atoms or molecules, which forces are temporary and fluctuate with time. • EXAMPLE : liquid H2

  31. DR.RABIUL HUSSAIN/966882081 POLAR MOLECULE-INDUCED DIPOLE BONDS • Permanent dipole moments exist in some molecules by virtue of an asymmetrical arrangement of positively and negatively charged regions; such molecules are termed polar molecules. EXAMPLE: HCl • Polar molecules can also induce dipoles in adjacent non-polar molecules , and a bond will form as a result of attractive forces between the two molecules. • The magnitude of dipole-induced dipole is greater than fluctuating induced dipoles.

  32. DR.RABIUL HUSSAIN/966882081 Permanent dipole bonds • Van der Waals forces will also exist between adjacent polar molecules. The associated bonding energies are significantly greater than for bonds involving induced dipoles. • The strongest secondary bonding type, the hydrogen bond, is a special case of polar molecule bonding. It occurs between molecules in which hydrogen is covalently bonded to fluorine (as in HF), oxygen (as in H2O), and nitrogen (as in NH3). • For each H—F, H—O, or H—N bond, the single hydrogen electron is shared with the other atom.

  33. DR.RABIUL HUSSAIN/966882081 Summary : bonding

  34. DR.RABIUL HUSSAIN/966882081 Bonding energies

  35. DR.RABIUL HUSSAIN/966882081 Bonding in solids

  36. DR.RABIUL HUSSAIN/966882081 THANK YOU FOR LISTENING

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