Bonding in Solids


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Bonding in Solids

  1. 1. bond in which one or more pairs of electrons are shared by two atoms.
  2. 2. bond in which one or more electrons from one atom are removed and attached to another atom, resulting in positive and negative ions which attract each other.
  3. 3. -Scientists use the percent ionic character of a bond to find out the difference between an ionic bond and polar covalent bond. -Ionic character increases with the electronegativity difference. -Between discrete pairs of atoms, there are no bonds that are considered ionic solids. -Compounds with more than 50% ionic character are usually considered ionic solids. -If a compound conducts an electric current after it’s been melted, it’s considered ionic.
  4. 4. A plot of the potential energy versus the separation distance. r= distance between the charges Constant ( )=9.0×109N·m2/C2
  5. 5. If the two charges have the same sign, the PE is positive for all values of r
  6. 6. If the two charges are of opposite sign, the PE is negative because the product is negative.
  7. 7. Binding Energy This is how much energy must be put into the system to separate the two atoms to infinity, where the PE=0.
  8. 8. It is also the energy equivalent of the “missing mass” of a nucleus.
  9. 9. Activation Energy The additional energy that must be injected into the system to get the atoms over the “hump” (or barrier) in the potential energy diagram.
  10. 10. Potential Energy Diagram For The Formation Of ATPfromADPandPhosphate(℗)
  14. 14. • Quantum mechanics has been a great tool for understanding the structure of solids. This active field of research today is called solid-state physics, or condensed-matter physics so as to include liquids as well. Although some solid materials are amorphous in structure (such as glass) in that the atoms and molecules show no longer range order, we will be interested here in the large class of crystalline substances whose atoms, ions, or molecules are generally accepted to form an orderly array in a geometric arrangement known as a lattice.
  15. 15. Figure 29-22 Arrangement of atoms in (a) a simple cubic crystal , (b) body- centered cubic crystal, and (c) face-centered cubic crystal . Each diagram shows the relationship of the bonds. Each of these “cells” is repeated in three dimensions to the edges of the macroscopic crystal.
  16. 16. Bonds in solid • Covalent Bonding (as between the carbon atoms of the diamond crystal)
  17. 17. • Ionic Bonding •
  18. 18. Diagram of NaCl crystal
  19. 19. • A different type of bond occurs in metals. Metal atoms have relatively loosely held outer electrons. Present-day metallic bond theories propose that in a metallic solid, these outer electrons roam rather freely among all the metal atoms which, without their outer electrons, act like positive ions. The electrostatic attraction between the metal ions and these negative electrons “gas” is at least in part responsible for holding the solid together. The binding energy of metal bonds is typically 1-3eV somewhat weaker than ionic or covalent bonds(5-10eV in solids). The free electrons are responsible for the high electrical and thermal conductivity of metals.
  20. 20. Comparison Of Important Strong Bonds • Ionic Bonding- an electron is stolen from one atom by another. • Covalent Bonding- electrons are shared by atoms within a single molecule • Metallic Bonding- electrons are shared by all atoms in the metal.
  21. 21. • When two hydrogen atoms approach each other, the wave functions overlap, and the two 1s states (one for each atom) divide into two states of different energy. • If a large number of atom comes together to form a solid, then each of the original atomic levels becomes a band. the energy levels are so close together in each band that they seem essentially continuous.
  22. 22. • The crucial aspect of a good conductor is that the highest energy band containing electrons is only partially filled. • The 3s band, is only half full. • Only two electrons can be in the 3s state , one with spin up and one with spin down. these two states have slightly different energy. • For a solid consisting of N atoms, the 3s band will contain 2N possible energy states. • When a potential difference is applied across the metal, electrons can respond by accelerating and increasing their energy.
  23. 23. • A current flows readily and sodium is a good conductor. • A characteristics of a good conductors is that the highest energy band is only partially filled. • In a material that is a good insulator, the highest band containing electrons, called the conduction band, is separated from the valence bond by a “ forbidden” energy gap (banded gap).
  24. 24. • The important class of materials known as semi conductors. • The bands for a semi conductor, such as silicon (Si ), or germanium(Ge). • At higher temperatures more electrons have enough energy to jump the gap, this effect can often more than offset the effects of more frequent collisions due to increased disorder at higher temperature. • Unfilled electron states are called holes.
  26. 26.  Semiconductor  Transistor
  27. 27. P-n junction diode Separately, the two semiconductors are electrically neutral . When joined, a few electron near the junction diffuse from the negative type into the positive type semiconductor. The n-type is left with a positive charge, and the p-type acquires a net negative charge.
  28. 28. The externally applied voltage opposes the internal potential difference and the diode is said to be forward biased. The positive type holes in the p-type semiconductor are repelled by the positive terminal of the battery and the electrons in the n-type are repelled by the negative terminal of the battery.
  29. 29. The positive terminal of the battery is continually pulling electrons off the p end, forming new holes, and electrons are being supplied by the negative terminal at the n end. consequently, a large current flows through the diode. When the diode is reversed biased, the holes in the p end are attracted to the battery’s negative terminal and the electrons in the n end are attracted to the positive terminal.
  30. 30. If a voltage across the diode connected in reverse bias is increasely greatly, a point is reached where breakdown occurs. The voltage remains constant over a wide range of currents, and a diode designed for this purpose is called a zener diode . Zener diode can be obtained corresponding to voltages of a few volts to a hundreds of volts. Rectifier circuits are important because most line voltage is ac and most electronic devices require a dc voltage for their operation.
  31. 31. Photodiodes and solar cells are p-n junctions used in the reverse way. Photons are absorbed , creating electron-hole pairs if the photon energy is greater than the band gap energy. A diode is called a nonlinear device because the current is not proportional to the voltage
  34. 34. Transistors and Integrated Circuits Transistors A small electronic device containing a semiconductor and having at least three electrical contacts, used in a circuit as an amplifier, detector, or switch. Integrated circuit a circuit of transistors, resistors, and capacitors constructed on a single semiconductor wafer or chip, in which the components are interconnected to perform a given function; microcircuit. Abbr.: IC
  35. 35. A simple Junction transistors • A bipolar transistor consists of a three-layer “sandwich” of doped (extrinsic) semiconductor materials, either P-N-P in Figure below(b) or N-P-N at (d). Each layer forming the transistor has a specific name, and each layer is provided with a wire contact for connection to a circuit. The schematic symbols are shown in Figure below(a) and (d). • BJT transistor: (a) PNP schematic symbol, (b) physical layout (c) NPN symbol, (d) layout.
  36. 36. The functional difference between a PNP transistor and an NPN transistor is the proper biasing (polarity) of the junctions when operating. For any given state of operation, the current directions and voltage polarities for each kind of transistor are exactly opposite each other. • Bipolartransistorsworkascurrent-controlledcurrentregulators. Inotherwords,transistorsrestricttheamountofcurrentpassed accordingtoasmaller,controllingcurrent.Themaincurrentthat is controlled goes from collector to emitter, or from emitter to collector,dependingonthetypeoftransistoritis(PNPorNPN, respectively). The small current that controls the main current goesfrombasetoemitter,orfromemittertobase,onceagain depending on the kind of transistor it is (PNP or NPN, respectively). According to the standards of semiconductor symbology, the arrow always points against the direction of electronflow.(Figurebelow)
  37. 37. Small electron base current controls large collector electron current flowing against emitter arrow.
  38. 38. Bipolar transistors are called bipolar because the main flow of electrons through them takes place in two types of semiconductor material: P and N, as the main current goes from emitter to collector (or vice versa). In other words, two types of charge carriers -- electrons and holes -- comprise this main current through the transistor. • As you can see, the controlling current and the controlled current always mesh together through the emitter wire, and their electrons always flow against the direction of the transistor's arrow. This is the first and foremost rule in the use of transistors: all currents must be going in the proper directions for the device to work as a current regulator. The small, controlling current is usually referred to simply as the base current because it is the only current that goes through the base wire of the transistor. Conversely, the large, controlled current is referred to as the collector current because it is the only current that goes through the collector wire. The emitter current is the sum of the base and collector currents, in compliance with Kirchhoff's Current Law.
  39. 39. • No current through the base of the transistor, shuts it off like an open switch and prevents current through the collector. A base current, turns the transistor on like a closed switch and allowsaproportionalamountof currentthrough the collector. Collector current is primarily limited by the base current, regardless of the amount of voltage available to push it.
  40. 40. REVIEW: • Bipolar transistors are so named because the controlled current must go through two types of semiconductor material: P and N. The current consists of both electron and hole flow, in different parts of the transistor. • Bipolar transistors consist of either a P-N-P or an N-P-N semiconductor “sandwich” structure. • The three leads of a bipolar transistor are called the Emitter, Base, and Collector. • Transistors function as current regulators by allowing a small current to control a larger current. The amount of current allowed between collector and emitter is primarily determined by the amount of current moving between base and emitter. • In order for a transistor to properly function as a current regulator, the controlling (base) current and the controlled (collector) currents must be going in the proper directions: meshing additively at the emitter and going against the emitter arrow symbol.