Transcript bond energy
Chapter 29: Molecular bonding Aileen, Jamie, Madeleine, and Ross Bonding in molecules • A molecule is two or more atoms that are held together so as to function as a single unit. When atoms make this attachment, it is called a chemical bond. • Two types of bond: covalent and ionic. Covalent bonding • Also called “sharing bond”. • When two atoms approach each other, the electron clouds begin to overlap, and the electrons from each atom orbits both nuclei. • For example, when using two hydrogen atoms to form a hydrogen molecule, there are two possibilities. Parallel with a spin of 1 and opposite with a spin of 0. • The exclusion principle tells us that since no two electrons can occupy the same state, if two electrons are the same they must be different is some other way. • The exclusive principle requires that when the spins are the same, there is destructive interference of the electrons wave functions between the two atoms. • A covalent bond results from constructive interference of the electron wave function. • A bond can be understood from the energy point of view. • If a H atom spin is opposite, electrons move freely, the wavelength is longer, momentum is less and energy is less. Ionic Bonding • Ionic bonds are a special type of covalent bonds • Instead of equal sharing of electrons, they are transferred or unequally shared • The electrostatic attraction between two atoms, usually a metal and a non-metal, keeps them together Ionic Bonding • Na+ + Cl- = NaCl • The Sodium loses an electron, forming a cation, and the Chlorine accepts the electron, forming an anion • These positive and negative charged ions then combine • When formed together they make the table salt Sodium Chloride • Reaction is endothermic • Only occurs if energy change is favorable – Bonded atoms have a lower energy than the free atoms • The larger the energy change indicates how strong the bond will be • The low electronegativity of metals and high electronegativity of non-metals means the energy change is favorable when metals lose an electron and non-metals gain them Weak bonds Intermolecular forces Forms between molecules due to electrostatic attraction (positive/negative charges) Energy required for bonding to occur is called the bond energy Typical weak bond energy = 0.04 to 0.3 eV To put things into perspective, a typical strong bond energy = 2 to 5 eV Avg. 12 times more energy than the strongest of the weak bonds (Hydrogen bonding) Types of weak bonds Dipole-dipole interaction: the (+) side of a dipole molecule attracts the (-) side of another dipole molecule. Dipole molecule : two elements with different amounts of electronegativity (attraction) NaCl – table salt Na: EN value = 0.9 Cl: EN value = 3.0 Chlorine has a higher EN value, thus pulling more electrons toward it and resulting in an partial (-) charge on Cl and a partial (+) charge on Na. This slight separation of charges is known as a dipole moment. Dipole-Dipole bonds www.sparknotes.com Stezlab1.unl.edu Hydrogen Bonding • A dipole-dipole bond that involves hydrogen and oxygen, nitrogen, or fluorine. • The strongest of the weak bonds • Simplest example: water molecules • Very important in the double helix shape of DNA Geneticengineering.org Junction transistors • Junction transistor consists of a crystal of one type of doped semiconductor sandwiched between two crystals of opposite type. • Figure 29-31 •npn arrow placed on emitter comes away from base while in pnp arrow goes toward the base •Arrow indicates the direction of current flow in normal operation • Transistors are the basic elements in modern electronic amplifiers • They also can act like a “gate” or switch in digital circuits. That is, they let a current pass or they block it off. • Transistors were a great advance in miniaturization of electronic circuits. • Individual transistors are very small compared to ones used in vacuum tubes but huge compared to integrated circuits or chips. • A tiny chip can contain millions of transistors and other circuit elements. • Integrated circuits are the main part of computers, televisions, calculators, cameras and electronic instruments in aircraft space vehicles and autos. • The miniaturization made by integrated circuits not only allows extremely complicated circuits to be placed in small places, but they do allow a great increase in speed of operation. Summary • Molecular bonding due to electrostatic forces • Strong bonds within molecules: – Covalent bonding is the sharing of electrons by two or more atoms while Ionic bonding is the donation of electrons by two or more atoms. • Weak bonds between molecules: – Hydrogen bonding: type of intermolecular bonding that occurs between H and O, N, or F. • An example of bonding in solids is a junction transistor, where impure crystals of silicon can be used as a semiconductor. • Junction transistors have lead to advances in electronics by creating a more compact circuit. Problems Concepts: 1. What is the main difference between ionic and covalent bonding? 2. How is hydrogen bonding exclusive? What elements are involved in this type of bond? Problems: 1. What is the energy range of a strong bond? A weak bond? 2. Explain how a transistor can behave like a switch in digital circuits.