1. Chapter 29: Molecular bonding Aileen, Jamie, Madeleine, and Ross

2. 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.

3. 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.

4. 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.

5. 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.

6. 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

7. Ionic Bonding

8. 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

9. 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

10. 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)

11. 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.

12. Dipole-Dipole bonds www.sparknotes.com Stezlab1.unl.edu

13. 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

14. 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

15. 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.

16. 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.

17. 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.

18. 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.