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Chapter 29 Molecules and Solids

Units of Chapter 29 Bonding in Molecules Potential-Energy Diagrams for Molecules Weak (van der Waals) Bonds Molecular Spectra Bonding in Solids Band Theory of Solids Semiconductors and Doping Semiconductor Diodes Transistors and Integrated Circuits

29.1 Bonding in Molecules Molecule: two or more atoms strongly held together to function as a unit This attachment is called a chemical bond Two types of bond: Covalent Ionic

29.1 Bonding in Molecules Hydrogen molecule, H2, is bound covalently. If the atoms have their spins in the same direction, so S = 1 for the molecule, the atoms will not bond due to the exclusion principle.

29.1 Bonding in Molecules The molecule will only form if S = 0. The two electrons are shared by both atoms: The energy needed to separate the atoms is called the binding energy.

29.1 Bonding in Molecules An ionic bond is created by the attraction of ions. For example, the outermost electron in the sodium atom spends most of its time around the chlorine atom in NaCl.

29.1 Bonding in Molecules The reason this happens is that sodium has a single electron outside a closed shell, and it is not tightly bound. Conversely, the chlorine atom has an empty space; there is only one electron where two can be accommodated.

29.1 Bonding in Molecules Pure covalent bonds are found in molecules consisting of only one type of atom. Otherwise, electrons are likely to spend more time around one type of atom than another, giving a partial ionic character. Water is one such molecule.

29.2 Potential-Energy Diagrams for Molecules Potential energy of two point charges:

29.2 Potential-Energy Diagrams for Molecules For the hydrogen molecule, the force between the atoms is attractive at large distances. If the atoms are too close, the electrons are too squeezed; therefore there is a minimum in the potential.

29.2 Potential-Energy Diagrams for Molecules Often, there is an activation energy required – atoms must be separated from existing molecules before they can be combined to make new ones.

29.2 Potential-Energy Diagrams for Molecules Sometimes the bond occurs in a configuration that is a local minimum of potential, but that takes energy to reach. This is important in living cells.

29.3 Weak (van der Waals) Bonds Weak bonds are electrostatic bonds between molecules (and not between atoms within a molecule). The binding energy is much less than that of strong bonds, about 0.04 to 0.3 eV. Weak bonds are usually the result of attraction between dipoles.

29.3 Weak (van der Waals) Bonds Weak bonds become important in liquids and solids where strong bonds are absent. They are also important in living cells, especially in DNA replication.

29.3 Weak (van der Waals) Bonds Protein synthesis involves the breaking of weak bonds and the formation of new ones through random collisions.

29.4 Molecular Spectra The overlap of orbits alters energy levels in molecules. Also, more types of energy levels are possible, due to rotations and vibrations. The result is a band of closely spaced energy levels.

29.4 Molecular Spectra A diatomic molecule can rotate around a vertical axis. The rotational energy is quantized.

29.4 Molecular Spectra These are some rotational energy levels and allowed transitions for a diatomic molecule.

29.4 Molecular Spectra Small-amplitude vibrations of a diatomic molecule will be simple harmonic. Again, the energy is quantized.

29.4 Molecular Spectra Here are some vibrational energy levels in a diatomic molecule, and allowed transitions.

29.5 Bonding in Solids Some solids are amorphous, but many are crystalline, having their molecules arranged in a regular lattice. Here are three cubic crystal lattices:

29.5 Bonding in Solids The NaCl lattice is face-centered cubic; here is what it looks like, with the atoms in their actual “packed” configuration.

29.5 Bonding in Solids Metallic bonds, where electrons are shared by all atoms in the metal, are neither ionic or covalent. The binding energy of metallic bonds is slightly weaker than that of ionic or covalent bonds – about 1 to 3 eV – but they are still strong bonds.

29.6 Band Theory of Solids The more atoms are bound together with overlapping wave functions, the more continuous the energy bands will become. Here is what happens with two, six, and many atoms:

29.6 Band Theory of Solids A good conductor has its highest energy band only partially filled, as in the figure. An insulator has its highest energy band completely filled, with a substantial gap separating it from the next level.

29.6 Band Theory of Solids A semiconductor also has its highest band filled, but the gap to the next level is small.

29.7 Semiconductors and Doping The most common semiconductors in use are silicon and germanium. A tiny amount of impurity gives the semiconductor useful properties – this is called doping. The doped semiconductor becomes slightly conducting; the conductivity can be controlled with great precision.

29.7 Semiconductors and Doping Arsenic-doped silicon is an n-type semiconductor, as the current is carried by negative charges.

29.7 Semiconductors and Doping Gallium-doped silicon is a p-type semiconductor – the current is carried by “holes,” or spots that are missing electrons.

29.7 Semiconductors and Doping The impurity provides additional energy states to the semiconductor.

29.8 Semiconductor Diodes When an n-type and a p-type semiconductor are joined, the result is a pn junction diode. This diode will conduct electricity in one direction but not the other.

29.8 Semiconductor Diodes A graph of the current vs. voltage shows this effect clearly. If the potential difference is large enough, current will flow in the reverse direction as well.

29.8 Semiconductor Diodes A diode can serve as a rectifier – a device that changes ac into dc. The simplest circuit is a half-wave rectifier:

29.8 Semiconductor Diodes A full-wave rectifier has a much smoother output.

29.9 Transistors and Integrated Circuits A junction transistor is one type of semiconductor sandwiched between layers of another – npn or pnp. These layers are called the collector, base, and emitter.

29.9 Transistors and Integrated Circuits A transistor can amplify a small current into a larger one, as the collector-emitter current is much larger than the base current. They are also used in digital circuits, and they can either let current pass or block it.

Summary of Chapter 29 Molecules form either covalent or ionic bonds Electron wave functions overlap Weak (van der Waals) bonds are dipole attractions between molecules Energy levels in molecules are altered Additional energy levels are possible, corresponding to rotational and vibrational states Energy levels become closely-spaced bands Rotational energy levels are quantized

Summary of Chapter 29 Vibrational energy levels are quantized too Solids can be bound by ionic, covalent, or metallic bonds Electron energy levels in crystals are bands, with gaps in between In conductors, the highest band is partially full In insulators, the highest band is completely full, and there is a large gap to the next band

Summary of Chapter 29 In semiconductors, the highest band is completely full but the energy gap is much smaller In doped semiconductors, small amounts of impurities allow current to be very precisely controlled Doped semiconductors can be either p-type or n-type A diode is a pn junction A transistor is a pnp or npn junction