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

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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

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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.