Lecture 11

Lecture 11 - EEE 352: LECTURE 11 BondingCovalent bonding...

Info iconThis preview shows pages 1–9. Sign up to view the full content.

View Full Document Right Arrow Icon
EEE 352: LECTURE 11 Bonding—Covalent bonding The Nearly Free Electron Model—Imposing Periodicity on our Particles * Combining free electrons: an approximation * Nearly free electron model Electron diffraction Energy gaps
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
We have discussed the formation of IONIC bonds and crystals by electron transfer Molecules and crystal structures may also form by COVALENT bonding * In these cases electrons are SHARED between different atoms RATHER than being transferred as in ionic crystals * By sharing electrons the atoms involved APPEAR to fill their shells EXAMPLES of covalent MOLECULES are H 2 , Cl 2 , H 2 O, CH 4 + + IN THIS CASE A COVALENT BOND IS FORMEDBETWEEN TWO HYDROGEN ATOMS THE 1s SHELL OF BOTH ATOMS THUS APPEARS FILLED AND STABLE Covalent Molecules and Crystals The covalent bond is the basic bond in ALL organic chemistry, which includes all living creatures, their DNA, RNA, etc.
Background image of page 2
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Stable CRYSTALS may also be formed by covalent bonding since the covalent bond is COMPARABLE in strength to an ionic bond A good covalent crystal is DIAMOND which forms when CARBON atoms bond * Each atom bonds covalently to FOUR other carbon atoms And so APPEARS to fill its outermost shell This gives rise to the distinctive TETRAHEDRAL bonding Covalent Molecules and Crystals EACH CARBON ATOM IN DIAMOND IS COVALENTLY BONDED TO FOUR OTHER CARBONS IN A TETRAHEDRAL BONDING ARRANGEMENT
Background image of page 4
Some properties of COVALENT crystals (continued) Their bonds are highly DIRECTIONAL in nature Covalent Molecules and Crystals CONTOURS OF ELECTRON DENSITY IN COVALENTLY BONDED GERMANIUM NOTE THE HIGH CONCENTRATION MIDWAY ALONG THE COVALENT Ge-Ge BOND Ge HAS A SIMILAR CRYSTAL STRUCTURE TO DIAMOND + +
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
http://cst-www.nrl.navy.mil/lattice/struk/a4.html
Background image of page 6
Some properties of COVALENT crystals They form relatively STABLE and HARD crystals Due to their HIGH cohesive energies They are POOR electrical conductors Since there are normally NO free electrons in the crystal Covalent Molecules and Crystals CRYSTAL DIAMOND Si Ge InAs SiC ZnS COHESIVE ENERGY (eV) 7.37 4.63 3.85 5.70 12.3 6.32
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
In a crystal, the electron wave will DIFFRACT from the atoms just as an X-ray diffracts from the atoms For what values of the electron wavelength will diffraction occur in a crystal? For wavelengths such that the BRAGG CONDITION is satisfied Electron Motion in a Periodic Potential CONSIDER THE PROBLEM WHERE AN ELECTRON OF WAVENUMBER k IS INCIDENT ON A CRYSTAL WITH A LATTICE SPACING d IN THE x and y DIRECTIONS Bragg Diffraction OccursWhen : 2 d sin θ = n
Background image of page 8
Image of page 9
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 35

Lecture 11 - EEE 352: LECTURE 11 BondingCovalent bonding...

This preview shows document pages 1 - 9. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online