DOC PREVIEW
SJSU PHYS 175A - Chapter 3

This preview shows page 1-2-3-18-19-36-37-38 out of 38 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 38 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 38 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 38 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 38 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 38 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 38 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 38 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 38 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 38 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

Chapter 3Crystal BindingPhys 175ADr. Ray KwokSJSU4 basis categoriesMolecular Bonds – Introduction To understand the crystal binding, one should understand how molecules bind together The bonding mechanisms in a molecule are fundamentally due to electric forces The forces are related to a potential energy function A stable molecule would be expected at a configuration for which the potential energy function has its minimum valueFeatures of Molecular Bonds The force between atoms is repulsive at very small separation distances This repulsion is partially electrostatic and partially due to the exclusion principle Due to the exclusion principle, some electrons in overlapping shells are forced into higher energy states The energy of the system increases as if a repulsive force existed between the atoms The force between the atoms is attractive at larger distances (e.g. due to shifted charge distribution, induced dipole-dipole interaction)Potential Energy Function The potential energy for a system of two atoms can be expressed in the form r is the internuclear separation distance m and n are small integers (usually) A is associated with the attractive force B is associated with the repulsive force( )n mA BU rr r= − +Graph – U(x) At large separations, the slope of the curve is positive Corresponds to a net attractive force (F = −dU/dr) At the equilibrium separation distance, the attractive and repulsive forces just balance At this point the potential energy is a minimum The slope is zero (F=0)Molecular Bonds – Types Simplified models of molecular bonding include Ionic Covalent van der Waals HydrogenIonic Bonding Ionic bonding occurs when two atoms combine in such a way that one or more outer electrons are transferred from one atom to the other Ionic bonds are fundamentally caused by the Coulomb attraction between oppositely charged ions (e.g. Na+Cl−)Ionic Bonding, cont. The energy required to remove an electron from an atom is called Ionization Energy. (Na+) The amount of energy gained by adding an electron (from far away E=0) to an atom (E<0) is called the Electron Affinity of the atom. (Cl−) The Binding Energy (or Dissociation Energy) is the amount of energy needed to break the molecular bonds and produce neutral atomsIonic Bonding, NaCl Example The graph shows the total energy of the molecule vs the internuclear distance The minimum energy is at the equilibrium separation distance Binding energy = 4.2 eVIonic Bonding,final The energy of the molecule is lower than the energy of the system of two neutral atoms It is said that it is energetically favorablefor the molecule to form The system of two atoms can reduce its energy by transferring energy out of the system and forming a moleculeCovalent Bonding A covalent bond between two atoms is one in which electrons supplied by either one or both atoms are shared by the two atoms Covalent bonds can be described in terms of atomic wave functions The example will be two hydrogen atoms forming H2 (bonding energy −4.48 eV)CovalentbondsBonding can occur without outright removal or addition of an electron. In these types of bonds, the connection occurs through orbital overlap.Wave Function – Two Atoms Far Apart Each atom has a wave function (1s1) There is little overlap between the wave functions of the two atoms when they are far away from each other131( )or asoψ r eπa−=Wave Function – Molecule The two atoms are brought close together The wave functions overlap and form the compound wave shown The probability amplitude is larger between the atoms than on either sideCovalent Bonding, Final The probability is higher that the electrons associated with the atoms will be located between them This can be modeled as if there were a fixed negative charge between the atoms, exerting attractive Coulomb forces on both nuclei The result is an overall attractive force between the atoms, resulting in the covalent bondVan der Waals Bonding Two neutral molecules are attracted to each other by weak electrostatic forces called van der Waals forces (typically 0.1 eV) Atoms that do not form ionic or covalent bonds are also attracted to each other by van der Waals forces The van der Waals force is due to the fact that the molecule has a charge distribution with positive and negative centers at different positions in the moleculeVan der Waals Bonding, cont. As a result of this charge distribution, the molecule may act as an electric dipole Because of the dipole electric fields, two molecules can interact such that there is an attractive force between them Remember, this occurs even though the molecules are electrically neutral e.g. Liquid nitrogen molecules N2Types of Van der Waals Forces Dipole-dipole force An interaction between two molecules each having a permanent electric dipole moment Dipole-induced dipole force A polar molecule having a permanent dipole moment induces a dipole moment in a nonpolar moleculeTypes of Van der Waals Forces, cont. Dispersion force An attractive force occurs between two nonpolar molecules The interaction results from the fact that, although the average dipole moment of a nonpolar molecule is zero, the average of the square of the dipole moment is nonzero because of charge fluctuations The two nonpolar molecules tend to have dipole moments that are correlated in time so as to produce van der Waals forcesHydrogen Bonding In addition to covalent bonds, a hydrogen atom in a molecule can also form a hydrogen bond (weak ≤ 0.5 eV) Using water (H2O) as an example There are two covalent bonds in the molecule The electrons from the hydrogen atoms are more likely to be found near the oxygen atom than the hydrogen atomsHydrogen Bonding – H2O cont. This leaves essentially bare protons at the positions of the hydrogen atoms The negative end of another molecule can come very close to the proton This bond is strong enough to form a solid crystalline structureHydrogen Bonding, Final The hydrogen bond is relatively weak compared with other electrical bonds Hydrogen bonding is a critical mechanism for the linking of biological molecules and polymers DNA is an exampleBonding in SolidsBonds in solids can be of the following typesIonicCovalentMetallicIonic Bonds in Solids The dominant interaction


View Full Document

SJSU PHYS 175A - Chapter 3

Download Chapter 3
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Chapter 3 and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Chapter 3 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?