MSEASUSlides: Muddiest Point: Bonding Slide Set
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Transcript of MSEASUSlides: Muddiest Point: Bonding Slide Set
Muddiest Point 1 -‐ Bonding
Topics covered in this screencast are:
• Metallic Bonding • Ionic Bonding • Covalent Bonding • Van der Waals Bonding • Van der Waals Bonding in Polymers • Bond Strength and a Material’s ProperAes
Metal ions are surrounded by delocalized electrons, or a “sea of electrons” (electron-‐glue).
Metallic bonds are moderately strong Bonding energies (E0): range: 850 kJ/mol (8.8 eV/atom) for W. 406 kJ/mol (4.1 eV/atom) for Fe. 324 kJ/mol (3.3 eV/atom) for Al MelAng temperatures (Tmelt~E0):
3410 C for W. 1493 C for Fe 660 C for Al
Stronger bonds lead to higher melAng temperature: atomic scale bonding ⇒ macroscale property.
Primary Bonding -‐ Metallic Bonding
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Student MisconcepAon – Sea of electrons surrounding protons
Metallic bonding – strong aXracAve force in metals due to “sea” of delocalized electrons that bind together metal caAon cores in a solid
3
• Occurs between metal (+) and nonmetal (-‐) ions.
• Mechanism is by electron transfer.
• Large difference in electronegaAvity (ΔeV > 1.7ev).
• Example: NaCl (Na = 0.9 eV; Cl = 3.0 eV; Δev = 2.1 eV)
Primary Bonding - Ionic Bonding
Na (metal) unstable
Cl (nonmetal) unstable
electron
+ - Coulombic AXracAon
Na (caAon) stable
Cl (anion) stable
Ionic bonding – strong aXracAve coulombic force between adjacent caAons and anions due to transfer of electrons between atoms
Primary Bonding - 0-D, 1-D, 3-D Covalently Bonded Materials
1-‐D Covalent Bonding -‐ Polymer -‐Polyethylene [CH2]n
3-‐D Covalent Bonding -‐ Ceramic Diamond
0-‐D Covalent Bonding – Fluorine Gas
4
Van der Waals Bonding
1-‐D Covalent Bonded Chain
1-‐D Covalent Bonded Chain
Covalent bonding – strong aXracAve force produced by sharing of electrons between neighboring atoms
Secondary Bonding – Van der Waals Bonding
secondary bonding
+ -‐ + -‐
Van der Waals Bonding – weak aXracAve force between molecules from weak columbic aXracAon due to temporary or permanent dipoles.
Dipole: A separaAon of posiAve and negaAve charges. Caused by unbalanced electron clouds.
2 types of Dipoles: Temporary (fluctuaAng) and Permanent
Temporary Dipole: Generally Random and fluctuaAng
Permanent Dipole: From electronegaAvity difference Van der Waals bonding is responsible for the unique proper7es of
polymers!
Van der Waals Bonding in Polymers
1D Covalently Bonded Chain
1D Covalently Bonded Chain
VAN DER WAALS BONDS
Polymer : A material made up of 1D covalently bonded molecules with Van der Waals bonding between the chains
Van der Waals bonds are what determines a polymer’s mel7ng temperature, elas7c modulus, and coefficient of thermal expansion because they are what holds
the material together.
Order of Strength: Van der Waals <<< Metallic < Covalent <Ionic
Property Polymer-‐ HDPE
Metal-‐ Copper
Ceramic-‐ Quartz (SiO2)
E (GPa) ~1 110 310
Tm (°C)
~130 1084 1405
Coefficient of Thermal Expansion (10-‐6 m/m-‐K)
~180 17 0.33
How Does Bond Strength Affect a Material’s ProperAes?
Polymer – 1D covalently bonded chains with Van der Waals bonds between the chains, holding the polymer together
Metal – Metallic bonding (posiAvely charged metal cores in a sea of delocalized electrons)
Ceramic – 3D Covalent or Ionic bonding. Both bond types are very strong and result in a crystal structure.
Higher bond strength = Higher E and Tm but lower coefficient of thermal expansion
Lower bond strength = Lower E and Tm but higher coefficient of thermal expansion
Polymers (VDW) <<< Metals < Ceramics
AddiAonal Videos
• Khanacademy – Van der Waals Bonding
• �kosasihiskandarsjah – Ionic and Covalent Bonding Anima7on
• bozemanbiology -‐ Chemical Bonds
• weiner7000 – Metallic Bonding and Metallic Proper7es