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Compounds and Bonding !• Chemical compounds!• Ionic bonding!– NaCl as a type example!– Electron transfer and shell completion!• Covalent bonding!– Diamond - pure C example!– Electron sharing!• Silicon Tetrahedron!– Structure (strong sp3 covalent bonds)!– Building block of silicate minerals!Halite Atomic Structure!Chemical Formula: NaCl!Note Cubic Symmetry and Closest Packing!Chemical compound: Two or more elements joined together by!a chemical bond. Most minerals are composed of at least two!elements.!Outer Shell Filling!1s2!1s22s22p6!Ionic Bonding and Electron Exchange!1s22s22p63s1!1s22s22p63s23p5!+1e-!17p+ + 18e- = -1!11p+ + 10e- = +1!Ionic Bonding: Electron Transfer !Cations (+) are always smaller than the neutral atom;!Anions (-) are always larger than the neutral atom.!The propensity of an element to gain or lose electrons may be!parameterized by a value called electronegativity (EN). The scale!is arbitrary and was defined by Linus Pauling in 1960.!Low EN elements tend to lose valence electrons to become cations.!High EN elements tend to gain valence electrons to become anions.!Ionic Bonding – NaCl example!Atoms that become ionized are!charged particles!- Like charges are repulsive!- Unlike charges are attractive!- Ionic bonds balance these forces!Fa ≈ (q+) (q-)/d2 – Coulomb Law!Fr ≈ -n/d1+n – Born Repulsion!Where q is charge (+ or -), d is the distance !between the atoms, and n is an integer that !depends on the number of filled shells!Ftotal = Fa + Fr!Covalent Bonding: Electron Sharing!Diamond and Graphite - Pure Carbon in complex 3D network!Covalent Bonding: Hybrid Orbitals!Diamond Example - Pure Carbon in complex 3D network!Diamond vs. Graphite - Polymorphs!Polymorphism is the ability of a chemical compound to crystallize in more than !one structure. The different forms are called polymorphs; they may show vastly !different physical and chemical behavior. Temperature and pressure are important !controls on which polymorph is stable. Diamond stabilization requires very high!pressures, which are normally only found deep in the Earth’s interior (the mantle).!Diamond has a hardness of 10 and is an electrical insulator, while Graphite has a hardness of 2 and is a electrical conductor.!Covalent Bonding: Sigma vs. Pi bonds!Graphite Example - Pure Carbon in layered 3D network!Note the hexagonal symmetry!and that graphite tends to form!in layered sheets!pi bonds are weaker than sigma bonds!Covalent Bonding: Diamond vs. Graphite!Diamond! Graphite!Metallic Bonding!Mg! Na!Extreme form of covalent bonding in which the electrons become!“decoupled” from the nuclei (low EN) and “move” through the crystal.!Occurs when Valence Band electrons overlap in energy with Conduction Band – Effect !is enhanced as interatomic spacing is decreased – that is with higher pressure (deep!interior of Earth and other planets – metallic H in Jupiter).!Partial Ionic Bonding and Electronegativity!Electronegativity defined by!Linus Pauling (1960) is a relative scale with Li fixed at a value of 1, !C at 2.5 and F at 4.0!Low values of EN yield cations (+)!High values of EN yield anions (-)!Na = 0.9 and Cl = 3.0 -> ∆E = 2.1!~68% ionic character!Si = 1.8 and O = 3.5 -> ∆E = 1.7 ~55% ionic character!Silicon Tetrahedron: SiO4 (net -4 charge)!1.30 Å!0.34 Å!1 angstrom = 1.0 × 10-10 metersSummary chemical bond types!• Bonds which involve valence electrons!– Ionic (transfer of electrons)!– Covalent (sharing of electrons in hybrid orbitals)!– Metallic (decoupled electrons)!– Partial ionic or mixed bonds!• Non valence electron bonds or molecular bonds!– Hydrogen (asymmetric charge must have H)!– van der Waals (short-term charge polarity and doesn’t require H)!Water, Ice and Snow!• Most important substance on Earth!• Essential for biological life as we know it!• Unique volumetric property!• Molecular symmetry and its relationship to crystal morphology!• Crystal growth and its effect on crystal morphology!Water Atomic Structure!Chemical Formula: H2O!Hydrogen Bonding in water and ice!In the H2O molecule,!charge is highly polarized.!Oxygen pulls electrons!from H toward itself and !leaves most of the positive!charge closer to the center !of the H atoms in the form of !a H+ proton.!The 3D network of ice forms bonds !because of this charge polarity in!the water molecule. This is!called “hydrogen bonding”.!Snowflake Morphology!1 - 5 mm!From: http://www.its.caltech.edu/~atomic/snowcrystals!Hexagonal Symmetry!Oddly, ice is less dense than liquid water, hence it floats and lakes freeze from the top down!!Snowflake Growth!From: http://www.its.caltech.edu/~atomic/snowcrystals!LT-SEM Images of Snow


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UT Arlington GEOL 2313 - Compounds and Bonding

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