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
Chemical compound: Two or more elements joined together by
a chemical bond. Most minerals are composed of at least two
elements.
Chemical Formula: NaCl
Note Cubic Symmetry and Closest Packing
Halite Atomic Structure
1s2
1s22s22p6
Outer Shell Filling
Ionic Bonding and Electron Exchange
11p+ + 10e- = +1
1s22s22p63s1
+1e-
1s22s22p63s23p5
17p+ + 18e- = -1
Ionic Bonding: Electron Transfer
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.
Cations (+) are always smaller than the neutral atom;
Anions (-) are always larger than the neutral atom.
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
Diamond has a hardness of 10 and is an
electrical insulator, while Graphite has a
hardness of 2 and is a electrical conductor.
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).
Covalent Bonding: Sigma vs. Pi bonds
Note the hexagonal symmetry
and that graphite tends to form
in layered sheets
pi bonds are weaker than sigma bonds
Graphite Example - Pure Carbon in layered 3D network
Covalent Bonding: Diamond vs. Graphite
Diamond
Graphite
Metallic Bonding
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).
Mg
Na
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 meters
Summary 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
Chemical Formula: H2O
Water Atomic Structure
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
Hexagonal Symmetry
1 - 5 mm
Oddly, ice is less dense than liquid
water, hence it floats and lakes
freeze from the top down!
From: http://www.its.caltech.edu/~atomic/snowcrystals
Snowflake Growth
From: http://www.its.caltech.edu/~atomic/snowcrystals
LT-SEM Images of Snow Crystals