Bonds

Bonding occurs when a lower energy state is possible between two atoms than if they are separate. In quantum mechanics, a bond is formed when the wave functions of two atoms overlap constructively. Bonds are categorized as either primary or secondary bonds. There are three types of primary bonds: metallic, ionic and covalent. These bonds are generally related to the interaction of valence electrons between atoms. Secondary bonds include Van der Walls interactions, hydrogens bonds polar bonds and others. These bonds weaker than primary bonds, on the order of 10kJ, and have effects on physical properties.

//Lewis structures// can be used to predict how atoms are bonded in a molecule based on nmber of shared bonds, lone pairs, valence elections and formal charges. //Valence Shell Electron Repulsion (VSEPR)// //theory// is used to understand the geometry to the molecules based on their bonds and atomic positions.

Types of Primary Chemical Bonds and Analogous Human Relationships

 * 1) Metallic Bonds - friendship (free exchange of resources)
 * 2) Ionic Bonds - courtship (exchange of resources and electric attraction)
 * 3) Covalent Bonds - marriage (double and triple bond)

Metallic Bonds
Metallic bonds have a sea of delocalized electrons.

Metallic bonding is exclusive for **metal** materials, unlike ceramics and polymers.

Ionic Bonds
Electrons are exchanged to satisfy the valence shell requirements of alkali and halide atoms. The resulting positive and negatively charged atoms, called ions, attract and form ionic bonds through //columbic forces//. The transfer of electrons between atoms are known as r//eduction-oxidation reactions (redox)//. In forming NaCl, chlorine (an electron theif, of sorts) oxidizes sodium by stripping away an electron and leaving a positive sodium cation. The following OIL RIG pnemonic can be used to recall reduction and oxidation as it pertains to loss and gain of electrons: **O**xidation **I**s **L**oss; **R**eduction **I**s **G**ain.

Ionic materials are generally hard and brittle materials as are most **ceramics**.

Covalent Bonds
Covalent bonds exist when two atoms share a pair of electrons. Covalent bonding is common in all metals, but it is the principal primary bond found in **polymers**, in addition to secondary bonds.

Bond length
[\Lennard-Jones Potential Energy Diagram between two atoms].

Two atoms approaching each other observe an attractive force between nuclei and electrons. At a specific distance, there is a minimum energy, between which at very close distances, electron-electron repulsion generates a highly positive, unfavorable energy. The dissociation energy is the difference between energies of atoms that are separated and bonded. The depth of the energy well suggests a more stable and shorter bond. Therefore, triple bonds will have deeper, narrower energy wells than double bonds.

Polar Covalent Bonds
Polar covalent bonds are a cross between polar and covalent bonds. They are determined when the difference in electronegativity between two atoms is greater than 0.4.

Energy Scale
Table I - Energy scale of common interactions.
 * ~ Interaction ||~ Energy (kcal/mol) ||~ Bond Length (Angstroms) ||
 * = Covalent Bond ||= 90 ||= 1.5 ||
 * = Ion-Ion ||= 60 ||= 5 ||
 * = Disulphide Bond ||= 40 ||= 2.2 ||
 * = Ion-dipole ||= 6 ||= 2.8 ||
 * = Salt bridge ||= 4-7 ||= 5 ||
 * = Hydrogen bond ||= 0.5-12 ||= 3-5 ||
 * = VdW ||= 1-4 ||= 3.5 ||
 * = kT ||= 0.6 ||= @310K ||
 * = GTP/ATP ||=  ||=   ||

Types of Secondary Bonds

-Induced dipole bonds: fluctuation in the position of charges can induce attractive forces in other atoms. This is seen in the liquification of inert gases such as H2 and Cl2. -Polar Molecule-Induced Dipole Bonds: permanent dipole moments exist in some molecules due to assymetrical structure. Such polar molecules can induce dipoles in nearby nonpolar molecules. -Permanent Dipole Bonds: these are Van der Waals forces between permanent dipoles. Hydrogen bonding is included, which is a strong bond that only exists between F, O or N.