Week 9

This week in AP chemistry we delved into greater detail on the electrostatic forces between atoms and their effects on the different characteristics of the substance they make up. Each of these separate classifications has its own particular set of rules determining which of its properties are affected by its particular structure. The separate classifications are atomic bonding structure, bonds formed, boiling/ melting point and vapor pressure.

The way in which a substance is made up overall will typically have the largest magnitude in determining the properties of that substance. First, with the lowest melting and boiling points in most cases, covalently bonded compounds are molecules made up of nonmetal elements who share their electrons in order to complete their octets. Such as carbon dioxide, nitrogen trifluoride and sulfur dichloride, these elements have the weakest total molecular forces, resulting in smaller quantities of energy required to pull them further apart. Next, substances with ionic bonds generally have much higher melting and boiling points than compounds with covalent bonds, and for this reason they are more commonly encountered as solids at room temperature, such as sodium chloride, better known as table salt, a combination of sodium cations and chlorine anions. The final group with the highest melting and boiling points, are often solids that can not be melted at all. These substances are known as network solids. They are composed of elements with covalent bonds, but no particular distinctions of individual molecules. Instead, these substances are composed of  "macromolecules," a single lattice structure of a very large quantity of atoms, all with covalent bonds between those next to it. Common examples of this include diamond and graphite, both structures of only carbon atoms bonded together in one piece, and quartz, composed of a large network of silicon dioxide. Their molecular bond strength comes from the continuous network of bonds within the entire substance, and that melting them would require breaking all of the covalent bonding (See figures below).

Quartz

Many characteristics of substances are further determined by the specific types of bonding that occur within the molecule. Especially present in covalent bonding, the presence of extra van der Waal forces each increase melting and boiling points. Typically the weakest, London Dispersion Forces (LDFs) are present in all substances, although to varying degrees. The strength of this force depends on the polarizability of the molecule, which increases with the size of the molecule, factoring in the number of elements and their sizes, which is ultimately the area with which they may form temporary dipoles with those next to it. Next, dipole interactions increase melting and boiling as a result of increasing the attraction between the positive and negative sides of separate atoms and requiring more energy to pull them apart. Most significant when present, hydrogen bonding can greatly increase the melting and boiling points of these compounds. The particular reasoning behind this trend was not explicitly described to us, but I have concluded that this may only occur between hydrogen and the elements nitrogen, oxygen and fluorine because only these elements have greater electronegativity values than hydrogen, making it negative and the hydrogen positive, and more likely to bond with the typically negative outside of another molecule.

For an additional recap of molecular trends, try the link below:
http://www.masterorganicchemistry.com/2010/10/25/3-trends-that-affect-boiling-points/