Polarity

Intermolecular Attractions

Anywhere molecules are located, they will have intermolecular forces (also known as Van der Waals forces), forces that interact between molecules and keep them together. Molecules can have positive and negative sides based on their electronegativity differences, becoming polar and attracting opposite polarities much like a bunch of magnets stuck together in a location. This polar force is known as a dipole-dipole force. The types of bonds that make up a molecule (ionic, covalent, and metallic) are instead known as intramolecular forces, or forces that keep molecules together.

There are two other intermolecular forces. The first one, hydrogen bonding, is pretty much the same as a dipole-dipole bond but is related to hydrogen creating polarity in a molecule. It is stronger than dipole-dipole forces thanks to the small size of hydrogen, allowing closer and more tightly-packed polar bonds. This occurs most often when a Hydrogen atom is directly bonded to either Oxygen, Nitrogen, or Fluorine.

The final type of intermolecular force is the London Dispersion Force. This type of force happens in all molecules (even nonpolar ones) and is the weakest of the intermolecular forces. Since electrons constantly move around atoms this movement makes temporary dipoles based on the locations the electrons are in.

Molecular Geometry

Solids are made of closely packed particles and have strong intermolecular forces (or even actual bonds between the particles) with not enough energy to break these bonds apart. This is what gives them a fixed structure no matter what type of chemical bond is holding them together. As solids gain more energy they can eventually overcome these forces and transform into liquids, with some bonds needing more energy to do so than others.

All 3 types of chemical bonds can create solids!

Questioned Documents (PSA)

Since different molecules have different polarities we can actually use this idea of polarity to separate inks and dyes, which tend to be a blend of different liquids to make a certain color. The process of separation to use in this is called paper (or thin layer) chromatography. Some dyes are attracted to the water more than the paper while others have the opposite happen, stretching out a dot of the ink or dye into its individual components. This information is then used to compare printer inks, pen ink, clothing dye, or the like, allowing more information to be gathered about evidence found at a crime scene (such as if they used a certain printer, who manufactured a pen, etc.). In addition to using polarity on documents we can also do handwriting analysis or using digital imaging with UV or IR light to determine information about documents.

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