Chemistry

The three major types of bonding are ionic bonds, covalent bonds, and metallic bonds.  In all bonds, electrons shift around as atoms try following the octet rule.

• Ionic Bonds: one atom gives up electrons to gain a positive charge, and another atom takes new electroncs to gain a negative charge.  These bonds tend to be incredibly strong, but also very brittle — the atoms are held together by opposing electric charges.

• Covalent Bonds: two or more atoms share electrons to create a single molecule.  These bonds can be very strong within the molecule, but the connections between molecules might not be as strong.  However, the massive size differences in covalent molecules lead to huge variations in the chemical properties.

• Metallic Bonds: a collection of metallic atoms will share their outer electrons.  Because the electrons are very mobile, these bonds tend to be both strong and flexible.

Basic Names: Ionic and Covalent Compounds

When atoms bond together, we need ways to name the resulting compound.  The name of the compound tells you the number of atoms and how they're bonded together.

Advanced Naming: Organic Compounds

Larger molecules require special naming conventions.

Typically, these are molecules built around carbon — and these molecules are everywhere.  We refer to them as organic molecules because carbon compounds form the basis of life on Earth.  From your DNA to your cell walls to the synapses connecting your neurons, you are largely built from these organic carbon molecules.

Carbon is an extremely versatile element, so our civilization is also highly dependent on carbon molecules.  From plastics to clothing to the gas you put in your car, much of our modern world is built on carbon compounds.

Lewis Structures offer a quick and easy way to draw the basic structures of molecules.  To make a Lewis Structure, you simply write the Periodic Table abbreviations of the atoms in their locations, and then add in dots and lines to represent the electrons and their bonds.

Now that you've seen the basics for naming and diagraming molecules, let's consider the actual shapes.  How do the atoms position themselves in space?

These arrangments affect the physical properties of the molecules:

• Polar molecules like water have negative charges on one side and positive charges on the other — this makes them stick together more easily, so they're more likely to be liquids or solids.

• Nonpolar molecules like methane have the same charge on all sides, so they don't stick together very easily, so they're more likely to be gases.

How do we know the shapes of molecules?  If you have a chain of carbon atoms with other atoms pasted on at intervals, how does that give us the twisted double helix of DNA?

To understand the shapes of molecules, we look at the positions of the electrons.  We use VSEPR Theory to predict how atoms will arrange themselves when bonded together.

VSEPR stands for Valence Shell Electron Pair Repulsion:

• Valence Shell: these are the outer electrons of an atom, the ones furthest away from the nucleus.  Note that these outer shells are also the ones involved in chemical bonding, so we're also looking at how the bonds arrange themselves.

• Electron Pair Repulsion: electrons are naturally grouped in pairs, and those pairs have negative charges.  Those negative charges repel each other.

So if we look at how the outer electrons of each atom repel each other, we can predict how the bonds will be positioned in order to reduce the repulsion.