Lewis Dot Structures
Lewis Dot structures help you predict how atoms will bond because they show the valence electrons available for bonding.
We're going to assume (for this class anyway) that bonding involves just the valence electrons. We already learned that atoms will ionize and transfer electrons (ionic bonds) or share electrons (covalent bonds) in order to reach the much coveted Noble Gas electron configuration. According to the Octet Rule* the magic number for such stability is "8" (2 electrons in an s orbital, 6 electrons in p orbitals).
Lewis Dot structures for Elements
We also learned that for Main Group (Representative) elements, the number of valence electrons is the same as the Group A number; all Group 1A elements have one valence electron, all Group 2A elements have two valence electrons and so on all the way over to the Noble Gases in Group 8A with 8 valence electrons. The picture below shows the Lewis Dot structure for the elements in Period (row) 2. Notice how the number of valence electrons for each Group matches its Group A number!
This should start your brain thinking... "yep," says your brain, "that explains why (in ionic bonding) metals give up electrons to gain a stable configuration while non-metals will gain electrons. If you're a metal it's easier to give up one electron than gain seven more; if you're a non-metal it's easier to gain one electron than give up seven." (Smart brain.) Lewis Dot structures can also help you predict how covalent bonds will form when electrons are shared between molecules.
If you have an iPad, check out Carlo Yuvienco's app Lewis Dots.
Lewis Dot structures for ions and simple compounds
So what does a Lewis Dot diagram look like AFTER an element becomes an ION? What do Lewis Dot diagrams for compounds look like?
IONS: Notice that once Sodium becomes a Sodium ION (1+) you do not draw the one valence electron because, well, it's gone! For chloride (the ion of chlorine) on the other hand, you draw 8 dots because when it ionizes chloride has one extra electron. The brackets and charges around the symbol indicate that you 1) have an ion and 2) show the charges.
MOLECULES: No ions means no brackets or charges to draw. Here the elements are SHARING electrons two at a time. The dashed line represents a single bond (sharing of two electrons). The remaining dots represent non-bonded pairs of electrons.
These resources will help you understand the Lewis Dot structures:
- Nice, simple introduction: http://www.ausetute.com.au/lewisstr.html
- This is a great tutorial but is built in Flash (sorry iPad/iPhone users!): http://www.wisc-online.com/objects/ViewObject.aspx?ID=GCH6404
This is a great little video on drawing basic Lewis Dot diagrams for elements and simple molecules.
How much do you need to know about Lewis Dot structures?
- Be able to determine the number of valence electrons for Main Group elements based on their position in the Periodic Table (Hint: It's the SAME as the Group A number!).
- Be able to draw Lewis Dot structures for Main Group elements.
- Be able to draw Lewis Dot structures for IONS of Main Group elements.
- Be able to draw Lewis Dot structures for simple molecular compounds.