By: Howard Andres

In Experiment 12, the purpose of the lab was to examine the geometrical structure of molecules by using molecular models.  With lab partners, the molecular models were constructed by using different colored balls with different amounts of holes, sticks, and springs.  The different colors represented different atoms, such as carbon, hydrogen, nitrogen, chlorine, oxygen, sulfur, and so on.  The amount of holes on each ball represented the amount of bonds that the different atoms were able to make, such as one hole on the hydrogen ball for hydrogen being able to make one bond, and four holes on the carbon for carbon being able to make four bonds with its valence electrons.  The sticks, when connected with other atoms, represented bonding pairs.  When sticking out of a ball and unconnected, the sticks represented lone pairs of electrons.  Springs were used to connect double or triple bonds between two atoms, or balls.

Before the models for the different molecules listed in the book were constructed though, the Lewis structure diagrams of the different molecules were made.  This was done by determining the total amount of valence electrons each atom in the molecule was able to have, and bonding them together with the other atoms in the molecule until each atom had its octet (hydrogen having a maximum of making one bond), and that the total amount of valence electrons in the diagram equaled the determined amount of valence electrons each molecule was able to have.  After this, the construction of the different molecules were made with the balls, sticks, and springs.  From these models, it was easier to see the kind of molecular geometry each molecule had because the holes in the balls were arranged in such a way so to see the kind of bond angles that were made between other atoms and lone electron pairs.  Shapes that were made in the experiment included tetrahedral (109.5^o), bent (<120^o, 105^o), trigonal pyramidal (107^o), linear (180^o), and trigonal planar (120^o).

The final part of the lab was the determination of whether the molecules were polar, and whether each molecule had any isomers or resonance structures.  Covalent bonds between different kinds of atoms from the central atom are said to be polar because of the different strength of pulls each different atom has on the surrounding electrons (electronegativity).  Lone pairs of electrons on a molecule also make it polar.  Resonance structures occur when more than one structure can be made for a compound as a result of double or triple bonds.  Isomers occur when there is a double or triple bond between two carbon atoms and two different pairs of atoms are able to be positioned around the carbons in a way to form -cis and -trans isomers.  -Cis isomers are where one pair of atoms are on one side of the carbons, and the other pair is on the other side.  -Trans isomers are where each pair of atoms are positioned diagonally from each other around the central carbon atoms.  This lab was a great aide in showing the kinds of molecular shapes different molecules make three-dimensionally.

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