Unit 2- Module 3-1

Unit 2- Module 3-1 - Module 3 Predicting Geometry John...

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Module 3 Predicting Geometry John Pollard University of Arizona The molecular structure of a compound is dependent on the position of the atoms, the atom connectivity and molecular geometry. All three factors are dependent on valency of the atoms involved and on the affinity that the atoms have for electrons which is related to their ionization potentials. To illustrate, let’s look at 3 cases and illustrate some simple rules that can be used to build the Lewis structures. Case 1 - H 2 O Rule 1 : Choose the central atom; never H as it only forms 1 bond. The central atom tends to be the atom with the lowest 1 st ionization energy. In the case of water, O is the central atom. It has a higher ionization potential than H but H is never going to be the central atom. O H H Rule 2: Count up the total number of valence electrons. In the case of H 2 O, there are 6 from O and 1 from each H giving a total of 8. Due to spin pairing, we can treat the electrons as pairs. 8 electrons is 4 pairs. Rule 3: Connect all the peripheral atoms to the central atom with single bonds (1 pair each) O H H This uses 2 pairs and leaves 2 left to be place. Rule 4: Use the remaining pairs to satisfy the octet rule in each atom. Start with the outside atoms and work in (H is excluded of course as it will always have 1 bond). Any left over pairs of electrons should be place as lone pairs on the central atom. Multiple bonds are formed from lone pairs to complete the unfilled octets of atoms. O H H This is now the complete Lewis structure for H 2 O. The octet for O is satisfied by placing 2 lone pairs of electrons on it. Periodic Behavior 1 st IE increases
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Case 2 - CH 2 O Rule 1 : Choose the central atom; never H as it only forms 1 bond. The central atom tends to be the atom with the lowest 1 st ionization energy. In CH 2 O, C has the lowest ionization potential (it is further to the left and down on the periodic table) so it is the central atom. The other atoms are then place around it. O C H H Rule 2: Count up the total number of valence electrons. 4 for C, 6 for O and 1 for each H makes 12 total or 6 pairs. Rule 3: Connect all the peripheral atoms to the central atom with single bonds (1 pair each) O C H H This uses 3 pairs and leaves 3 left to use. Rule 4: Use the remaining pairs to satisfy the octet rule in each atom. Start with the outside atoms and work in (H is excluded of course as it will always have 1 bond). Any left over pairs of electrons should be place as lone pairs on the central atom. Multiple bonds are formed from lone pairs to complete the unfilled octets of atoms. O C H H As is always the case, one bond to H completes its bonding. According to the rule, the remaining 3 pairs are placed on O to complete its octet. The only problem is that C still does not have a complete octet. In the case where a central atom cannot complete its octet and there are no more electron pairs to distribute, lone pairs from the outside atoms
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should be moved in to form multiple bonds. In this case, moving 1 lone pair to a bonding pair creates a C=O double bond while completing the octets of both.
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