Lewis Structures - Assignment For Chemistry

Lewis Structure Fundamentals In previous lessons, we have investigated the composition of chemical compounds; that is, which elements are present in each, and in what atomic proportions. Knowledge of the chemical formula for a compound is important, but it is not the whole story in determining the identity of a compound (many may have the same chemical formula as one another!) or in predicting the properties of a compound. For a complete picture, we want to know not just the composition of a compound, but also its constitution; that is, which specific atoms are chemically bonded to one another in a molecule of the compound. There are many ways to represent the constitution of a compound; we will start in this course with the Lewis structure (named after Gilbert N. Lewis, who originated the idea of the covalent bond as a way for atoms to complete their valence electron shells without forming ions). The Lewis structure is the most detailed structural convention that chemists tend to use – the position of every atom and every valence electron (whether bonding or non-bonding) is accounted for explicitly, as are any non-zero formal charges present in the molecule. In this exercise, we will practice drawing reasonable Lewis structures for a variety of compounds. Some general tips for drawing Lewis structures consisting mostly of p-block elements are as follows: 1. Count the valence electrons in the compound based on the chemical formula and the number of valence electrons each neutral atom should contribute. Adjust our valence electron count appropriately if your compound is charged. Add one electron for a negative charge, and subtract one for a positive charge. 2. Draw a reasonable skeletal structure showing which atoms are connected to one another, using single bonds only. Elements with lower group number or lower electronegativity tend to be central atoms. Hydrogen atoms are virtually always terminal atoms, because they can only accommodate two electrons (a single bond). Subtract the number of electrons in your skeletal structure (two for each bond) from the total number of valence electrons you have to work with to see how many remain. 3. Add electrons to peripheral atoms to complete their octets. Subtract the number of electrons added in this way from your electron tally. If you still have valence electrons that need placing, add them to the central atom. 4. Once all valence electrons have been accounted for, check that all atoms have complete valence shells (duets for hydrogen, octets for other p-block elements). If any atom does not have a complete octet, you might need to use lone pairs on adjacent atoms to form double or triple bonds to complete the valence shell. Give it a try! Go through the process with the compound phosphorus trifluoride (PF3). PF3: Total Valence Electrons: __26____________ Skeletal Structure Complete Peripheral Octets Final Structure Valence Electrons Remaining: Valence Electrons Remaining: Valence Electrons Remaining: 2 ZERO 20 Page 1 of 6 When dealing with ions, first calculate the number of valence electrons you would have without any charges, then adjust your total to account for charges. Don’t forget to label formal charges! AlCl4–: Total Valence Electrons: __32____________ Skeletal Structure Valence Electrons Remaining: 24 Complete Peripheral Octets Final Structure Valence Electrons Remaining: Valence Electrons Remaining: ZERO ZERO NH4+: Total Valence Electrons: ___8___________ Skeletal Structure Complete Peripheral Octets Final Structure N/A Valence Electrons Remaining: Valence Electrons Remaining: Valence Electrons Remaining: ZERO ZERO ZERO CH3+: Total Valence Electrons: __6____________ Skeletal Structure Complete Peripheral Octets Final Structure N/A Valence Electrons Remaining: Valence Electrons Remaining: Valence Electrons Remaining: ZERO ZERO ZERO Consider electronegativity differences to decide whether atoms ought to participate in covalent or ionic bonds. Generally, electronegativity differences of ~2 or more result in ionic bonds. NaNH2: Total Valence Electrons: __8____________ Skeletal Structure Complete Peripheral Octets Final Structure Valence Electrons Remaining: FOUR Valence Electrons Remaining: ZERO Valence Electrons Remaining: ZERO Page 2 of 6 When proposing a skeletal structure, keep in mind how many bonds each element typically prefers to form in order to minimize formal charge. CH4O: Total Valence Electrons: __14____________ Skeletal Structure Complete Peripheral Octets Final Structure N/A Valence Electrons Remaining: FOUR Valence Electrons Remaining: FOUR Valence Electrons Remaining: ZERO It may prove necessary to introduce double or triple bonds in order to complete octets and minimize formal charges. N2H2: Total Valence Electrons: __12____________ Skeletal Structure Complete Peripheral Octets Valence Electrons Remaining: Valence Electrons Remaining: 6 ZERO Final Structure Valence Electrons Remaining: ZERO C2HBr: Total Valence Electrons: ___16___________ Skeletal Structure Complete Peripheral Octets Final Structure Valence Electrons Remaining: 10 Valence Electrons Remaining: ZERO Valence Electrons Remaining: ZERO It may not always be possible to have no formal charge at all for an overall neutral compound. HNO3: Total Valence Electrons: ___24___________ Skeletal Structure Complete Peripheral Octets Valence Electrons Remaining: Valence Electrons Remaining: 16 ZERO Final Structure Valence Electrons Remaining: ZERO Page 3 of 6 Remember, the octet rule is not ironclad. BH3: Total Valence Electrons: __6____________ Skeletal Structure Complete Peripheral Octets Final Structure N/A Valence Electrons Remaining: Valence Electrons Remaining: Valence Electrons Remaining: ZERO ZERO ZERO HIO4: Total Valence Electrons: __32____________ Skeletal Structure Complete Peripheral Octets Valence Electrons Remaining: Valence Electrons Remaining: 22 ZERO Final Structure Valence Electrons Remaining: ZERO XeF5+: Total Valence Electrons: __42____________ Skeletal Structure Complete Peripheral Octets Final Structure Valence Electrons Remaining: Valence Electrons Remaining: Valence Electrons Remaining: 32 TWO ZERO CaSO4: Total Valence Electrons: __32____________ Skeletal Structure Complete Peripheral Octets Final Structure Valence Electrons Remaining: 24 Valence Electrons Remaining: ZERO Valence Electrons Remaining: ZERO Page 4 of 6 Sometimes you will encounter species with an odd number of valence electrons, which will have unpaired electrons (called “radicals”) in the final structure. My favorite way to deal with this situation is to imagine that the compound has one more electron (to get to the next even number), build the structure, then remove one electron from the least electronegative atom that has non-bonding electrons on it. NO2: Total Valence Electrons: __17____________ Skeletal Structure Complete Peripheral Octets Final Structure Valence Electrons Remaining: Valence Electrons Remaining: Valence Electrons Remaining: ZERO ZERO 13 CFCl2: Total Valence Electrons: ___25___________ Skeletal Structure Valence Electrons Remaining: 19 Complete Peripheral Octets Final Structure Valence Electrons Remaining: Valence Electrons Remaining: 1 ZERO You may find that there is more than one good way to draw a valid Lewis structure starting by different positioning of double/triple bonds and non-bonding electrons on the same skeleton. This phenomenon is called “resonance,” and all such Lewis structures contribute to the real “hybrid” structure of the compound, with more “important” resonance structures making a greater overall contribution to the hybrid structure. How to tell which structures are more important? When charges are present, a good way is to consider which atoms will better accommodate those charges. Draw two good Lewis structures for the cyanate anion in the last box. Which would be more important, and why? CNO–: Total Valence Electrons: __16____________ Skeletal Structure Complete Peripheral Octets Final Structures (more important) (less important) Valence Electrons Remaining: Valence Electrons Remaining: Valence Electrons Remaining: 12 ZERO ZERO The more important structure places the negative charge on an oxygen atom, which is more electronegative than a nitrogen atom, so it is reasonable to expect that the negative charge will be “happier” on oxygen than on nitrogen. Page 5 of 6 Once there are more than a handful of atoms present in a molecule, there may be multiple good skeletal structures to start from, with different atomic partners bonded to one another. This phenomenon is called isomerism, and it is the reason that organic chemistry, which typically deals only in a handful of s- and p-block elements, is nevertheless a field of great diversity. Even the cyanate anion from the previous problem, with only three elements, has an isomer, the fulminate anion, which is explosive. Draw fulminate. Hint: it doesn’t obey the rule that the least electronegative element is central. CNO– : Total Valence Electrons: __16____________ Skeletal Structure Complete Peripheral Octets Final Structure Valence Electrons Remaining: Valence Electrons Remaining: Valence Electrons Remaining: 12 ZERO ZERO Can you come up with three neutral isomers with formula C2H5N? Two start from very similar skeletons, differing in the placement of one hydrogen. You might have to think outside the box to get that third one. C2H5N: Total Valence Electrons: __18____________ Skeletal Structure Complete Peripheral Octets Final Structure Valence Electrons Remaining: Valence Electrons Remaining: Valence Electrons Remaining: 4 ZERO ZERO Skeletal Structure Complete Peripheral Octets Final Structure Valence Electrons Remaining: Valence Electrons Remaining: Valence Electrons Remaining: 4 ZERO ZERO Skeletal Structure Complete Peripheral Octets Final Structure Valence Electrons Remaining: Valence Electrons Remaining: Valence Electrons Remaining: 4 ZERO ZERO I can think of one more. Can you? Page 6 of 6