Experiment #2. Lewis Structures

Transcription

1 Experiment #2. Lewis Structures A Lewis structure shows how the valence electrons are arranged and indicates the bonding between atoms in a molecule. We represent the elements by their symbols. The shared electron pair is shown as a line/bond between the two atoms. All the other valence electrons are shown as dots or lines around the symbol of the element. For Example: The Lewis structure for l 2 is Let us now see how to draw the Lewis structure for O 2 Steps for drawing Lewis Structures Example with O 2 1. Sum the valence electrons of all the atoms. For anions, add one electron for each negative charge. For cations, subtract one electron for each positive charge. 2. hoose the least electronegative element (other than ) as the central atom. For a molecule of type XY n, X is always the central atom. The total number of valence electrons is 16 (4 from carbon and 6 from each oxygen). hoose carbon as the central atom since it is less electronegative and also follows the XY n rule. 3. Use single bonds (lines) to connect the central atom to the surrounding atoms. 4. Subtract 2 electrons for each bond from the original total number of electrons. 5. omplete the octet for all outer atoms (other than ). Each bond to an atom will count as 2 electrons for that atom's octet. Attach the two oxygens to carbon by single bonds. O O We subtract 4 electrons (2 for each bond) from 16 leaving 12 electrons to distribute to the remaining atoms. omplete the octet for each of the oxygen atoms which uses the last 12 electrons. (We need to add 6 electrons to each oxygen.) 6. omplete the octet for the central atom. There are no more electrons, so we cannot add more to the central atom to complete its octet. 7. If you run out of electrons before the octet for the central atom can be completed, start forming multiple bonds until the central atom has a complete octet. When you form a multiple bond, remember to remove an electron pair from the outer atom. Since carbon does not have an octet, form multiple bonds by taking the lone pairs. becomes The central atom is still electron deficient, so share another pair. becomes 8. Make sure the correct number of electrons has been used and that every atom s octet is complete. The exceptions are atoms of group 2 and 3 which can have incomplete octets and atoms in period 3 or greater which can have expanded octets. 4 bonds + 2 electron pairs equal 16 electrons. All atoms have an octet.

2 Molecular Geometry the Valence Shell Electron Pair Repulsion (VSEPR) Theory The electron groups around the central atom repel each other and therefore prefer to be as far apart from each other as possible. This is the main idea of the VSPER theory. We can apply the VSEPR theory to predict the molecular shape/geometry of a molecule. 1. Draw the Lewis structure for the molecule in question. 2. ount the total number of electron groups on the central atom. Add the number of atoms bonded to the central atom and the number of lone pairs on the central atom this is the total number of electron groups. Note that multiple bonds to one outer atom still count as one electron group. 3. The arrangement of the electron groups is determined by minimizing the repulsions between them. 4. Remember that lone pairs require more space than bonding pairs. Therefore, choose an arrangement that gives lone pairs as much room as possible. Electron Groups 2 0 Lone Pairs 1 Lone Pair 2 Lone Pairs 3 Lone Pairs 4 Lone Pairs linear (180 o ) 3 trigonal planar (120 o ) bent tetrahedral (109 o ) trigonal pyramid bent 4 trigonal bipyramid (120 o, 90 o ) seesaw T- shaped linear 5 octahedral(90 o ) square pyramid square planar T- shaped linear 6

3 Polarity of Molecules A covalent bond is polar if there is a difference in electronegativity between the bonded atoms. A molecule like l has a polar covalent bond since there is a difference in electronegativity between the two atoms (the difference is greater than 0.4 and less than 1.8). Thus l possesses a permanent dipole moment because the molecule has a distinct negative end and a distinct positive end. owever, just because there is a polar bond present in a molecule does not necessarily mean that the molecule is polar. If all the dipoles in the molecule cancel each other out, then the molecule will be non- polar. For example, O 2 has two polar bonds, but they point in opposite directions and cancel each other out. For simple molecules, polarity can usually be determined by looking for lone pairs on a central atom or non- identical outer atoms. Both of these things cause uneven electron distribution and generally lead to polar molecules. Polar molecules bond dipoles do not cancel. ave a lone pair on the central atom AND/OR different outer atoms. Non- polar molecules bond dipoles cancel. ave no lone pairs on the central atom AND have all outer atoms the same. ondensed Structural Formulas Organic (carbon- based) molecules can contain chains of several central atoms. ondensed formulas are often used to indicate how the atoms are connected. To draw a Lewis structure from a condensed structure, connect any elements that are not hydrogens or halogens in a chain in the order written. onnect the hydrogens and halogens with single bonds to the atom they are written next to. ount valence electrons and add lone pairs and double/triple bonds to complete octets as needed. Molecular Formula ondensed Formula Lewis structure 3 8 O 3 2 O 3 O 3 8 O O O F F N 3 2 NF 3 F 3 2 N F

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5 Name M112 Lab Lewis Structures Grading Rubric riteria Points possible Points earned Question 1 (1.33 points each question) 8 Question 2 (1 point each question) 4 Question 3 (1 point each question) 2 Question 4 (1 point each question) 4 Question 5 (1 points each question) 2 Total 20 Subject to additional penalties at the discretion of the instructor.

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7 M 112: Lewis Structures Lab Name 1. Fill in the table below. 2 l 2 Lewis Structure (redraw to reflect geometry, show net dipole direction if polar) Polar? Y/N Geometry Br 4 Br 2 O NBr O at? at O? 5 10 O (connect s & O in a 6- membered ring. s are attached to s ) at? at O?

8 2. Draw the Lewis Structures of each of the following acids: (These structures, like most acids, have each attached to one of the oxygens. (A) 2 SO 4 (B) IO 2 () 3 PO 3 (D) F 3 O 2 (Both O atoms are attached to second ) 3. Draw the Lewis Structure for these anions. Each anion is a conjugate base of an acid. 2 (E) O 3 ( attached to one of the O s) (F) SO 3 4. Weak bases frequently contain N: (nitrogen with a lone pair of electrons). Draw the Lewis Structure for each of the following weak bases. (A) N 3 (B) 3 ON 2 (onnectivity is N ) O () 3 N 2 (D) 5 5 N (six membered ring made of and N, s attached to s) 5. Draw the Lewis structures of the following cations. Each cation is the conjugate acid of a weak base: + + (A) N 4 (B) 3 N 2 3