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UW CHEM 110 - Chem 110 Lecture 9 REPRESENTATION chemical bonds and lewis structures

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10/14/20131Wed, Oct 16• Lecture 9 (Representation)– Chemical Bonds (4.11, 12.1)– Lewis Structures (12.6)• Questions we’ll answer:– What is a chemical bond?– How can we use the electronic structure of atoms to represent the bonding patterns in simple molecules?Reading the Periodic Table“B” groups“A” groups13 4 5 6 7 8 9 10 11 12 13 14 15 16 17 181A3B 4B 5B 6B 7B 8B 1B 2B3A 4A 5A 6A 7A 8ANumber of electrons increases with Z: • left to right across a period, • top to bottom down a group.The A group headings will be most useful for our purposes. 10/14/20132HeNeArKrXeRnMetals tend to lose electrons to form cations.Non‐metals tend to gainelectrons to form anions.• Atoms in groups 1A and 2A will lose electrons to attain an electron structure like the noble gas in the previous period (row).− Ca (Z = 20), period 4: Ca2+has electronic structure lik e Ar (Z = 18), period 3• Atoms in groups 5A, 6A, and 7A, will gain electrons to attain an electron structure like the closest noble gas in the same period (row). − N (Z = 7), period 2: N3‐has electronic structure lik e Ne (Z = 10), period 2• Metalloids and nonmetals in groups 3A, 4A, and 8A don’t typically form ions.• Many transition metals are capable of forming more than one kind of cation, but there is no easy trend we can identify.Chemical BondsIonic bonds form when electrons are “transferred” from a metal to a non‐metal.Covalent bonds form when two non‐metals share electrons.The driving force for the formation of chemical bonds is the attainment of a complete “octet” of electrons in the valence shell.Note: In the case of H and He, we refer to a complete “duet” instead, because the n = 1 level can only contain two electrons.10/14/20133Ionic CompoundsSodium + Chlorine: http://youtu.be/Ftw7a5ccubs , http://youtu.be/Mx5JJWI2aawAn ionic compound contains a cation (formed from a metal) and a anion (formed from a nonmetal).The transfer of an electron from sodium to chlorine results in two oppositely‐charged ions. The electrostatic attraction between Na+and Cl–causes them to “stick” together. An ionic solid consists of an extended network of anions and cations.Ionic CompoundsWhen ionic compounds form, the overall charge of the compound must be zero.This requirement determines the relative number of cationsto anions in the formula unit.10/14/20134A compound contains an unknown ion X and has the formula XCl2. Ion X contains 20 electrons. What is the identity of X?Covalent CompoundsH.H.HHNote that this line represents a pair of electrons.In a covalent bond, electrons are shared between the atoms rather than transferred. The driving force for covalent bond formation is to attain a full outer‐most electron shell. 10/14/20135Lewis Dot StructuresThree basic steps for drawing Lewis structures:1. Sum the valence electrons for all atoms to determine total number of electrons.2. Use pairs of electrons to form a bond between each pair of atoms (bonding pairs).3. Arrange remaining electrons around atoms (lone pairs and/or multiple bonds) to satisfy the “octet rule” for each atom (“duet” rule for hydrogen). A Lewis structure is a 2D representation of the bonding pattern in a molecule.Central idea: the most stable arrangement of electrons is one in which all atoms have a “noble” gas configuration.An atom typically forms as many bonds as it has “holes” in its valence shell.Bonding PairLone PairLDS Mechanics (cont.)• An example: CH48 e-0 e-HCH HHHCHHHDone!–8 e-(bonding)CHHHH10/14/2013620 e-OClCl16 e-4 e-–4 e-(bonding)– 12 e-(lone, Cl)0 e-–4 e-(lone, O)OClClAn example: Cl2ODrawing Lewis StructuresOCl ClDraw the Lewis structure for H2O10/14/20137Draw the Lewis structure for OH–Draw the Lewis structure for H2CCl210/14/20138Multiple bonds• Sometimes atoms have to share more than one pair of electrons in order to fulfill the octet rule, like O2.OOOOO OO O12 e-10 e-4 e-–2 e-(bonding)–6 e-(lone, O)0 e-–4 e-(lone, O)OCOOCO16 e-12 e-0 e-–4 e-(bonding)– 12 e-(lone, O)Example: Draw the Lewis structure for CO2.OCOMultiple BondsCOO10/14/20139Draw the Lewis structure for CODraw the Lewis structure for H2CO10/14/201310LDS Guidelines1. In a polyatomic molecule, the atom that can make the largest number of bonds typically goes in the center.2. H can only form one bond, so it goes on the outside of the molecule...H is a terminal atom.3. F is also a terminal atom. It never forms more than one bond.– Halogens in period 3 and beyond can form more than one bond in certain situations. 4. The –OH (“hydroxy” ) group is a common motif in chemistry.5.


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UW CHEM 110 - Chem 110 Lecture 9 REPRESENTATION chemical bonds and lewis structures

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