How does QM Model of Atoms relate to chemical bonding (Ch. 9) ? Coulomb’s Law predicts strength of “ionic bonding” (between ions) –“lattice energy” Finally answer the question “Why do two neutral atoms have an attraction for one another?” –“covalent bonding” Simple model (tool) for describing covalent bonding: Lewis Dot Structures –More sophisticated model done afterward (Ch. 10) Copyright © Houghton Mifflin Company. All rights reserved.1

What is a bond? A bond is a state of lowered energy (PE) –Technically, an electron is “bonded” to a nucleus (lower PE when close) –Two water molecules are “bonded” to each other in a liquid (lower PE than when separated in a gas) –Na + ions and Cl - ions are “bonded” to each other in a formula unit of NaCl –C atoms are “bonded” to O atoms in a molecule of CO 2 Copyright © Houghton Mifflin Company. All rights reserved.2

But…in chemistry, the word “bond” is generally reserved for the latter two “cases” “Ionic bonding” occurs between ions –in the lattice of an ionic compound (solid state) “Covalent bonding” occurs between atoms –in a molecule of a molecular compound (or atoms in a polyatomic ion) Copyright © Houghton Mifflin Company. All rights reserved.3 o Interactions between molecules are called “intermolecular forces”—Chapter 11! o Each of these, in different circumstances, must lower energy relative to “being separated” or they would not “occur”.

Strength of Ionic Bonding Is Measured Experimentally by “Lattice Energy” Lattice Energy: The energy change associated with the formation of one mole of a crystalline ionic solid from its gaseous ions. Na + (g) + Cl - (g)  NaCl (s) ;  H = lattice energy for NaCl (  H lattice ) Copyright © Houghton Mifflin Company. All rights reserved.4 o Is lattice energy positive or negative? o What do you think determines the magnitude of this energy? Negative. Energy is released when things that attract get to come closer together. Force of attraction between the ions! Stronger force  larger negative lattice energy

Can you Rationalize This Trend? Copyright © Houghton Mifflin Company. All rights reserved.5 Hint 1: Recall the trend in ionic radii (Li + vs Na + ) Hint 3: Apply Coulomb’s Law! If charges on ions are the same in two compounds: smaller distance…  stronger force of attraction  larger negative lattice energy Hint 2: Sketch an “ion pair” of each.

Can You Predict Relative Value for CaO? 6 If interionic distance is similar in two compounds: greater charges on ions…  stronger force of attraction  larger negative lattice energy More negative or less negative? Hint 1: Na + and Ca 2+ have similar radii, and F - and O 2- also have similar radii Hint 2: Sketch an “ion pair” of each and apply Coulomb’s law!

Noble Gases are “stable” because of shielding effects: It takes energy to oxidize OR reduce them

(Small IE 1 for alkali metals) (Large IE 1 for noble gases) (no attraction for added e - ; PE won’t be lowered) (attraction for added e - ; PE will be lowered for halogens) A Few Generalizations For Monatomic Ions (recall Ppt 21) Result: Many cations adopt an s 2 p 6 configuration (like noble gas) Result: Anions tend to adopt an s 2 p 6 configuration (like noble gas)

A Few Generalizations For Monatomic Ions (recall Ppt 21) Cations tend to adopt a noble gas configuration (s 2 p 6 ) –It becomes “prohibitively costly” to remove electrons once you “hit” the core (Z eff huge) –E.g., Na +, Mg 2+, Al 3+ Copyright © Houghton Mifflin Company. All rights reserved.9 Anions tend to adopt a noble gas configuration (s 2 p 6 ) –Adding electrons is relatively favorable only until you start filling the next energy level (which occurs after the s and p sublevels of a level are full!); once you start the next level, Z eff is ~0 –E.g., Cl -, O 2-, N 3- Tentative “conclusion”? Noble gas configurations (s 2 p 6, not “filled shells” mind you!) seem to be favored or “stable” –But this is not a “law”!! It is a tendency. –It is wrong to use this argument as “reasoning”!! –Many exceptions!

Recall (PS3!): M + NM = Ionic Compound NM + NM = Molecular Compound M + NM results in electron TRANSFER from M to NM –To form cations and anions, which are attracted to one another by Coulomb’s Law forces (lattice energy idea). –IDEA: Transferring electrons lowers energy overall if ions get to be close to each other afterward [large negative lattice energy reflects large energy lowering]. Copyright © Houghton Mifflin Company. All rights reserved.10 NM + NM won’t result in electron transfer! –Why not? NOBODY “WANTS” TO BE THE CATION! (Takes too much energy!) –Any other way to lower energy?  SHARE VALENCE ELECTRONS (when it lowers energy)  Covalent bond (“Co-valent” = “sharing valence”) is a “shared pair” of electrons between atoms

Why (and when) does sharing electrons lower energy? Only happens if the electrons get to be inside the other atom’s valence shell –Otherwise, Z eff ~ 0; won’t be attracted! (Recall He - !) Copyright © Houghton Mifflin Company. All rights reserved.11  Limit to sharing is often “when s and p orbitals are full” Consider H 2 as an example –Why not H 3 ? (See board) If shared electrons get to “see” two nuclei, that’s “more positive charge” than just one.  Lowers potential energy!

Lewis Dot Structures (LDS) are Simple Models Showing Covalent Bonding Very useful, although not very rigorous theoretically Need to learn how to create and interpret LDS’s! Start with simple idea now –Add some complexity later Copyright © Houghton Mifflin Company. All rights reserved.12

Characteristics of “Good” LDS’s (I) 1.Skeleton Structure is Reasonable –No H’s in “middle” –Generally element lower and left in center (more on this later) 2.The correct # of valence e - ’s is shown –No more and no less!! 3.Atoms of non-metal elements from the second row will have 8 e - ’s around them (unless there is an odd # of v. e - ’s) –s & p sublevels filled totals 8 –[Can go past 8 if d orbitals are present (n valence ≥ 3) ] Copyright © Houghton Mifflin Company. All rights reserved.13

Procedure: Generating an Initial “Good” LDS 1.Create a Skeleton Structure (if one not provided) 2.Calculate total valence e - ’s for one F.U. 3.Add pairs of electrons to outer atoms until each has 8 electrons around it (unless H) Shared electrons count toward the 8 4.If you have “extra” electrons after Step 3, put them on the central atom (no matter what!) 5.Check central atom. If you have fewer than 8, make double or triple bonds until you get 8 If you already have 8 or more, do NOTHING!! If MORE than 8, the central atom had better be from 3 rd row or below!! Those atoms have d orbitals available in their valence shell. 6.If an ion, add brackets and the overall charge. Copyright © Houghton Mifflin Company. All rights reserved.14

Examples: Lewis Structures For the Following (done on board) PCl 3 N 2 CO 2 SO 2 SO 4 2- PO 4 3- XeF 2 15

Practice: Draw Lewis Structures For the Following Br 3 - O 3 ClF 3 HCN