1. 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)
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2. 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 CO2
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3. 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)
Each of these, in different circumstances, must lower energy relative to “being separated” or they would not “occur”.
Interactions between molecules are called “intermolecular forces”—Chapter 11!
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4. 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); DH = lattice energy for NaCl (DHlattice)
Is lattice energy positive or negative?
Negative. Energy is released when things that attract get to come closer together.
What do you think determines the magnitude of this energy?
Force of attraction between the ions! (Coulomb’s Law!)
Stronger force  larger negative lattice energy
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5. Can you Rationalize This Trend?
If charges on ions are the same in two compounds:
smaller distance…
 stronger force of attraction
Hint 1: Recall the trend in ionic radii (Li+ vs Na+)
Hint 2: Sketch an “ion pair” of each.
 larger negative lattice energy
Hint 3: Apply Coulomb’s Law!
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6. Can You Predict Relative Value for CaO?
If interionic distance is similar in two compounds: greater charges on ions…
More negative or less negative?
 stronger force of attraction
Hint 1: Na+ and Ca2+ have similar radii, and F- and O2- also have similar radii
 larger negative lattice energy
Hint 2: Sketch an “ion pair” of each and apply Coulomb’s law!

7. Q: Is Lattice Energy a Periodic Property?
No! The Periodic Table contains elements.
Periodic Properties are properties of elements or atoms (or monatomic ions, in families)
Lattice Energy is a property of a compound.
However…
L.E. depends on some periodic properties of ions: radii and charges!
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8. Noble Gases are “stable” because of shielding effects: It takes energy to oxidize OR reduce them
Figure: UN
Title:
The Octet Rule
Caption:
Atoms tend to gain or lose electrons in order to attain an electron configuration resembling a noble gas. Since the outermost shell in these cases is comprised of an s orbital and three p orbitals, the maximum number of electrons in the valence shell is eight (the octet).

9. A Few Generalizations For Monatomic Ions (recall Ppt 21)
(Small IE1 for alkali metals)
(Large IE1 for noble gases)
Result: Many cations adopt an s2 p6 configuration (like noble gas)
Figure: UN
Title:
The Octet Rule
Caption:
Atoms tend to gain or lose electrons in order to attain an electron configuration resembling a noble gas. Since the outermost shell in these cases is comprised of an s orbital and three p orbitals, the maximum number of electrons in the valence shell is eight (the octet).
(no attraction for added e-; PE won’t be lowered)
(attraction for added e-; PE will be lowered for halogens)
Result: Anions tend to adopt an s2 p6 configuration (like noble gas)

10. A Few Generalizations For Monatomic Ions (recall Ppt 21)
Cations tend to adopt a noble gas configuration (s2 p6)
It becomes “prohibitively costly” to remove electrons once you “hit” the core (Zeff huge)
E.g., Na+, Mg2+, Al3+
Anions tend to adopt a noble gas configuration (s2 p6)
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, Zeff is ~0
E.g., Cl-, O2-, N3-
Tentative “conclusion”? Noble gas configurations (s2 p6, 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!
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11. 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].
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
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12. Why (and when) does sharing electrons lower energy?
If shared electrons get to “see” two nuclei, that’s “more positive charge” than just one.
 Lowers potential energy!
Only happens if the electrons get to be inside the other atom’s valence shell
Otherwise, Zeff ~ 0; won’t be attracted! (Recall He-!)
Consider H2 as an example
Why not H3? (See board)
Limit to sharing is often “when s and p orbitals are full”
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13. 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
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14. Characteristics of “Good” LDS’s (I)
Skeleton Structure is Reasonable
No H’s in “middle”
Generally element lower and left in center
(more on this later)
The correct # of valence e-’s is shown
No more and no less!!
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 (nvalence ≥ 3)]
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15. Procedure: Generating an Initial “Good” LDS
Create a Skeleton Structure (if one not provided)
Calculate total valence e-’s for one F.U.
Add pairs of electrons to outer atoms until each has 8 electrons around it (unless H)
Shared electrons count toward the 8
If you have “extra” electrons after Step 3, put them on the central atom (no matter what!)
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 3rd row or below!! Those atoms have d orbitals available in their valence shell.
If an ion, add brackets and the overall charge.
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16. Examples: Lewis Structures For the Following (done on board)
PCl3 N2
CO2
SO2
SO42-
PO43-
XeF2 16

17. Practice: Draw Lewis Structures For the Following
Br3- O3
ClF3
HCN 17