1. Basic Concepts of Chemical Bonding
Topics Covered
Chemical bonds
Lewis symbols
Octet rule
Ionic bonds
Lattice Energy
Homework
Exercises 8.5, 7, 13, 17, 20, 23, 24
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2. Chemical Bonds
Whenever atoms or ions are strongly attracted to on another, we say that there is a chemical bond between them
An ionic bond is an electrostatic attraction that exists between ions of different charges
A covalent bond results from the sharing of electrons between two atoms
A metallic bond is where metal atoms are bound to the surrounding atoms but electrons are free to move throughout the structure

3. Lewis Symbols
The Lewis symbol for an element consists of the chemical symbol for the element plus a dot for each valence e-
Sulfur has the e-config. [Ne]3s23p4, so its Lewis symbol shows 6 valence e-

4. The Octet Rule
Atoms tend to gain or lose e- to achieve the same number of e- as the nearest noble gas to them
Because all noble gases have 8 valence e-, the guideline that atoms tend to gain, lose, or share e- until they have 8 valence e- is referred to as the octet rule
An octet of e- consists of full s and p subshells

5. The Octet Rule

6. Transition Metal Ions
Because of their d orbitals, transition metals often can form multiple ions
Fe [Ar]4s23d6
Can lose 2 e- from 4s
Fe2+ [Ar] 3d6
Can lose one more from 3d to have a stable half-full sublevel
Fe3+ [Ar]3d5
Transition metals generally do not form ions with noble-gas configurations, which shows the limitations of the octet rule

7. Ionic Bonding
When sodium atoms transfer electrons to chlorine atoms, crystals of the ionic solid sodium chloride are formed
In the sodium chloride crystal, each Na+ ion is surrounded by six nearest-neighbor Cl- ions, and each Cl- ion is surrounded by six nearest-neighbor Na+ ions

8. Ionic Bonding

9. Ionic Bonding
NaCl is an example of a very typical ionic compound because it consists of a metal with low ionization energy and a nonmetal with high electron affinity
Reactions to form ionic compounds from their elements are generally very exothermic
Reaction of sodium metal with chlorine gas

10. Ionic Bonding
A measure of how much stabilization results from arranging oppositely charged ions in an ionic solid is given by the lattice energy
Lattice energy is the energy required to completely separate one mole of solid ionic compound into its gaseous ions
NaCl(s)  Na+(g) + Cl-(g)
∆Hlattice = +788 kJ/mol

11. Ionic Bonding
The magnitude of the lattice energy of a solid depends on the charges of the ions, their sizes, and their arrangement
For a given arrangement of ions, the lattice energy increases as the charges on the ions increase and as their radii decrease
Ionic radii do not vary over a very wide range, so the lattice energies depend primarily on the ionic charges

12. Ionic Bonding

13. Sample Problems
Arrange the following ionic compounds in order of increasing lattice energy: NaF, CsI, and CaO
Answer: CsI < NaF < CaO
Which substance would you expect to have the highest lattice energy, AgCl, CuO, or CrN?
Answer: CrN

14. Sample Problems
One of the following pictures represents NaCl and one represents MgO. Which is which, and which has the larger lattice energy?
Answer: (a) is NaCl, (b) is MgO, MgO has greater lattice energy

15. Ionic Compounds
There is no way to calculate lattice energies experimentally
They can, however, be calculated using Hess’s law
The sequence of five steps that will be summed is best shown by constructing a Born-Haber cycle

16. The Born-Haber Cycle for NaCl

17. The Born-Haber Cycle for NaCl
Energy to vaporize sodium metal (standard heat of formation)
Energy to create monatomic chlorine (standard heat of formation)
Energy needed to ionize sodium gas (ionization energy)
Energy released when chlorine ionizes (electron affinity)
Energy released when gaseous ions form ionic crystal (negative lattice energy)

18. -411 kJ = 107 kJ + 122 kJ + 496 kJ - 349 kJ - ∆Hlattice
∆Hlattice = 787 kJ/mol

19. Classwork Problems
Use this data to estimate the lattice energies of NaBr and MgCl2
Answers:
NaBr = 747 kJ/mol
MgCl2 = 2524 kJ/mol