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Chapter 9

CHEM 1F92 Chapter Notes - Chapter 9: Lewis Structure, Lattice Energy, Ionic Compound

6 pages71 viewsFall 2013

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Lydia W.L.Chen

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Chapter 9: Chemical Bonding
9.1 – Lewis Dot Symbols
Gilbert Lewis stated that atoms combine with one another to achieve a more stable electron
configuration that is isoelectronic with that of a noble gas.
Lewis dot symbols consist of the symbol of an element surrounded by dots, representing the valence
electrons in one atom of the element.
We cannot draw Lewis dot structures for the transition metals, lanthanides, and actinides since they
have inner shells that are not filled.
oFor example, since the 4s orbital fills before the 3d orbitals do, the fourth shell begins to fill
before the inner, third shell is filled.
The unpaired dots in the Lewis dot structures represent the maximum number of bonds that can form.
9.2 – The Ionic Bond
An ionic bond is the electrostatic force that holds ions together in an ionic compound.
oUsually, they involve a Group 1A/2A metal cation and an oxide or halide cation.
When LiF forms, Li atoms lose an electron (from outer 2s orbital) to F, forming Li+. Consequently, the F
atom, which just gained the electron (in 2p orbital), forms F-. The attraction of these ions forms LiF.
In ionic compounds involving oxygen, the O2 splits into individual O atoms and then gains electrons to
form anions.
Two Li atom loses an electron from its 2s orbital to the 2p orbital of a single O atom. This forms 2 Li+
and 1 O2-.
9.3 – Lattice Energy of Ionic Compounds
Ionization energy and electron affinity do NOT define the stability of an ionic compound since they
depend on the compound being in gaseous state, and all ionic compounds at 1 atm and 25 ºC are solid.
Since many anions interact with a single cation (and vice versa) in the 3D network of an ionic compound,
the stability of n ionic compound is related to many interactions, not just one interaction.
Lattice energy is the amount of energy required to separate one mole of a solid ionic compound into its
gaseous ions.
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Lattice energy determines the stability of the ionic compound (↑ Lattice Energy = ↑ Stability).
Lattice Energy can be calculated using Coulomb’s law OR the Born-Haber Cycle
Coulomb’s law can be used to calculate the potential energy (E) between two ions in an ionic compound.
Since one ion is (+) and the other one is (-), E < 0 and the formation of the bond between 2 ions is an
exothermic process. Hence, separating an ionic compound into ions (lattice energy) is endothermic (-E).
The Born-Haber cycle assumes that the formation of an ionic compound takes place in a series of steps,
and each step is involves ionization energies/electron affinities/other properties. An example of the Born-
Haber cycle for the formation of LiF is shown in the PPT slide.
Sublimation is the conversion of a solid to a
gas (endothermic process). Dissociation is the
splitting of F2 (g) into F (g) (endothermic
process). It is 150.6 kJ/mol. Since only ½ mol
of F2 are converted to F, the value is 0.5(150.6
kJ) = 75.3 kJ. The ionization of Li to Li+
requires energy and is endothermic. The
electron affinity of F is 328 kJ. However,
since enthalpy is the negative of electron
affinity, the energy change is (electron
affinity) or -328 kJ. The energy released when
bonding the gaseous Li+ and F- into solid LiF
(∆H5º) can be calculate from the other values.
Since this value is the energy to for LiF(s), the energy to break LiF(s), or the lattice energy is -∆H5º is 1017
For a compound to be stable, the lattice energy must be greater than the sum of the ionization energies and
electron affinities involved in the formation of ions.
oThe lattice energy of LiF(s) is greater than the net energy required to form Li+ (ionization energy)
and the F- (electron affinity).
9.4 – The Covalent Bond
The attraction of the nucleus to the electrons of both atoms involved in the covalent bond helps to maintain
the bond.
The Lewis structures use dots for lone pairs. Bonded pairs can be shown using dots for the electrons, or
lines representing the bonds.
Octet Rule states that all atoms other than H form covalent bonds so that they have 8 valence electrons.
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