Post on 06-May-2018
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IONIC electrons traded to form separate ions
Mary J. Bojan Chem 110
COVALENT electrons shared between a few nuclei distinct molecules
METALLIC electrons shared among all nuclei
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Atoms (or ions) combine (bond) to form molecules (or compounds).
There are three forms of bonding.
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1. Valence electrons are involved in bonding
2. Octet rule: elements tend to gain, lose or share electrons so as to gain an inert gas configuration. (Duet for H and He) Ø
IONIC COMPOUNDS share some Characteristic properties
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1. low electrical conductivity as
solids (high as liquids) 2. very high melting, boiling
points 3. hard but brittle 4. soluble only in polar solvents
(water)
• distinct +/− ions • ∞ crystal lattice • strong omnidirectional
electrostatic bonds
By knowing properties, it is easier to distinguish Ionic vs. Molecular
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Ionic compound: a compound that consists of positive and negative ions. Metal + nonmetal (usually)
Molecular compounds: compounds consisting of individual molecules All nonmetals or nonmetals and metalloids.
Molecular compounds usually have low melting points
Ionic compounds have high melting points and are brittle.
Eg. NaCl K2SO4 Mg(NO3)2 NH4Cl
Eg. HCl H2O CH3COOH NH3
Ionic bonding occurs when ions combine to form compounds (salts).
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Electrons are exchanged to form separate ions with complete octets.
Ionic bond (interaction between the ions) is due to electrostatic attraction.
Coulomb's Law is place to start.
Lattice energy is the change in energy when an ionic solid is separated into
isolated ions in the gas phase.
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Lattice energy cannot be determined experimentally
How do we determine Lattice energy? Born-Haber cycle (10.8)
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Using known properties of atoms, lattice energy can be calculated. Process is known as the Born-Haber cycle.
Use Coulomb’s Law (charge and size) to explain trends in Lattice Energy
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LiF E = 1017 kJ MgO E = 3890 kJ Why the big difference???
dQQE 21!
Q1 +1 +2 Q2 −1 −2
d (0.76+ 1.33)/2 d= 1.045Å
(0.72 + 1.40)/2 d =1.06Å
E
Use Coulomb’s Law (charge and size) to explain trends in Lattice Energy
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What if the charge is the same? Q1Q2 = −1 for all of these salts d
QQE 21!
LiI NaI KI d= 1.48Å d= 1.61Å d= 1.79Å
Ion sizes are important in ionic bonds.
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Which has the greatest lattice energy? (How do you know?) NaF NaCl NaBr NaI
lattice energy can’t be measured: what can we measure that is related to lattice energy?
Melting point increases as lattice energy increases.
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mp NaF 993oC NaCl 801oC NaBr 747oC NaI 661oC MgO 2800oC
TRANSITION METAL IONS are exceptions to octet rule.
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• s electrons are part of valence electrons
• transition metals can have variable charges.
When forming an ion: • s electrons are lost first • then maybe d electrons