Download Ionic bonding

Lecture 7: Chemical bonding
Lecture 7 Topics
1. Basic bonding background information
• Types of chemical bonds
• Valence electrons & Lewis structures
• The ‘octet rule’
Brown, chapter 8
8.1
2. Ionic bonding
• Lattice energy
• Charge & size affect lattice energy
• Transition metal ions
8.2
3. Covalent bonding
• Lewis structures & unequal sharing
• Unequal sharing & electronegativity
• Bond polarity
• Guidelines for Lewis Structures
• Dipole moment & overall molecular polarity
• Multiple bonds & formal charge
8.3
4. Resonance structures
8.6
1.
8.7
Exceptions to the octet rule
8.4
8.5
Ionic bonding joins metals & non-metals
Electron transfer satisfies octets.
Lattice energy measures the strength of ionic bonds.
Ionic bonding
Ionic bonding…
…the bonding of a metal to a non-metal via transfer of electrons &
electrostatic attraction. Ionic compounds are commonly called ‘salts’.
• Salt is a generic term referring to all ionic compounds.
Na + 1/2 Cl2  NaCl
(solid)
(gas)
H°f = - 410.9 kJ/mol
(solid)
NaCl is a 3D array of Na+ and Cl- ions that form
a repeating array, aka a crystal.
• The size of the crystal depends on the
number of salt units. Size can increase!
How does NaCl form? Na loses an e-, and that same e- is transferred to Cl.
• Both atoms become ions and are oppositely charged: Na+1 & Cl-1.
• Electrostatic force causes the ions to come together to form salt crystals.
..
.. 1
.
.
+1
Na. + .Cl.
Na + .Cl: ..
..
Energy is released as sodium bonds to chloride; shown by (-) ΔHf.
• Because energy state is lower with a full valence shell.
p.291-300
Lattice energy
How much energy is stored in the ionic bonds of a salt?
Lattice energy: the energy needed to separate 1 mole of salt into elemental gases.
NaCl  Na+1(g) + Cl-1(g)
H = +788 kJ/mol
Why isn’t lattice energy equal in magnitude but opposite in sign to the energy
(enthalpy) of formation (ΔHf)?
Because here the products are in a gas state, a higher energy state.
• The sodium used to form salt, the reactant, was a solid.
• Remember, it takes energy to break bonds
There’s another way to express lattice energy, E:
E = k(Q1Q2)
Q is ionic charge
d
d is distance between ions
k is a constant = 8.99x109 J-m/C2
So how do charges and distance effect lattice energy?
• Increasing magnitude of charges increases lattice energy.
• Increasing distance decreases lattice energy.
• So… large lattice energies mean that ions are strongly attracted to each other.
p.291-300
Charge & size affect lattice energies
Remember how to assess charge & size?
Charge from marked-up periodic table or column number.
Size? Cations < atoms < anions
p.293
Examples: lattice energies
Which have the highest and lowest lattice energies? NaF, CsI, CaO?
1) What about charges?
All are +1/-1 except Ca+2, O-2.
 CaO has largest E
2) So NaF and CsI differ only in distance between atoms = atomic radius.
The atomic radii of Na & F are smaller than Cs & I.
3) Thus, in rank order highest to lowest E:
CaO > NaF > CsI
Which has the highest E: AgCl, CuO, or CrN?
Based on charge CrN; +3/-3.
Predict the charges on ions that form when Mg reacts with N.
Mg+2 and N-3
p.293