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LECTURE 7: IONIC COMPOUNDS
LECTURE 7: IONIC COMPOUNDS
Ionic Compounds
• The nucleus of an atom is unchanged by chemical reactions (number of protons never changes)
• However, electrons are readily added and lost and ions are formed
• When a metal reacts with a nonmetal, ions form and attract. The result is an ionic compound, more commonly known as a salt.
The most important thing to know about chemical reactions is that atoms undergoing a reaction will always seek to reach a noble gas configuration
Na: [Ne] 3s1 Cl: [Ne] 3s2 3p5
• Consider the formation of NaCl. For Na, the nearest noble gas is Ne. To reach the Ne configuration, it needs to lose a single electron. We can see that Na+ is much more stable than Na.
Na ( [Ne] 3s1 ) ---> Na+ ([Ne]) + e-
11 p+
11 e-
1st Ionization Energy
11 p+
Neutral Na atom Na+ cation
10 e-
Mechanism of an Ionic Reaction
Na: [Ne] 3s1 Cl: [Ne] 3s2 3p5
• For Cl, the nearest noble gas is Ar. To reach the Ar configuration, it needs to gain a single electron. Cl- is much more stable than Cl.
Cl ([Ne] 3s2 3p5) + e- ---> Cl- ([Ar])• Electron affinity describes the energy change that occurs when an electron is added.
17 p+1st Electron Affinity 17
p+
Neutral Cl atom
Cl- anion
17 e-18 e-
Mechanism of an Ionic Reaction
• Na and Cl can simultaneously achieve a noble gas configuration if an electron is transferred from the metal to the nonmetal
[Ne] 3s1 + [Ne] 3s2 3p5 ---> Na+ Cl-
[Ne] [Ar]
IONIC COMPOUND
Cl-Na+
Lewis dot structure of the product
Mechanism of an Ionic Reaction
• So now, we understand that ionic compounds form when metal and nonmetal ions interact
• We also see why sodium chloride is NaCl, not NaCl2 or Na2Cl, etc.
The overall charge of any complete molecule must be zero.
• In ionic compounds, the metal is always positively charged (cation) and the nonmetal is always negatively charged (anion)
Predicting and Balancing Charge
1+
2+ 3+ 3- 2- 1-
metals nonmetals
Predicting and Balancing Charge
• Write the chemical formulas of the following ionic compounds:– Calcium oxide– Magnesium Chloride– Sodium Sulfide– Potassium Phosphide
• Determine the ionic product and balance. • Mg + O2 ?• Na + N2 ?
• Write the following reaction:• Lithium sulfide (s) + Strontium fluoride Lithium fluoride
(s) + Strontium sulfide(s)
Group Examples
• Ionic compounds completely dissociate in water, forming individual ions. Ions become completely ‘hydrated’.
Na+ Cl- Na+(aq) + Cl- (aq)
H2O (L)
• Here, NaCl is the solute, water is the solvent
Dissolving Ionic Compounds in Water
• Water molecules “solvate” ionic compounds, ripping the ions apart.
• The negative oxygen atoms (red) attracted to the positive Na+, and the positive hydrogens are attracted to the negative Cl-
Na+
Cl-
Dissolving Ionic Compounds in Water
• Ions in solution are capable of conducting electric current (hence, the term electrolyte). Ions are able to transport charge across the water.
– Non-ionic solutions (covalent) do not exhibit this property because they do not dissociate
Electrolytes
• Cations tend to be smaller than their neutral atom counterparts, and anions seem to be larger
• Excess negative charge (anions) causes repulsion in the electron cloud, which causes expansion of atom.
• Excess positive charge (cations) draws the electron cloud closer to the nucleus, shrinking the atom
Neutral X
Anion, X-
Cation, X+
+ e- - e-
Ionic Radii
• The electrostatic attraction (also called coulombic attraction) between positive and negative ions is what holds an ionic compound together. There are no actual chemical bonds in
• When two ions form an ionic compound, there is an overall change in energy.
• We can calculate this energy by considering:1. the ionization energy of the metal2. the electron affinity of the nonmetal3. the coulombic energy of attraction between the
cation and anion
Energy Changes In Reactions
• Lets revisit the reaction: Na(g) + Cl(g) NaCl(s) – Ignore the monatomic chlorine
• To form NaCl, there are 3 steps1. Form Na+ (ionization energy)2. Form Cl- (electron affinity)3. Join them together (coulombic energy)
Energy Changes In Reactions
1. (Ionization of Na) Na(g) Na+(g) + e- EIE > 0 *Positive value means energy is absorbed.
2. (Ionization of Cl) Cl(g) + e- Cl-(g) EEA < 0 *Negative value means energy is released.
3. (Coulombic energy) Na+(g) + Cl-(g) Na+Cl-(s) Ec = ?
Energy Changes In Reactions
Total energy change = sum of 1, 2 and 3
• The third step is to join the two ions, as shown below.
rNa = 102 pm rCl = 181 pm
• Q1 and Q2 are the charges of the metal and nonmetal• d is the distance between the nuclei. This is the sum of
the ionic radii.• k is a constant. (2.31 x 10 -16 J•pm)
• The equation shown above is Coulomb’s Law, which gives the energy change (Ec) that results when two ions come together.
Coulombic Energy
Ionic Radii
• Given the following data, calculate the energy of reaction to form CsCl given that the first ionization energy of Cesium is 0.624 aJ
Example
• When a transition metal forms an ion, electrons are first removed from the preceding s-orbital.
Fe: [Ar] 4s2 3d6
Fe2+: [Ar] 3d6
Fe3+: [Ar] 3d5
• If the ionization of a transition metal results in an unpaired s-electron, that electron will move into the valence d orbital
Ni: [Ar] 4s2 3d8
Ni+: [Ar] 4s1 3d8 ---> [Ar] 3d9
Electron Configurations of Transition Metals
• Transition metals can have multiple positive ionic charges. To distinguish, a roman numeral is placed in front of a transition metal in a compound to identify its charge.
• Ex. FeCl2 ---> Here, Fe is 2+. So, we name this compound:
Iron (II) chloride FeCl3 ---> Here, Fe is 3+.
Iron (III) chloride
• Name the following: TiO2, WCl6 Titanium (IV) oxide, Tungsten (VI)
chloride
Electron Configurations of Transition Metals
• Polyatomic ions are covalent molecules that possess charge and behave as normal ions in solution.
• When a salt containing a polyatomic ion is dissolved in water, the polyatomic ions themselves DO NOT break apart. They are simply separated from the counter-ion.
• Example: Phosphate (PO43-)
Na3PO4 (s) 3Na+ (aq) + PO43-(aq)
Sodium Phosphate
Sodium cations
Phosphate anion
H2O (L)
About Polyatomic Ions
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Charge Name Structure
-1
Hydroxide OH-
Cyanide CN-
Bicarbonate HCO3-
Acetate CH3COO-
Nitrate NO3-
Nitrite NO2-
Perchlorate ClO4-
Charge Name Structure
-2
Carbonate CO32-
Oxalate C2O42-
Sulfate SO42-
Sulfite SO32-
Charge Name Structure
-3 Phosphate PO43-
Charge Name Structure
+1 Ammonium
NH4+
KNOW YOUR POLYATOMIC IONS !!!!!
• Write out chemical formulas for the following:– Calcium nitrate– Sodium sulfate– Ammonium hydroxide– Iron (III) acetate– Tungsten (VI) cyanide– Aluminum perchlorate
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Group Examples
