IONIC COMPOUNDS

Ionic Compounds The nucleus of an atom is unchanged by chemical reactions (number of protons never changes, except for radioactive decay) 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. Let’s consider the formation of a very common ionic compound, NaCl (s)

Ionic Compounds We know that Na(s) and Cl2(g) react together to form NaCl (s), but how? The most important thing to know about chemical reactions is that atoms undergoing a reaction will always seek to reach a noble gas configuration Let’s look at the electron configurations of Na and Cl

Mechanism of an Ionic Reaction Na: [Ne] 3s1 Cl: [Ne] 3s2 3p5 For Na, the nearest noble gas is Ne. To reach the Ne configuration, it needs to lose a single electron. We know that metals have low ionization energies and can readily give up electrons. In this example, the Na atom is oxidized. Na ( [Ne] 3s1 ) ---> Na+ ([Ne]) + e- 11 e- 10 e- 1st Ionization Energy 11 p+ 11 p+ Neutral Na atom Na+ cation

Mechanism of an Ionic Reaction Electron affinity describes the energy released when an electron is added. Na: [Ne] 3s1 Cl: [Ne] 3s2 3p5 Now, in order for Na to lose its electron, another element must be available to accept it. For Cl, the nearest noble gas is Ar. To reach the Ar configuration, it needs to gain a single electron. In doing so, the Cl atom is reduced. Every oxidation process requires a simultaneous reduction Cl ([Ne] 3s2 3p5) + e- ---> Cl- ([Ar]) 18 e- 17 e- 1st Electron Affinity 17 p+ 17 p+ Cl- anion Neutral Cl atom

Mechanism of an Ionic Reaction There is an energy change in Cl that corresponds to the addition of the new electron. This is called electron affinity. If an element wants an electron, this value is negative, meaning that the atoms energy is decreased (stabilized). Na and Cl can simultaneously achieve a noble gas configuration if an electron is transferred from the metal (Na) to the nonmetal (Cl) [Ne] 3s1 + [Ne] 3s2 3p5 ---> Na+ Cl- [Ne] [Ar] IONIC COMPOUND

Predicting and Balancing Charge 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. Since the Na loses an electron to become Na+, and Cl gains an electron to become Cl-, only one of each ion is needed to balance the charge. In ionic compounds, the metal is always positively charged (cation) and the nonmetal is always negatively charged (anion)

Predicting and Balancing Charge metals nonmetals 1+ 2+ 3+ 3- 2- 1-

Group Examples 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)

Dissolving Ionic Compounds in Water Most ionic compounds dissolve in water by dissociation, forming hydrated ions. Na+ Cl- Na+(aq) + Cl- (aq) H2O (L) Here, NaCl is the solute, water is the solvent

Dissolving Ionic Compounds in Water Na+ Cl- 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-

Ionic Radii Cations tend to be smaller than their neutral atom counterparts, and anions seem to be larger Electrons in anions repel, which leads to expansion of the electron cloud. The excess positive charge in cations draws the electron cloud closer to the nucleus + e- - e- Cation, X+ Neutral X Anion, X-

Ionic Radii

Energy Changes In Reactions The electrostatic attraction, or the electrical attraction between positive and negative ions, is what holds an ionic compound together When two ions form an ionic compound, there is an overall change in energy. We can calculate this energy by considering: the ionization energy of the metal the electron affinity of the nonmetal 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 steps Form Na+ (ionization energy) Form Cl- (electron affinity) Join them together (coulombic energy)

Energy Changes In Reactions 1. (Ionization of Na) Na(g)  Na+(g) + e- ΔEI = +0.824 aJ *Positive sign means energy is added. 2. (Ionization of Cl) Cl(g) + e-  Cl-(g) ΔEEA = -0.580 aJ *Negative sign means energy is released. 3. (Coulombic energy) Na+(g) + Cl-(g)  Na+Cl-(s) ?

Coulombic Energy The third step is to join the two ions, as shown below. rNa = 102 pm   rCl = 181 pm The equation shown above is Coulomb’s Law, which gives the energy change (Ec) that results when two ions come together. 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. (231 aJ•pm)

Electron Configurations of Transition Metals 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

Polyatomic Ions 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) H2O (L) Sodium Phosphate Sodium cations Phosphate anion

KNOW YOUR POLYATOMIC IONS !!!!! 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 +1 Ammonium NH4+ Charge Name Structure -3 Phosphate PO43-

Group Examples Write out chemical formulas or names for the following: Calcium nitrate Sodium sulfate NH4OH Fe(CH3COO)3 WS2 Vanadium (V) carbonate Silver (I) phosphate Aluminum perchlorate