Chapter 18Chapter 18

The Representative The Representative Elements: Groups 1A Elements: Groups 1A

Through 4AThrough 4A

Chapter 18: The Representative Elements: Groups 1A through 4A

18.1 A Survey of the Representative Elements

18.2 The Group 1A Elements

18.3 The Chemistry of Hydrogen

18.4 The Group 2A Elements

18.5 The Group 3A Elements

18.6 The Group 4A Elements

The ignition of hydrogen soap bubbles.

Figure 18.1: The periodic table.

Figure 18.2: Some atomic radii (in picometers).

Figure 18.3: Structure of quartz

Sand, such as that found in the massive sand dunes bordering the desert plain near Namib, Namibia, is composed of silicon and oxygen.

Source: Getty Images

Properties Based Upon ns1 Electron Configuration

1. Low heat of atomization ( Hatom) (Energy needed to convert the solid to individual gaseous atoms) M(s) M(g) Hatom ( L i > Na > K > Rb > Cs )

2. Low IE and High charge density a great decrease in size occurs when the one valence electron is lost!

Vol of Li+ is < 13% of that of Li - thus M+ ions are small with a great charge density.3. High Lattice Energy

small cations pull very close to large anions, yielding large quantities of energy liberated upon crystallization Trend - as the cation becomes larger, the lattice energy becomes smaller (less negative)

4. Hydration Energy E+(g) + H2O(l) E+

(aq)

- HHydr ( Li+ > Na+ > K+ > Rb+ > Cs+ )

The smaller ions attract water molecules strongly enough to formlarger hydrated ions: Li+

(aq) is larger than Cs+(aq)

Sodium reacts violently with water

Reactions of the Alkali Metals with water

All of the alkali metals react with water to form hydrogen and the hydroxide ion. Li, Na, K, Rb, and Cs react I order of reactivity, The largest atom Cs is the most reactive, and the reactivity decreases as theyget smaller and smaller, with Li being the least reactive.

Lithium and Sodium are less dense than water, and react on the water surface, Potassium is so reactive it catches the hydrogen released catchesFire from the exothermic reaction. Rubidium and Cesium literally explode when they come in contact with water. They all produce solutions of the alkali metal hydroxides, but the reactivity increases down the group. This reactivity increase is due to the increasing diameter of the atoms, So that the attractive forces on the one valence electron is much less, and It is free to leave.

2 M(s) + 2 H2O(l) H2 (g) + 2 OH-(aq) + 2 M+

(aq)

Fig. 14.3 (P 547)

Reactions of Alkali Metals with Oxygen - I

All alkali metals will form oxides in the form of M2O, but onlylithium will form this oxide in excess oxygen:

4 Li(s) + O2 (g) 2 Li2O(s)

Sodium in an excess of oxygen will form the peroxide:

2 Na(s) + O2 (g) Na2O2 (g)

The Peroxide anion reacts with water to from Hydrogen peroxide and hydroxide anions.

Na2O2 (s) + 2 H2O(l) 2 Na+(aq) + 2 OH-

(aq) + H2O2 (aq)

Reactions of Alkali Metals with Oxygen - II

Potassium, Rubidium and Cesium react with oxygen to form super-oxidesWith the general formula of MO2 for example potassium:

K(s) + O2 (g) ____________

The superoxides release oxygen gas when they react with water or carbon dioxide gas. For this reason they are very useful in emergency breathers for fireman or emergency workers.

2 MO2 (s) + 2 H2O(l) 2 M+(aq) + 2 OH-

(aq) + O2 (aq) + H2O2 (aq)

4 MO2 (s) + 2 CO2 (g) 2 MCO3 (s) + 3 O2 (g)

Important Reactions of the Alkali Metals - I

1. The alkali metals reduce hydrogen to form ionic (salt like) hydrides:

NaH is an industrial base and reducing agent that is used to prepare other reducing agents, such as NaBH4.

2. The alkali metals reduce halogens to form ionic halides:

3. Sodium chloride is the most important alkali halide. a) In the Downs process for the production of sodium metal, by the electrolysis of molten NaCl:

2 M(s) + H2 (g) 2 MH(s)

2 M(s) + X2(-) 2 MX(s) ( X = F, Cl, Br, I)

2 NaCl(L) 2 Na(L) + Cl2 (g)

electricity

Important Reactions of the Alkali Metals - II

3. continued b) In the chlor-alkali process, NaCl(aq) is electrolyzed to form several key industrial chemicals:

c) In its reaction with sulfuric acid, NaCl forms two major products:

Sodium sulfate is important in the paper industry; HCl is essential in steel, plastics, textiles, and food production.4. Sodium hydroxide is used in the formation of bleaching solutions:

5. In an ion-exchange process water is “softened” when Na+ is displaced by “hard-water” ions (M2+):

2 NaCl(aq) + 2 H2O(l) 2 NaOH(aq) + H2 (g) + Cl2 (g)electricity

2 NaCl(s) + H2SO4 (aq) Na2SO4 (aq) + 2 HCl(g)

2 NaOH(aq) + Cl2 (g) NaClO(aq) + NaCl(aq) + H2O(l)

Mg2+(aq) + Na2(resin)(s) Mg(resin)(s) + 2 Na+

(aq)

Important Compounds of the Alkali Metals1. Lithium chloride and lithium bromide, LiCl and LiBr. Because the Li+

ion is so small, Li salts have a affinity for H2O and yet a positive heat of solution, so they are used in dehumidifiers and air-cooling units.2. Lithium carbonate, Li2CO3. Used to make porcelain enamels and toughened glasses and as a drug in the treatment of manic-depressive disorders.3. Sodium chloride, NaCl. Millions of tons used in the industrial production of Na, NaOH, Na2CO3/NaHCO3, Na2SO4, HCl, and purified for use as table salt.4. Sodium carbonate and sodium bicarbonate, Na2CO3 and NaHCO3. Carbonate used as an industrial base and to make glass. Bicarbonate, which releases CO2 at low temperatures (500 to 1000), used in baking powder and in fire extinguishers.5. Sodium hydroxide, NaOH. Most important industrial base; used to make bleach, sodium phosphates, and alcohols.6. Potassium nitrate, KNO3. Powerful oxidizing agent used in gunpowder and fireworks.

Where Does Hydrogen Belong?

Hydrogen gas being used to blow soap bubbles

As the bubbles float upward, they are lighted by using a candle on a long pole.

Hydrides of Alkali Metals - I

Ionic Hydrides are formed with Group IA and IIA metals.

2 Li(s) + H2 (g) 2 LiH(s)

Ca(s) + H2 (g) CaH2 (s)

These Hydrides react with water to release hydrogen gas.

LiH(s) + H2O(l) Li+(aq) + OH-

(aq) + H2 (g)

CaH2 (s) + 2 H2O(l) Ca+2(aq) + 2 OH-

(aq) + 2 H2 (g)

Covalent Hydrides are formed by the reaction of hydrogen with non-metals.

Examples are: HCl, NH3, CH4, and H2O

Hydrides of Alkali Metals - II

Metallic or interstitial Hydrides These are hydrides formed with metals and vary a great deal.

Hydrogen gas can occupy the interstitial holes in metals due to their small size.

Palladium can absorb 900 times it’s volume of hydrogen gas

Some metallic Hydrides can be formed such as: UH3 and FeH6

Most metallic hydrides are interstitial or non-stoichiometric compositions. such as: LaH2.76 or VH0.56

Figure 18.5: Structure of ice, showing the hydrogen bonding

Standing in Group 2A(2), Looking Backward to 1A(1) and Forward to 3A(13)

Fig. 14.6 (P 551)

Important Reactions of the Alkaline Earth Metals - I

1. The metals reduce O2 to form the oxide:

Barium also forms the peroxide BaO (s).2. The Metals of higher atomic weight reduce water to form hydrogen gas:

Be and Mg form an adherent oxide coating that allows only slight reaction.3. The metals reduce halogens to form ionic halides:

4. Most of the metals reduce hydrogen to form ionic hydrides.

2 M(s) + O2 (g) 2 MO(s)

M(s) + 2 H2O(l) M(OH)2 (aq) + H2 (g)

M = Ca, Sr and Ba

M(s) + X2(-) MX2 (s) X = F, Cl, Br, I

M(s) + H2 (g) MH2 (s) all except Be

Calcium metal reacting with water to form bubbles of hydrogen gas.

Important Reactions of the Alkaline Earth Metals - II

5. Most of the metals reduce nitrogen to form ionic nitrides:

6. Except for amphoteric BeO, the oxides are basic:

7. All carbonates undergo thermal decomposition to the oxide:

MCO3 (s) ____________________

This reaction is used to produce CaO (lime) in huge amounts from naturally occurring limestone, and was the reaction used to generate carbon dioxide to smother the graphite fire in the Chernobyl reactor.

3 M(s) + N2 (g) M3N2 (s) all except Be

MO(s) + H2O(l) M(OH)2 (aq)

Important Compounds of the Alkaline Earth Metals

1. Beryl, Be3Al2Si6O18. Beryl occurs as a gemstone with a variety of colors. It is chemically identical to emerald, except for the trace of Cr+3 that gives emerald its green color. Beryl is the industrial source of Be metal.2. Magnesium oxide, MgO. Because of its high melting point (28520C), it is used as a refractory material for furnace brick and wire insulation.3. Alkylmagnesium halides, RMgX (R = hydrocarbon group; X = halogen). These compounds, called Grignard reagents, are used to synthesize many organic compounds. Organotin agricultural fungicides are made by treating RMgX with SnCl4.

4. Calcium carbonate, CaCO3. Occurs as enormous natural deposits of limestone, marble, chalk and coral. Used as a building material, to make lime, and, in high purity, as toothpaste abrasive and antacid.

3 RMgCl + SnCl4 3MgCl2 + R3SnCl

Figure 18.6: The structure of solid BeH2

Crystalline Beryl - BeCl2

Source: Photo Researchers, Inc.

Figure 18.7: Solid BeCl2 can be visualized as consisting of many BeCl2 molecules

A) At high temperatures, BeCl2 occurs as a gaseous molecule with only four electrons around Be.B) In the solid state, BeCl2 occurs in long chains with each Cl bridging two Be atoms, which gives each Be an octet.

Figure 18.8: (a) A schematic representation of a typical cation exchange resin.(b) and (c)

hard water

Gallium melts in the hand

Figure 18.9: The structure of B2H6

Diborane A borane

The Two Types of Covalent Bonding in Diborane

The DimericStructureof GaseousAluminumChloride

Important Reactions of Boron GroupElements - I

1. The elements react sluggishly, if at all, with water:

2. When strongly heated in pure O2, all members form oxides:

Oxide acidity decreases down the group:B2O3 (weakly acidic) > Al2O3 > Ga2O3 > In2O3 > Tl2O (strongly basic) for Tl, the +1 oxide is more basic than the +3 oxide.

2 Ga(s) + 6 H2O(Hot) 2 Ga3+(aq) + 6 OH-

(aq) + 3 H2 (g)

2 Tl(s) + 2 H2O(steam) 2 Tl+(aq) + 2 OH-

(aq) + H2 (g)

4 M(s) + 3 O2 (g) 2 M2O3 (s) M = B, Al, Ga, In

4 Tl(s) + O2 (g) 2 Tl2O(s)

Important Reactions of Boron Group Elements - II

3. All members reduce halogens (X2) :

BX3 are volatile covalent molecules. Trihalides of Al, Ga, and In are (mostly) ionic solids but occur as covalent dimers in the gas phase; in this way, the 3A atoms attains a filled outer level.

4. Acid treatment of Al2O3 is important in water purification:

2 Al2O3 (s) + 3 H2SO4 (l) Al2(SO4)3 (s) + 4 H2O(l)

In water, Al2(SO4)3 and CaO form a colloid that aids in removing suspended particles.

2 M(s) + 3X2 (-) 2 MX3 (-) (M = B, Al, Ga, In)

2 Tl(s) + X2 (-) 2 TlX(s)

Important Reactions of Boron Group Elements - III

5. The overall reaction in the production of aluminum metal is a redox process:

This electrochemical process is carried out in the presence of cryolite (Na3AlF6), which lowers the melting point of the reactant mixture and takes part in the change.

6. A displacement reaction produces gallium arsenide, GaAs:

2 Al2O3 (s) + 3 C(s) 4 Al(s) + 3 CO2 (g)

(CH3)3Ga(g) + AsH3 (g) _________________

Important Compounds of Boron Group Elements - I

1. Boron oxide, B2O3. Used in the production of borosilicate glass.

2. Borax, Na2[B4O5(OH)4] 8H2O. Major mineral source of boron compounds and B2O3. Used as a fireproof insulation material and as a washing powder (20-Mule Team Borax).

3. Boric acid, H3BO3 [ or B(OH)3]. Used as external disinfectant, eyewash, and insecticide.

4. Diborane, B2H6. A powerful reductant for possible use as a rocket fuel. Used to synthesize higher boranes, compounds that led to new theories of chemical bonding.

5. Aluminum sulfate (alum), Al2(SO4)3 18H2O. Used in water purification, tanning leather, and as an antiperspirant.

.

.

Important Compounds of Boron Group Elements - II

6. Aluminum oxide, Al2O3. Major compound in natural source (bauxite) of Al metal. Used as abrasive in sandpaper, sanding and cutting tools, and toothpaste. Large crystals with metal ion impurities often of gemstone quality. Inert support for chromatography. In fibrous forms, woven into heat-resistant fabrics; also used to strengthen ceramics and metals.

7. Tl2Ba2Ca2Cu3O10. Becomes a high-temperature superconductor at 125 K, which is readily attained with liquid Nitrogen. (77K)

C-C : ethane(C2H6), benzene (C6H6), graphite, diamondB-N : amine-borane(BNH6), borazine (B3N3H6), boron nitride, borazon

Standing in Group 4A(14), Looking Backwardto 3A(13) and Forward to 5A(15)

Important Reactions of the Carbon Family - I

1. The elements are oxidized by halogens:

The +2 halides are more stable for tin and lead, SnX2 and PbX2.

2. The elements are oxidized by O2:

Pb forms the +2 oxide, PbO. Oxides become more basic down the group. The reaction of CO2and water provides the weak acidity of natural unpolluted waters:

M(s) + 2 X2(-) MX4(-) (M = C, Si, Ge)

M(s) + O2(g) MO2(-) (M = C, Si, Ge, Sn)

CO2(g) + H2O(l) [H2CO3(aq)] H+(aq) + HCO3

-(aq)

carbonic acid

Important Reactions of the Carbon Family - II

3. Air and steam passed over hot coke produce gaseous fuel mixtures (producer gas and water gas):

4. Hydrocarbons react with O2 to form CO2 and H2O. the reaction for methane can be adopted to yield heat or electricity:

The gas is used to make other organic compounds and as a fuel in welding.

C(s) + air(g) + H2O(g) CO(g) + CO2(g) + N2(g) + H2(g)

(not balanced)

CH4(g) + 2 O2(g) __________________

Important Reactions of the Carbon Family- III

6. Freons (chlorofluorocarbons) are formed by fluorinating carbon tetrachloride:

Production of trichlorofluoromethane (Freon-11), the major refrigerant in the world, is being eliminated because of its severe effects on the environment, the ozone destruction in the stratosphere.

7. Silica is reduced to form elemental silicon:

This crude silicon is made ultrapure through zone refining for the manufacture of computer chips.

CCl4(l) + HF(g) CFCl3 (g) + HCl(g)

SiO2(s) + 2 C(s) Si(s) + 2 CO(g)

Important Compounds of the Carbon Family - I

1. Carbon monoxide, CO. Used as a gaseous fuel as a precursor for one- carbon organic compounds, and as a reactant in the purification of nickel. Formed in internal combustion engines and released as a toxic air pollutant.2. Carbon dioxide, CO2. Atmospheric component used by photosynthetic plants to make carbohydrates and O2. The final oxidation product of all C - based fuels; its increase in the atmosphere is leading to global warming. Used industrially as a refrigerant gas, blanketing gas in fire extinguishers, and effervescent gas in beverages. Combined with NH3

to form urea for fertilizers and plastics manufacture.3. Methane, CH4. Used as a fuel and in the production of many organic compounds. Major component of natural gas. Formed by anaerobic decomposition of plants (swamp gas) and by microbes in termites and certain mammals. May contribute to global warming.

Important Compounds of the Carbon Family - II

4. Silicon dioxide, SiO2. Occurs in many amorphous (glassy) and crystalline forms, quartz being the most common. Used to make glass and as an inert chromatography support material.

5. Silicon carbide, SiC. Known as carborundum, a major industrial abrasive and a highly refractory ceramic for tough, high-temperature uses. Can be doped to form a high-temperature semiconductor.

6. Organotin compounds, R4Sn. Used to stabilize PVC (polyvinyl chloride) plastics and to cure silicone rubbers. Agricultural biocide for insects, fungi, and weeds.

7. Tetraethyl lead, (C2H5)4Pb. Once used as a gasoline additive to improve fuel efficiency, but now removed because of its inactivation of auto catalytic converters. Major source of lead as a toxic air pollutant.

Roman baths such as these in Bath, England, used lead pipes for water

Source: Getty Images

Lead (II) oxide