chapter 19 the representative elements: group 1a through 4a

Chapter 19The Representative Elements:

Group 1A through 4AReviewing the Periodic Table Regions:

• Representative elements: Groups 1A - 8A - filling s and p orbitals (s and p valence electrons).

• Transition elements: Center of the table - filling d orbitals.

• Lanthanides, Actinides: listed separately, on the bottom of the table - filling 4f and 5f orbitals.

• Metalloids: separate metals from nonmetals, exhibit both metallic and nonmetallic properties.

Figure 19.1 The Periodic Table

Metals and Nonmetals• Metals:

– Lose one or more valence electrons– Form cations (positive ions)– Low ionization energy– Found on the left side of the periodic table

• Nonmetals:– Gain one or more electrons– Form anions (negative ions)– Large ionization energy– Most negative electron affinity– Found on the right side of the table

Atomic Size and Group Anomalies

• Atomic size increases down a group.• Large increase in atomic radius in going from first to

second member.• First element shows properties that are quite different

from the others.• Hydrogen is a nonmetal but lithium is a very active

metal.• BeO is amphoteric but other group 2A oxides are basic.• Usually C forms C-C bonds but Si forms Si-O bonds• N forms :NN: but P forms tetrahedral P4 molecules

• O form O2 molecule but S forms bigger aggregates, S8.

Figure 19.2 The Atomic Radii of Some Atoms in Picometers

Figure 19.3 The Structure of Quartz

Metallurgy. . . the process of obtaining a metal from its ore.This always involves reduction of the ions to the elemental metal (oxidation state = 0).Reducing agents are used to produce the metals from their ores.

2SnO(s) + C(s) 2Sn(s) + CO2(g)

2PbO(s) + C(s) 2Pb(s) + CO2(g)

SnO(s) + H2(g) Sn(s) + H2O(g)

WO ( ) 3H ( ) W( ) 3H O( )3 2heat

2s g l g

heat heat heat

Alkali Metals• The group 1A elements with their ns1 valence electron

configurations are very active metals. They lose their valence electrons very readily. They have low ionization energies and react with nonmetals to form ionic solids.

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

• The expected trend in reducing ability, Cs>Rb>K>Na>Li

• Alkali metals all react vigorously with water to release hydrogen gas.

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

(aq) + H2(g)

• Observed reducing abilities: Li>K>Na

Group 1A Oxides

In the presence of ample oxygen,4Li + O2 2Li2O (regular oxide)

2Na + O2 Na2O2 (peroxide)

K + O2 KO2 (superoxide)

Rb + O2 RbO2 (superoxide)

Cs + O2 CsO2 (superoxide)

Hydrogen• Hydrogen is a color less, odorless gas composed of H2

molecules.• Because of its low molar mass and nonpolarity,

hydrogen has a low boiling point (-253oC) and melting point (-260oC).

• Industrial source: methane reacts with water at high temp.(800-1000oC) and high pressure (10-50 atm) in presence of nickel catalyst

CH4(g) + H2O(g) CO(g) + 3H2(g)

• Hydrogen can be produced by electrolysis of water which is not economically feasible because of the high cost of electricity.

• The major industrial use of hydrogen is in the production of ammonia.

Hydrides

. . . are binary compounds containing hydrogen.

• ionic hydrides: hydrogen + the most active metals (e.g, LiH, CaH2)

• covalent hydrides: hydrogen + other nonmetals (e.g, H2O, CH4, NH3)

• metallic (interstitial) hydrides: transition metal crystals treated with H2 gas.

Figure 19.5 The Structure of Ice, Showing the Hydrogen Bonding

Alkaline Earth Metals• Group 2A elements are very reactive• Valence electron configuration ns2

• Lose 2 valence electrons to form noble gas configuration

• Form ionic compounds containing M2+ cations• Oxides are basic:

MO(s) + H2O(l) M2+(aq) + 2OH-

(aq)

• Ca, Sr, and Ba reacts vigorously with water:M(s) + 2H2O(l) M2+

(aq) + 2OH-(aq) + H2(g)

• Be form covalent bond because of small size and high electronegativity

The Structure of Solid BeH2

Figure 19.6 Preparation for the Group 2A Elements

Ion Exchange• Ca2+ and Mg2+ ions cause the hardness of water.

These ions of hard water interfere with the action of detergents and form precipitate with soap. Those ions are removed by ion exchange with ion exchange resins.

• Ca2+ and Mg2+ are often removed during ion exchange, releasing Na+ into solution.

• Ion exchange resin: large molecules that have many ionic sites.

Group 3A Elements• Valence electron configuration ns2np1.• Increase metallic character in going down the

group.• B is a nonmetal and its compounds are covalent.• Boranes are hydride of B: diborane B2H6

• Al is the most abundant metal on earth.• Al forms covalent bond with nonmetals• Gallium has unusually low melting point at

29.8oC (Al:660oC)

Figure 19.8 The Structure of B2H6

Group 4 Elements

• Valence electron configuration ns2np2

• C and Si are the two most important elements on earth

• Increase metallic character in going down the group

• C is a nonmetal, Si & Ge are semimetals, Sn and Pb are metals

• All elements form four covalent bond to nonmetals

Group 4 Elementscontinued…

• Form tetrahedral molecules because of sp3 hybridization

• Carbon occurs as graphite and diamond• Si forms silica and silicates are the most

abundant of the earth crust• Ge used in the manufacture of semiconductors• Sn used in various alloys such as bronze (20%

Sn and 8% Cu)

Inert Pair Effect

• The tendency for the heavier elements of Groups 3A and 4A to exhibit lower oxidation states as well as their expected oxidation states.

• Group 3A +1 and +3 oxidation states• Group 4A +2 and +4 oxidation states