Chapter 19 The Representative Elements: Groups 1A Through 4A
19.1 Survey of the Representative Elements • Elements in group 1A through 8A are called representative elements because they display a wide range of physical and chemical properties. • Representative elements display the range of possible valence electrons from one in group 1A to eight in group 8A. • The valence electrons of representative elements are in s or p orbitals. • Metals tend to lose their valence electrons to form cations with a configuration of the noble gas from the preceding period • Nonmetals tend to gain electrons to form anins with a configuration of the noble gas in the same period
Atomic size and Group anomalies 1. H, Be, B, C, N, O & F have properties that distinguish them from their groups due to their relatively small sizes 2. H is nonmetal and forms covalent bonds; Li is metal and forms ionic bonds 3. All oxides of G 2A are ionic except that of Be (BeO) it is covalent also amphoteric. 4. Same is applicable for G 3A 5. G 5A: carbon forms readily C-C but Si forms readily Si-O 6. Si-Si exists but less stable the C-C 7. O=C=O exists but O=Si=O is not stable; Si-O-Si is stable. 8.Si does not form bonds. Si 3p valence orbitals do not overlap with the O- 2p orbitals 2.
bonding is important for relatively small elements of the 2nd period. • N exists as N≡N due to tendency to form bonds • P exists as P4; P large atoms are like Si do not form strong bonds. They prefer to achieve noble gas configuration but forming single bonds. • O (G 6A) exists as O=O; tendency to form bonding • S does not form bonding thus it exists as S8. • F has smaller electron affinity than Cl (not expected) the small size of F in F-F with 6 lone pairs of electrons leads to much greater repulsion compared to Cl Abundance and Preparation (P. 917): Self study
Distribution (Mass Percent) of the 18 Most Abundant Elements in the Earth's Crust, Oceans, and Atmosphere
19.2 Group 1A Elements • ns1 valence electron configuration. • Comprise H and alkali metals Alkali Metals
Selected Physical Properties of the Alkali Metals Group 1: M(s) + H2O(l) 2M+(aq) + 2 (OH)-(aq) + H2(g) • Li has the highest ionization potential and the highest oxidation potential. Due to its small size and large energy of hydration, thus Li+ attracts water effectively and large energy is released when the ion is formed. Thus formation of Li+ is favored and Li behaves as a strong • Reducing agent.
19.3 Hydrogen Also prepared from the electrolysis of water
Covalent hydrides Hydrides Metallic/interstitial hydrides
Group 2A Elements • Ns2 metals. They are called “Alkaline Earth metals” • Their oxides are basic MO(s) + H2O M(OH)2 • The differences in reactivity among them are shown by their reaction with water: M(s) + 2H2O M(OH)2 + H2(g) • Ca, Ba, Sr react easily with cold water • Mg reacts with hot water • Be does not react with water
Selected Physical Properties, Sources, and Methods of Preparation for the Group 2A Elements
BeCl2 and BeF2 • BeCl2 and BeF2 melts are poor conductors: • Therefore they are covalent rather than ionic solids.
Ions in Natural Waters: Hard Water • Rainwater is not chemically pure water. • Contains dissolved atmospheric gases. • Once on the ground it may pick up a few to about 1000 ppm of dissolved substances. • If the water contains Ca2+ and or Mg 2+ ions we say that the water is hard. • Hardness may be permanent or temporary.
Temporary Hard Water • Contains HCO3- ion. • When heated gives CO32-, CO2 and H2O. • The CO32- reacts with multivalent ions to form precipitates. (for example CaCO3, MgCO3) • Water softening on a large scale is carried out by precipitating the multivalent ions using slaked lime Ca(OH)2. CaCO3 would be precipitated
Permanent Hard Water • Contains significant concentrations of anions other than carbonate. • For example SO42-, HSO4-. • Usually soften by precipitating the Ca2+ and Mg2+using sodium carbonate leaving sodium salts in solution.
Water Softening by ion-exchange • Ion exchange. • Undesirable cations, Mg2+ Ca2+ and Fe3+ are changed for ions that are not as undesirable, ex. Na+. • Resins or zeolites.
A typical cation-exchange resin. when hard water is passed over the cation-exchange resin, the ca2+ and mg2+ bind to the resin
19.5 Group 3A Elements • ns2np1 elements • Boron is a nonmetal that forms covalent bonds; other elements are metals • Al forms significant covalent bonding with nonmetals; that is why Al2O3 has amphoteric nature.
Selected Physical Properties, Sources, and Methods of Preparation for the Group 3A Elements
19.6 Group 4A Elements • They have the valence electron configuration: ns2np2 • Contain the two most important elements on the earth, C and Si which form the basic of geologic world. • They all form covalent bonds with nonmetals: CH4, SiF4, GeBr4, SnCl4, PbCl4( ِAll sp3 hybridization)
Selected Physical Properties, Sources, and Methods of Preparation for the Group 4A Elements
Chapter 20 The Representative Elements: Groups 5A Through 8A
20.1 Group 5A ElementsThe Nitrogen Family • ns2np3 valence electrons configuration • Nitrogen can exist in many oxidation states. • N and P are nonmetallic. • As and Sb are metalloid. • Bi is metallic.
Bi and Sb tend to be metallic • But no ionic compounds containing Bi5+ and Sb5+ are known • BiF5, SbF5 and SbCl5 are molecular rather than ionic • G 5A elements can form molecules or ions that involve 3, 5 or 6 covalent bonds to the G 5A atom • NH3, PH3, NF3, and AsCl3. They all behave as Lewis base. • All G 5A elements except N can form molecules (MX5) with 5 covalent bonds. • The ability of G 5A elements to form bonds decreases dramatically after N. • This is why N exists as N2 molecules; while other elements in the group exist as larger aggregates containing single bonds: P4, As4, Sb4
20.2 The Chemistry of Nitrogen • Since N2 molecule contains a triple bond, most binary compounds (except NH3) containing N decompose exothermically to the elements • In the preparation of NH3 from N2 and H2, too much energy is needed to disrupt the N≡N bond. • Thus, though K (106) is high the reaction is very slow at room temperature. • Haber process is used to prepare NH3 (high pressure, high temperature and a catalyst are needed) • Nitrogen fixation: The process of transforming N2 to other nitrogen containing compounds
Nitrogen fixation can be carried out by: • Haber process (ammonia can be applied to the soil as a fertilizer) • High temperature combustion process in automobile engines. NO produced is converted into NO2 that with moisture is concerted into NO3- that reaches soil. • Natural. Lightning produces the energy that disrupt N2 andO2molecules producing reactive N and O atoms that attack other molecules to form nitrogen oxides that convert eventually to NO3- • Nitrogen-fixing bacteria that reside on the root of nodules of plants such as beans and peas. This converts N2 toammonia and other nitrogen containing compounds. • Denitrification: return of N element to the atmosphere as N2 gas. Bacteria changes NO3- to N2
20.3 The Chemistry of Phosphorus • Chemical properties of P are significantly different from N for the following reasons: • Nitrogen’s stability to form much stronger bonds • Grater electronegativity of N • Larger size of P atom • Availability of empty valence d orbitals on P • White phosphorus exists as P4: very reactive and bursts into flames on contact with air. • It is commonly stored under water • Black P and Red P are network solids
(a) The P4 Molecule Found in White Phosphorus (b) The Crystalline Network Structure of Black Phosphorous (c) The Chain Structure of Red Phosphorus
P is essential for plant growth • Soluble phosphate fertilizer is made by treating phosphate rock with sulfuric acid to make superphosphate of lime, that is a mixture of : CaSO4. 2H2O and Ca(H2PO4)2. H2O • A reaction of NH3 and P produces NH4H2PO4 a very efficient fertilizer
20.4 The Group 6A Elements • The valence electron configuration is ns2np4 • Non of these elements behaves as a metal • They achive the noble gas configuration by adding 2 electrons to become 2- anion • G 6A elements can form covalent bonds with other nonmetals • Due to the presence of empty d orbitals (except O), they form molecules in which central atom is surrounded by more than 8 electrons: SF4 and SF6
Group 7A • ns2 np5 valence electron configuration. • All nonmetals • Reactive. Not free in nature. Found as halide (X-) ions. • Astatine radioactive with t1/2 = 8.3 hrs for its longest living isotope • Very high electronegativities (4, 3, 2.8, 2.5 and 2). • Ionic bonds with metals and covalent bonds with nonmetals in low oxidation states & polar covalent in metals in high oxidation states.
Hydrogen Halides • HCl is the most important acid. • HF is used for etching glass. SiO2(s) + 4HF(aq) SiF4(g) + 2H2O(l)
Group 8A • ns2p6 configuration; Un-reactive. • He. Component of the sun. • Present in natural gas (from decay of radioactive elements). • Used as Coolant and a rocket pressurizing gas. • Ne. Used in Luminescent lighting. • Ar. Used as a non-corrosive atmosphere in light bulbs. • Xe & Kr form compounds with O and F.