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 Metalloids or semi metals 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 2. 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 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 Abundance of Elements in the Human Body 19.2 Group 1A Elements ns1 valence electron configuration. Comprise H and alkali metals Alkali Metals Sources and Methods of Preparation of the Pure 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. Types of Compounds Formed by the Alkali Metals with Oxygen Selected Reactions of the Alkali Metals 19.3 Hydrogen Also prepared from the electrolysis of water Covalent hydrides Hydrides Metallic/interstitial hydrides 19.4 Group 2A Elements 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. Selected Reactions of the Group 2A Elements 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 Selected Reactions of 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 Selected Reactions of the Group 4A Elements Chapter 20 The Representative Elements: Groups 5A Through 8A 20.1 Group 5A Elements The 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 The Molecules of the Types MX3, MX5, and MX6 Formed by Group 5A Elements The Structures of the Tetrahedral MX4+ and Octahedral MX6- Ions 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 and O2 molecules 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 to ammonia and other nitrogen containing compounds. Denitrification: return of N element to the atmosphere as N2 gas. Bacteria changes NO3- to N2 The Nitrogen Cycle Some Common Nitrogen Compounds 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) H–X X ( g ) H X (aq) 2O Bond energy Ho Ho Go X (kJ/mol (kJ/mol) (J/mol.K) (kJ/mol) F 565 -510 -159 + 102.4 Cl 427 -366 -96 90 Br 363 -334 -81 53 I 295 -291 -64 23 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.