The Main Group Elements

Where Are The Main Group Elements? Groups 1A - 8A on the periodic table 1A and 2A: valence electron configurations of ns 1 or ns 2 3A through 8A: valence electron configurations of ns 2 np 1-6 Very abundant in the universe…

A. A Review of General Properties and Periodic Trends Remember where to find metals and nonmetals on the periodic table Periodic tablePeriodic table Semimetals: elements with intermediate properties Going right across the periodic table: effective nuclear charge (Z eff ) increases -- you’re adding more electrons the same distance out from the nucleus (in the same shell) but you’re adding more protons, so the outermost electrons will feel more positive charge.

Periodic Trends cont. Elements on left side of table form cations; elements on right side form anions Atomic radius increases going down a group (filling additional shells) But farther down in a group, EN and IE will decrease Ionic vs. covalent compounds…

The Second Row Elements

B. Distinctive Properties of the Second-Row Elements Small, and high EN (this is why HF can form hydrogen bonds, but not HCl, HBr, or HI) No d orbitals. Only four valence orbitals (2s, 2p x, 2p y, 2p z ) so they generally form a maximum of four covalent bonds. (Third-row elements can accommodate more bonds)  overlap of 2p orbitals allows them to form multiple bonds. In third-row elements, too much distance between p orbitals does not allow for overlap.

Comparison of p-orbital overlap: C vs. Si

example Sulfur forms SF 6, but oxygen bonds to a maximum of two F atoms, yielding OF 2. Explain. Sulfur: third-row element, can use d orbitals to form more than 4 covalent bonds. Oxygen can form a maximum of 4 covalent bonds,as it is a second-row element and has no d orbitals available. So, it will form 2 to satisfy its octet.

C. Boron Boron halides are highly reactive Lewis acids (BX 3 ). Can react with a Lewis base such as ammonia to form a Lewis acid-base adduct (B has an empty p orbital) X 3 B - NH 3 Boranes (boron hydrides) have the formula B n H m example: diborans is B 2 H 6

D. Carbon Carbon exists in many forms… Diamond: three-dimensional covalent network in all directions Graphite: composed of many two-dimensional sheets, where each sheet consists of covalently bonded carbon atoms. The sheets are attracted to each other by LDFs. Fullerene: spherical C 60 molecules shaped like a soccer ball. Cage- like structure, led to development of carbon nanotube. Structures

Carbon Compounds Oxides of Carbon: Most important are carbon monoxide (CO) and carbon dioxide (CO 2 ). Why is CO so toxic? It strongly bonds to the iron(II) atom of hemoglobin, which carries oxygen in red blood cells. As a result, our tissues won’t get enough oxygen (they will get CO instead) and the heart has to work harder to supply enough oxygen.

Carbon Compounds Carbonates (Na 2 CO 3 ) are used to make glass Sodium bicarbonate is used to make bread Hydrogen cyanide (HCN) is highly toxic Carbides: compounds where C has a negative oxidation state such as CaC 2, Al 4 C 3, SiC

example The equilibrium between oxyhemoglobin and carboxyhemoglobin suggests an approach to treating mild cases of CO poisoning. Explain. The reversible reaction: Hb-O 2 + COHb-CO + O 2 To decrease the amount of Hb-CO and restore the amount of Hb-O 2, using Le Chatelier’s Principle, you might administer O 2 to the patient to cause more of the reverse reaction.

E. Silicon Just below C on the periodic table, but larger atomic radius does not allow for  bonding. Generally naturally found combined with oxygen and in various silicate materials Silicates: ionic compounds containing silicon oxoanions as well as a cation to balance the negative charge (examples: Be 3 Al 2 Si 6 O emerald; CaMgSi 2 O 6 )

F. Nitrogen Gas at room temp; 78% of the earth’s atmosphere by volume N 2 gas is unreactive -- high strength of triple bond N 2 + O 2 2 NO K = 4.5 * This reaction doesn’t happen at 25 o C However, at higher temperatures, K increases, and the equilibrium shifts to the right (reaction is endothermic) -- forming NO in car engines, causing air pollution

G. The Halogens Valence electron configuration ns 2 np 5 How do they complete their octets? Consider the halogen oxoacids HXO n Acid strength increases as the oxidation state of the halogen increases (HClO 4 stronger than HClO)