The Main Group Elements

Where Are The Main Group Elements?Groups 1A - 8A on the periodic table1A and 2A: valence electron

configurations of ns1 or ns2

3A through 8A: valence electron configurations of ns2np1-6

Very abundant in the universe…

A. A Review of General Properties and Periodic TrendsRemember where to find metals and nonmetals

on the periodic table Periodic tableSemimetals: elements with intermediate

propertiesGoing right across the periodic table: effective

nuclear charge (Zeff) 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 anionsAtomic radius increases going down a group

(filling additional shells)But farther down in a group, EN and IE will

decreaseIonic vs. covalent compounds…

The Second Row Elements

B. Distinctive Properties of the Second-Row ElementsSmall, 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, 2px,

2py, 2pz) 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

exampleSulfur forms SF6, but oxygen bonds to a

maximum of two F atoms, yielding OF2. 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. BoronBoron halides are highly reactive Lewis acids

(BX3). Can react with a Lewis base such as ammonia to form a Lewis acid-base adduct (B has an empty p orbital)

X3B - NH3

Boranes (boron hydrides) have the formula BnHm

example: diborans is B2H6

D. CarbonCarbon 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 C60 molecules shaped like a soccer ball. Cage-like structure, led to development of carbon nanotube.

Structures

Carbon CompoundsOxides of Carbon: Most important are carbon

monoxide (CO) and carbon dioxide (CO2).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 CompoundsCarbonates (Na2CO3) are used to make glassSodium bicarbonate is used to make breadHydrogen cyanide (HCN) is highly toxicCarbides: compounds where C has a negative

oxidation state such as CaC2, Al4C3, SiC

exampleThe equilibrium between oxyhemoglobin and

carboxyhemoglobin suggests an approach to treating mild cases of CO poisoning. Explain.

The reversible reaction:Hb-O2 + CO Hb-CO + O2

To decrease the amount of Hb-CO and restore the amount of Hb-O2, using Le Chatelier’s Principle, you might administer O2 to the patient to cause more of the reverse reaction.

E. SiliconJust 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 materialsSilicates: ionic compounds containing silicon

oxoanions as well as a cation to balance the negative charge (examples: Be3Al2Si6O18 -- emerald; CaMgSi2O6)

F. NitrogenGas at room temp; 78% of the earth’s

atmosphere by volumeN2 gas is unreactive -- high strength of triple

bondN2 + O2 2 NO K = 4.5 * 10-31

This reaction doesn’t happen at 25oCHowever, 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 HalogensValence electron configuration ns2np5

How do they complete their octets?Consider the halogen oxoacids HXOn

Acid strength increases as the oxidation state of the halogen increases (HClO4 stronger than HClO)