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Lecture Notes for

CHEM 364

Chemistry of the Elements I Main Groups

Developed by

Marcel Schlaf (2000-04; 2008-09) and Kathryn Preuss (2005-07; 2010)

Department of Chemistry

University of Guelph

© 2010

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Learning the language …

• Inorganic Chemistry is NOT an isolated branch of chemistry!

• In studying the elements and their compounds, we must also study the physical principles

that govern their structure and behavior.

• We will be learning how to interpret / predict structure, and chemical and physical behavior

using a variety of MODELS: e.g., VSEPR, VB, and MO theories.

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The Periodic Table

The Periodic System of the Elements (PSE) and What It Means … a little bit of history

Mendeleyev, Dmitri Ivanovich (1834-1907) Russian chemist who framed the periodic law in

chemistry 1869, which states that the chemical properties of the elements depend on their relative atomic masses. This law is the basis of the periodic table of the elements, in which the elements are arranged by atomic number and organized by their related groups. Mendeleyev was the first chemist to understand that all elements are related members of a single ordered system. From his table he predicted the properties of elements then unknown, of which three (gallium, scandium, and germanium) were discovered in his lifetime. Meanwhile Lothar Meyer in Germany presented a similar but independent classification of the elements. Mendeleyev was born in Tobol'sk, Siberia, and studied at St. Petersburg and in Germany at Heidelberg. He became professor at the Technical Institute in St Petersburg 1864. But in 1890, for supporting a student rebellion, he was retired from the university and became controller of the Bureau for Weights and Measures. Mendeleyev was convinced that the future held great possibilities for human flight, and in 1887 he made an ascent in a balloon to observe an eclipse of the Sun. His textbook Principles of Chemistry 1868-70

was widely adopted. (Source: http://b.cartage.org.lb/web/bio/bio/M/Mendeleyev/1.html)

Mendeleyev’s PSE in 1871 – he realized that the most important chemical property of an element is its valence and ordered them accordingly. The PSE is the result!

- The shaded elements were

unknown at the time, but Mendeleyev predicted the existence and properties of several of them based on his realization.

- Note the main and transition

elements are still “mixed up” in this table as the Aufbau Principle was unknown before the advent of quantum mechanics in the 1920’s.

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The periodic system orders the elements according to the number of their valence electrons. Structure, i.e. molecular shape → VSEPR

Number of valence electrons Reactivity: How does the molecule behave? (Acid/Base? Oxidant/Reductant?)

Source: Shriver & Atkins, Inorganic Chemistry, 3rd Edition, Freeman 1999 Group number = number of valence electrons Some very general but important trends:

Down a group: Increase in size and thus length of bonds to other atoms. Decrease in ionization energy, electron affinity, electronegativity and thus strength of bonds with other atoms. Across a period: Increase in ionization energy, electron affinity, electronegativity and

maximum valence. Decrease in size. All these properties correlate with the metallic/non-metallic character – how?

If you combine an element from the left side with from the right side, what are you bound to get?

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Which are the so-called “Main Group” elements?

They are the “s” and “p” block collectively.

Why are we studying them separately from the rest of the periodic table?

• The frontier atomic orbitals are s & p.

• Typical properties/reactivities are different from the elements of the d-block. (Chem365)

• Interestingly, the typical chemistry of the f-block elements (lanthanides and actinides) is similar to the chemistry of the alkali and alkaline earth metals (Groups I & II).

Why are main group elements important?

• They are the most abundant elements at the surface of this planet!

Earth’s atmosphere (dry): N2 78.084% O2 20.946% Ar 0.9340% CO2 0.039% Note that there is also a lot of water in the atmosphere. H2O ~0.40% over full atmosphere ~1% - 4% at surface

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Earth’s oceans: At an average salinity (PSU = 35), the molar composition of seawater is… H2O 53.6 mol/kg Cl- 0.546 mol/kg Na+ 0.469 mol/kg Mg2+ 0.0528 mol/kg SO4

2+ 0.0528 mol/kg Ca2+ 0.0103 mol/kg K+ 0.0102 mol/kg

Earth’s crust:

“Inorganic Chemistry: An Industrial and Environmental Perspective”, T.W. Swaddle, Academic Press, San Diego, 1997.

• Silicates are minerals containing silicon and oxygen (i.e. silicon oxides) as well as numerous other elements. Silicates, including the important subclass of aluminosilicates, are the major rock-forming minerals (ca. 90% of Earth’s surface).

Feldspars KAlSi3O8, NaAlSi3O8, CaAl2Si2O8

~60% of Earth’s crust

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Silicates can be amorphous.

Kaolin (a clay mineral) Al2Si2O5(OH)4 i.e., Al2O3 ▪ 2(SiO2) ▪ 2(H2O)

Original ingredient in Kaopectate!

Borosilicate (PYREX 7740) Glass as sold by Pegasus (Cambridge, ON)

SiO2 (81%), Na2O (4.0%), K2O (0.5%), B2O3 (13.0%), Al2O3 (2.0%)

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Silicates are often crystalline.

Some varieties are inexpensive and quite useful.

Talc Mg3Si4O10(OH)2

Soapstone is primarily talc.

Some varieties are expense and less useful.

Topaz Al2SiO3(OH,F)2 Beryl Be3Al2Si6O18

Emeralds are beryl with trace Cr or V.

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• Zeolites (naturally occurring and synthetic) are an important class of aluminosilicate.

Zeolites are comprised of an anionic Al-O-Si framework forming cavities and channels occupied

by cations such as Na+, Ca2+, Mg2+ (and water). The cavity/channel system makes the zeolite

porous and the cations can often be easily replaced without destroying the crystal.

Uses:

• A zeolite in its sodium form (say, Na2Z) can be added to a laundry detergent in order to improve its efficacy by preventing the formation of soap scum (insoluble Ca2+ and Mg2+ stearates, palmitates and oleates) in hard water. It does so by exchanging its Na+ ions with the Ca2+ and Mg2+ ions. (Note that sodium stearates, palmitates and oleates are water soluble).

• Zeolites can be dried (all the water removed) and can then be used as desiccants to reabsorb

water from the surrounding environment (e.g., molecular sieves for anhydrous chemistry!) • Zeolites are widely used as solid (heterogeneous) catalysts and catalyst support systems

owing to their very large surface area. The effective area of the anionic aluminosilicate framework in the pores of a zeolite is at least 100 times the external surface area (as high as 1000 m2 g-1!)

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• Silica (SiO2) is present in the natural environment as quartz and is often the primary component of sand.

UV-transparent tubing

The siloxane (Si-O-Si) linkage is the key to silicate chemistry.

When atoms of two different elements (e.g., Si and O) catenate (i.e., form chains) in alternation,

the chain gains strength from the disparity in electronegativity between them.

**(Look ahead in the notes to Pauling’s observations on bond strengths and his definition of

electronegativity.)

It is also important that discrete Si4+ ions are unlikely to form because such ions would be so

small and highly charged as to polarize any neighboring anion (e.g., O2-) into covalency.

**(This is essentially a statement of Fajan’s rules.)

Thus, polysiloxane chains are very stable. Linear chains, branched chains, rings, and cages exist.

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• Silicones (organopolysiloxanes) [R2SiO]n can take the form of oils, greases, waxes, waxy solids and rubbers, with several very useful and adjustable properties: high thermal stability resistance to oxidation excellent electrical insulation remarkable water repellency good biocompatibility low chemical reactivity (although aqueous OH- or HF

can break the Si-O-Si linkage) Liquids have surprisingly low viscosity for such large molecules and are widely used as

very thermally stable high temperature heat transfer fluids. Solids are much more permeable to gases than are most plastics. The physical properties can be “tuned” by appropriate choices of the side groups R.

Solid polydimethylsiloxane (which is transparent) is used for soft contact lenses, as the cornea of the eye must have access to the oxygen in air to its metabolic processes.

Silicones are made by hydrolysis of organochlorosilanes RnSiCl4-n, which are produced from elemental silicon by the Rochow process: RCl(g) + Si(s) → RnSiCl4-n

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• Alumina (Al2O3) occurs in nature as crystalline corundum, but is typically prepared commercially by the calcination (thermal decomposition) of bauxite.

Corundum Al2O3 Bauxite Al(OH)3 + AlO(OH)

Rubies are corundum with trace Cr Crude aluminum ore

Alumina is has a very high melting point (2072 °C) and is therefore used a solid support medium for heterogeneous catalysis. It is very hard and therefore used as a ceramic for grinding applications. Because aluminum metal is readily oxidized by atmospheric oxygen, all aluminum metal (under ambient conditions) is coated with a layer of aluminum oxide. This very thin layer of aluminum oxide is extremely insoluble in water (at intermediate pH values) and does not have any readily accessible redox states. Thus, it protects the very reactive aluminum metal against aqueous corrosion…making it possible to use Al(s) as a building material. It should be noted that aluminum oxide is amphoteric, meaning it is water soluble at very high and very low pH (i.e. in both acids and bases.)

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• Gypsum (CaSO4•2H2O) is a common water-soluble mineral.

Gypsum is very soft. Most people are familiar with gypsum in its commercial application as drywall. Plaster of Paris (calcium sulfate hemihydrate) is obtained by heating gypsum (ca. 150 °C) and driving off 1.5 equivalents of water (as steam). 2 CaSO4•2H2O → 2 CaSO4•0.5H2O + 3 H2O When Plaster of Paris is mixed with water, it rehydrates exothermically and sets as gypsum (reverse process).

• Dolomite (CaMg(CO3)2) is a common mineral (rock) similar to limestone (see below). Magnesium ions and calcium ions often occur interchangeably in natural minerals.

(Why does this make chemical sense?) Dolomite Alps (Italy)

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• Calcium Carbonate (CaCO3) is also known as limestone or as chalk. It is the primary component of pearls, seashells, eggshells, coral and marble.

Calcination (heating) of calcium carbonate releases carbon dioxide generating calcium oxide (CaO) also known as lime.

CaCO3(s) → CO2(g) + CaO(s) Lime (CaO) reacts with water to form calcium hydroxide (Ca(OH)2) also known as hydrated lime or slaked lime.

CaO(s) + H2O(l) → Ca(OH)2 Calcium hydroxide is somewhat soluble in water and is basic. A solution or suspension of hydrated lime in water will react with the dissolved atmospheric CO2 and precipitate as CaCO3. This is the reverse process of the above equations. Lime-sand cements (a.k.a. mortar) in use since Roman times gain mechanical strength from the slow reaction of Ca(OH)2 with CO2 of the air to form interlocking crystals of CaCO3 (can take centuries to harden properly!) The sand acts primarily as a matrix around which this process occurs.

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• Other less abundant minerals that have been in the news recently include potash (“K2O”) and lithium brine (Li2CO3 + LiCl + …).

Potash “K2O”

Check out new on the Potash Corp of Saskatchewan

http://www.potashcorp.com/

Lithium Brine Li2CO3 + LiCl …

Check out news on Bolivia’s lithium deposits and Afghanistan’s lithium deposits.

http://news.bbc.co.uk/2/hi/business/7707847.stm

http://www.nytimes.com/2010/06/14/world/asia/14minerals.html

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• Main group elements are involved in numerous important industrial processes.

Haber process: synthesis of ammonia (synthetic nitrogen fixing!)

Contact process: production of sulfuric acid (NB: H2O + SO3 generates dangerous aerosols)

• Environmental Chemistry involves a significant amount of main group chemistry.

Atmosphere and Atmospheric Pollution:

-the chemistry of N2, O2, CO2, O3, NOx, CFCs, SO2, etc.

Lakes and Rivers:

-the chemistry of Al (Al3+ kills fish by displacing Ca2+ that regulates gill permeability)

-the chemistry of Cl and S regarding the pulp and paper industry and water treatment

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• Main group chemistry has significant biological importance.

Phosphates PxOyn-

-DNA : sugar (deoxyribose) + phosphate unit + basic sidegroup (A, C, D, T)

-Ca5(PO4)3OH : hydroxyapatite is the essential structural material of bones and teeth

-ADP ↔ ATP : phosphate-phosphate bonds store energy

• ATP is a tetraprotic acid.

• It is always found as an Mg2+ complex!

Alkali and alkaline earth metal ions Na+, K+, Ca2+, Mg2+

-ATP is a tetraprotic acid and is always found as an Mg2+ complex (see text p.714)

-Na+/K+ pumps and channels

-calcium signaling in proteins

-Chorophyll a is a Mg2+ complex: see figure →

Iodine

-concentrated in humans by the thyroid gland to

form thyroid hormones, such as the iodo-amino

acid thyroxine.