6 Basic Inorganic Chemistry for Environmental Sciences

Dr. Y. P. Singh

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Contents

  1. Introduction
  2. Elements
  3. Periodic Table of Elements
  4. Classification of Elements on the basis of their properties
  5. Non-Metals
  6. Types of Bonds
  7. Coordination compounds
  8. Free radicals
  9. Lithosphere and Inorganic Chemical Reactions
  10. Weathering of Rocks
  11. Coordination Compounds in Soil
  12. References

 

Introduction

The learners of environmental science come with diverse academic backgrounds. Some have studied chemistry as a major subject at graduate or 10+2 level, others have hardly any insight in the subject. Planets and their environments are composed of chemical systems. For example, air is composed of oxygen, nitrogen gases as major constituents and other gases and aerosols as minor constituents. Major component of clouds, raindrops, fog, ocean, river and for that all aquatic systems, is water. Likewise, earth crust is made of rocks, minerals and soil etc., which are heterogeneous mixture of variety of chemical compounds. So for understanding the properties of chemical systems in environment, it is necessary to get a basic understanding of the nature of chemical elements and their compounds. This module attempts to provide a birds’ eye view of chemical elements and their compounds essential for understanding the chemistry of the environment presented in subsequent modules.

 

Elements

The chemical elements, the fundamental substances, are made from atoms. And atom is the smallest particle of an element which can take part in a chemical reaction. The main components of an atom are protons, which are positively charged, electrons, which are negatively charged and neutrons, which are uncharged. The all atoms of an element have the same atomic number (Z), which is equal to the number of protons in an atom. In an atom the number of protons and electrons is equal and therefore an atom is electrically neutral. The elements may exist in mono-atomic or polyatomic forms. For example in air, argon exists as a single atom, Ar, and nitrogen (N) exists as N2.

A given element may exist in more than one polyatomic (molecular) forms. These forms are known as allotropes. For example, in atmosphere oxygen is found both as dioxygen, O2, and ozone,O3.

The elements and their compounds are building block of every material found in the universe. The relative abundance of the elements by mass in whole Earth is: 35% iron, 30% oxygen, 15% silicon. On the other hand Earth’s crust has 40% oxygen, 28% silicon, 8% aluminum 6% iron, etc.

 

Periodic Table of Elements

At present 118 elements are known including some synthetic ones. The realization dawned that instead of studying the properties of each element individually, it would be more convenient to divide these elements into a few groups in a table such that each group contains a number of elements with similar properties. After several attempts, finally a table was constructed by arranging the elements in the order of their increasing atomic numbers. Elements with similar properties reappear at regular intervals, in agreement with periodic law modified by Moseley that the physical and chemical properties of the elements are the periodic function of their atomic numbers. Periodic table consists of 18 vertical columns called groups, and 7 horizontal rows known as periods.

 

Periodic table

Based on electronic configuration, the elements have been classified in the blocks as follows.

  1. s- block elements belong to Groups 1 & 2. The elements of Group 1 are characterized by an electron configuration of [ ] ns1 and are known as alkali metals. The elements of Group 2 are characterized by [ ] ns2 (where [ ] represents the core with inert gas configuration, for example, K has the configuration: [Ar], 4s1. Elements of Group 2 are collectively called alkaline earth metals. Hydrogen, although a non-metal, belongs to Group 1.
  2. d-block elements  belong  to  Groups  3-12.  The  general  electronic  configuration  of  these elements is [ ], ns0-2, (n-1)d1-10. However, the transition elements, although part of d-block have partially filled d-orbitals. Among these elements, iron, manganese, copper and zinc have great environmental significance. Most of these metals and their ions/compounds are good catalysts and many of them are involved in biochemical processes.
  3. f-block elements– belong to Group 3 comprising lanthanide and actinide series. In these elements, the f- orbitals are in the process of being filled. Uranium is the heaviest element of terrestrial occurrence. The post uranium elements are synthetic elements and all of these are radioactive elements.p-block elements(Groups  13-18)-  have  general  outer  most  electronic  configuration  of ns2,np1-6. All the nonmetallic elements belong to this block. The most important elements belonging to this group are C, Si, O, N, P, Cl, S, As, etc.

Classification of Elements on the Basis of Their Properties

  1. Metals – These elements have low ionization energies, low electron affinities and low electronegativities, and form cations. Out of 118 elements known at present, about 81 are metals. Metals are further classified as follows.
  • (1) Alkali Metals

 Alkali metals, viz., Li, Na, K, Rb, Cs are very reactive and react with O2, S, and other nonmetals to form compounds. That is why these are never found in nature in native state. There outermost electronic configuration is [noble gas], ns1. These elements are easily ionized to form cations.

Alkali form strongly alkaline oxides, e.g., Na2O and hydroxides, NaOH, all of which react with water to produce hydroxide ions, OH-, which make the aqueous solution alkaline.

2. Alkaline Earth Metals

The elements belonging to Group 2 are also reactive metals and commonly known as alkaline earth metals. The alkaline earth metals have the outermost electronic configuration, ns2. Thus these metals form dipositive ions. The oxides and hydroxides and carbonates of Mg, Ca, Sr and Ba are alkaline and their aqueous solutions are alkaline.

3. Transition metals

There are four series of these elements. The first series includes Sc, Ti, V, Cr, Mn, Fe, Co, Ni and Cu. Chemistry of these elements is of great environmental significance. These elements exhibit the variable oxidation states. For example Mn shows all the oxidation states starting from -1 to +7. These elements form large number of complexes and also act as catalysts in all types of chemical systems.

4. Rare earth elements

The elements of lanthanide group are called the rare earths, which comprise the fourteen elements from Ce(z = 580 to Lu (Z = 71), but sometimes La, Sc and Y are also included. The prominent member of actinide series are U and Pu, which are used in production of nuclear energy.

5. Heavy metals

Because of their high relative atomic masses, As, Be, Cd, Pb, Mn, Hg, Ni and Se are called heavy metals. These elements concern us because of occupational or residual exposure. They persist in nature and can cause damage or death in animals, humans and plants, even at a very low concentration (1 or 2 microns in some cases). Industrial processes release these into air and water. Since heavy metals have a property to accumulate in the selected body organs, such as brain and liver, long term exposure may result in slowly progressing physical, muscular and neurological degenerative processes that mimic Alzheimer’s diseases, Parkinson’s diseases, Muscular dystrophy and multiple sclerosis, Allergies are not uncommon. In all water analyses, measurement of heavy metals is necessary.

6. Base metals

The non-ferrous metals, excluding precious metals, are called base metals, e. g., iron, steel, aluminium, tin, tungsten, molybdenum, tantalum, cobalt, bismuth, cadmium, titanium, zirconium, antimony, manganese, beryllium, chromium, germanium, vanadium, gallium, hafnium, indium, niobium, rhenium, thallium, Copper and Lead.

7. Ferrous and Nonferrous metals.

Ferrous metals contain iron, for example carbon steel, stainless steel (both alloys; mixtures of metals) and wrought iron, Non-ferrous metals are metals that do not contain iron, for example aluminium, brass, copper and titanium brass. Al, Be, Cu, Pb, Mg, Ni, Sn, Zn, and precious metals.

8.Metalloid-

There is no rigorous definition of metalloids, but the elements having the properties of both metals and nonmetals are called metalloids. The metalloids often form amphoteric oxides (B, Si, Ge, As and Sb and often behave as semiconductor (B, Si, Ge, As).

 

Non-Metals

About 22 elements, which are members of p-block, behave as non-metals. They are usually poor conductor. They are found as gases, liquids or solids.

1. Carbon Family

The elements of Group 14, C, Si, Ge, Sn, Pb constitute the carbon family. Carbon is nonmetal, Si and Ge are semimetal, and tin and lead are metals. The chemistry of carbon is vast and studied separately as organic chemistry. This is due to the property of catenation, i. e., ability to carbon chains. The carbon cycle is an important cycle in environment.

 2. Nitrogen family

Nitrogen family consists of group 15 elements, viz., N, P, As, Sb, Bi, etc. The first two elements, N and P are very important and their biogeochemical cycles play a very important role in the chemistry of the environment

3. Oxygen family

Group 16 contain 6 elements, viz., O, S, Se, etc. O and S are nonmetals whereas Se is a metalloid. The importance of oxygen and sulfur cycles in environment shall be presented in a subsequent module.

4. Halogen

The elements of group 17 are known as halogen, viz., F, Cr, Br, I. The halogens have a very strong tendency to pick up one electron to acquire the stable noble gas configuration. These are among the most reactive elements. Chlorine easily undergoes photochemical dissociation and forms Cl atom/free radical, which is highly reactive and responsible for stratospheric ozone depletion. Chlorine is widely used for disinfection drinking water.

5. Noble/Inert/Rare Gases

Group 18 elements such as He, Ne, Ar, Kr, Xe and Rn are known as noble, rare or inert gases. Due to their stable ns2, np6 electronic configuration, they have very little tendency to form compounds with other elements. For this reason they exist in mono-atomic form, e. g., He, Ar, Ne, etc. In air, argon is the third most significant gas (0.9%) after N2 and O2. Radon is a radioactive element having serious environmental concerns as discussed another module.

 

Types of Bonds

Ionic Bonding/ Ionic compounds:

Ions are formed by the complete transfer of electron(s) from an atom of low electronegativity and low ionization energy to an atom of high electronegativity and high electron affinity. The formation of cation, Na+ and anion, Cl- , is shown below.

These oppositely charged ions get bonded together with electrostatic force of attraction to form ionic

Na+ + Cl-    NaCl (ionic compound)

bond and ionic compounds. Since ionic bond is very strong, these ionic compounds have some characteristic properties like high melting point, boiling point and low volatility, soluble in water and sparingly soluble in organic solvents and are brittle and bad conductor of electricity in solid state. However, these are good conductor in aqueous solution.

 

Covalent Compounds

These compounds are formed due to covalent bonding. A covalent bond is formed by the sharing of electrons between bonded atoms as in hydrogen and chlorine share an electron pair in H : Cl. These compounds can exist in gas, liquid or solid states. These compounds are usually soluble in organic solvents. However, covalent compounds like HCl dissolve in water to yield ions. Covalent compounds are of two type, polar and nonpolar.

 

Polar Compounds

When the covalently bonded atoms have significantly different electronegativities, shared electron pair shifts towards more electronegative atom. Consequently, partial positive charge develops on less electronegative and a partial negative charge on the more elecrtonegative atom, as in: Hδ+Clδ- . Similarly H2O, NO, NO2, NH3, CO2, SO3, N2O5, SO2, etc. have polar bonds. It may be pointed out that most of these are air pollutants. The polar compounds are generally soluble in water.

Non polar compounds

Non polar compounds are formed by the sharing of electrons between the atoms of nearly equal electronegativities. For example, H2, N2, O2, CH4, C2H6, C6H6, polycyclic aromatic hydrocarbons are non polar compounds.

 

Coordination compounds

A coordination compound or a complex is formed when one or more molecules (ligands) donate a pair of electrons to the central atom by forming a coordinate covalent bond. For example, when AgCN and KCN are mixed, CN- ions form coordinate bond with Ag + ion to form complex ion, Ag(CN)2- .

In environmental chemistry, particularly in soil and aquatic chemistry, we come across a large number of coordination compounds. For example, humic substances are the most important complexing ligand, which occur in nature. These are polyelectrolytic macro-molecules having high molecular weight. These ligands bind the metals, such as iron, aluminum, nickel, lead and zinc and magnesium, etc. through carboxylic (–COOH) and phenolic hydroxyl (-OH) groups. This results in mobilization of these metals in water and soil.

 

Free Radicals

The chemistry of atmosphere is basically governed by the radicals formed through photochemical reactions initiated by solar radiation. The radicals are chemical species that have an unpaired or free unbound electron. These radically generally formed via the homolysis of a bond.Some of the important free radicals are: H, Cl, OH, HO2, O, etc. It may be pointed out a dot is not placed as a superscript to denote a radical.

 

Lithosphere and Inorganic Chemical Reactions

A dense shell (the solid portion) of material that surrounds the entire Earth is known as the lithosphere. Though two third of the Earth’s surface is covered with water, underneath that water is a solid layer of rock. Basically, any solid non-organic material, except ice, seen on the Earth is from the lithosphere. This includes soil, rocks, sand, etc.

 

Composition

It is important to know that elements present in the lithosphere are in the form of various minerals. There are two different ways to express the composition of earth crust by mass and by volume. The most abundant element in the Earth’s crust (lithosphere) is oxygen; by both mass and volume. Silicon is the second most abundant element by mass. Potassium is the second most abundant by volume. Aluminum is third most abundant element by mass.

 

Average Chemical Composition of Earth’s Crust

Weathering of Rocks

Weathering is the process of disintegration of rock from physical, chemical, and biological stresses. Weathering is influenced by temperature and moisture and general by climatic conditions. As rock disintegrates, it becomes more susceptible to further physical, chemical, and biological weathering due to the increase in exposed surface area. During weathering, minerals that were once bound in the rock structure are released. The degree of weathering depends upon the resistance of the minerals in the rock, as well as to the degree of the physical, chemical, and biological stress. Weathering is usually confined to the top few meters of geologic material, because physical, chemical, and biological stresses generally decrease with depth. Weathering of rocks occurs at a place, but the disintegrated weathering products can be carried by water, wind, or gravity to another location (i.e., erosion ).Two important type of weathering processes are physical and chemical weathering . Each sometimes involves biological component.

 

Physical weathering

Physical weathering is the weakening and subsequent disintegration of rock by physical forces. These physical forces include temperature fluctuation, abrasion, frost action (freezing and thawing), and salt crystal growth. Temperature fluctuation can cause expansion or contraction of rock. When the temperature of rock increases, the rock expands, and when the temperature of rock decreases, the rock contracts. This process of expansion and contraction is a physical stress and can crack or break the rock. Abrasion of rock is caused by the friction of water, wind, or ice upon the rock. The continuous exposure to these elements slowly breaks down the exposed surface of the rock.

 

Frost action is the repeated cycle of ice formation and ice melt in the pore spaces and fractures of rocks causing disintegration of the rock. When water in rock pores freezes, its volume increase by about 10%. This can create a significant amount of pressure on rocks. The magnitude and extent of frost action is dependent on the frequency, duration and intensity of the freezing and thawing cycles. Salt crystal growth can cause the break-up of rock materials. Crystal growth often occurs when groundwater moves into empty pores or spaces of rock by capillary action. As the water evaporates, salt crystals grow and accumulate, putting pressure on the rock and causing it to break apart. Salt crystallization is common in drier climates.

 

Chemical weathering

Chemical weathering is the weakening and subsequent disintegration of rock by chemical reactions, which include oxidation, hydrolysis, and carbonation. These processes results in the formation or destruction of minerals, so the nature of the rock’s mineral composition changes. The critical parameters for chemical weathering are temperature and moisture. Hot, humid climatic regions favor the chemical weathering of rock minerals.

 

Oxidation

The reaction of rock minerals with oxygen leads to oxidation, which changes the mineral composition of the rock. The oxidation makes the minerals less resistant to weathering. Iron, a common mineral, on oxidation becomes red or rust colored.

The reaction of carbonic acid with rock minerals is called Carbonation. Carbonic acid is formed when carbon dioxide combines with. Carbonic acid dissolves or breaks down minerals in the rock.

Chemical reaction of water with a chemical is called Hydrolysis. Water changes the chemical composition and size of minerals in rock, making them less resistant to weathering. For example when the mineral feldspar is completely hydrolyzed, clay minerals and quartz are produced and the ions such as K+, Ca2+ or Na+ are released.

A hydrolysis reaction of orthoclase (alkali feldspar), a common mineral found in igneous rock, yields kaolinite, silicic acid, and potassium ion:

Hydration is the absorption of water into the mineral structure. A good example of hydration is the absorption of water by anhydrite, resulting in the formation of gypsum. Hydration expands volume and also results in rock deformation.

 

Dehydration is the removal of water from rock or mineral structures. A good example of dehydration is the removal of water from limonite, resulting in the formation of hematite.

 

Biological weathering

Biological weathering is the weakening and subsequent disintegration of rock by plants, animals and microbes. Growing plant roots can exert stress or pressure on rock. Although the process is physical, the pressure is exerted by a biological process, i.e., growing roots. Biological processes can also produce chemical weathering. For example, when plant roots or microorganisms produce organic acids, the latter help dissolve minerals.

 

Microbial activity breaks down rock minerals by altering the rock’s chemical composition, thus making it more susceptible to weathering. One example of microbial activity is lichen; lichen is fungi and algae, living together in a symbiotic relationship. Fungi release chemicals that break down rock minerals; the minerals thus released from rock are consumed by the algae. As this process continues, holes and gaps continue to develop on the rock, exposing the rock further to physical and chemical weathering. Burrowing animals can move rock fragments to the surface, exposing the rock to more intense chemical, physical, and biological processes and so indirectly enhancing the process of rock weathering.

Although physical, chemical, and biological weathering are separate processes, but some or all of the processes can act together in nature.

 

Coordination Compounds in Soil

The major organic constituents of soil is humus other being peat, coal, many upland streams, dystrophic lakes, and ocean water. Humus is produced by biodegradation of dead organic matter. The major constituent of humus is humic acid. Part of the organic matter of soil consists of organisms (mainly bacteria and fungi) and plant roots and root hairs. The remainder is largely in the form of fulvic and humic acids. These acids of indefinite composition are classified on the basis of their solubility behavior; both humic and fulvic acids are soluble in strong alkali solution. When pH of solution is adjusted to 2 with hydrochloric acid, fulvic acid remains in solution but humic acid having molecular weights of 20,000 to 100,000, is precipitated. Humic acid and fulvic acid are not single acids. These acids are the complex mixture of many different compounds containing carboxylic and phenolate groups. A wide variety of smaller molecules such as alkanes, amino acids, amino sugars, sulfur and phosphorus derivatives of sugars, etc. are associated in these acids. Model structure of humic acid is given in Figure 1.

 

Due to the presence of carboxylic and phenolate groups, the fulvic and the humic acids behave functionally as a dibasic or, sometimes, as a tribasic ligands. These ligands interact strongly with inorganic ions such as Mg2+, Ca2+, Fe2+ and Fe3+ and form complexes. Many humic acids have two or more of these groups arranged so as to enable the formation of chelate complexes. The formation of complexes (chelates) is an important aspect of the biological role of humic acids in regulating bioavailability of metal ions in water and soil.

Figure 1. The model structure of humic acid

The most important thing to realize is that oxygen is by far the most abundant element in the Earth’s crust. But this refers to oxygen in compounds, not oxygen as a gas. This oxygen is present as part of solid chemical compounds in the rocks and soil.

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References

  1. Basic Inorganic Chemistry, 3rd Edition; F. Albert Cotton, Geoffrey Wilkinson, Paul L. Gaus, 1994 Wiley.
  2. Concise Inorganic Chemistry, 5th Edition; J. D. Lee, 1998, Wiley-Blackwell
  3. https://www.google.co.in/search?q=chemical+composition+of+lithosphere&espv=2&biw=1024& bih=667&tbm=isch&tbo=u&sou
  4. http://digital.csic.es/bitstream/10261/56164/1/Humic%20substances.%20Introduction.pdf