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. 

Classification of Elements on the Basis of Their Properties

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 di positive ions. The oxides and hydroxides and carbonates of Mg, Ca, Sr and Ba are alkaline and their aqueous solutions are alkaline. The carbonates of these metals are found in Earth crust. The last member of the group, Radium, is radioactive and has several applications.

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.

Chemical Bonding

Chemical bonding is the process by which atoms or molecules combine to form more complex structures, held together by attractive forces. These forces arise from the interactions between electrons and nuclei of different atoms. Chemical bonds can be categorized into several types, including ionic bonds, where electrons are transferred between atoms, covalent bonds, where electrons are shared between atoms, and metallic bonds, which involve a 'sea' of shared electrons among a lattice of metal atoms. The nature of the chemical bond determines the physical and chemical properties of the resulting compounds.

1. Ionic Bonding

Ionic bonds are formed through the transfer of electrons from one atom to another, resulting in the formation of ions. This typically occurs between metals and non-metals. Metals lose electrons to become positively charged cations, while non-metals gain those electrons to become negatively charged anions. The electrostatic attraction between oppositely charged ions creates the ionic bond.

Example Reaction: Formation of Sodium Chloride (NaCl)

• Sodium (Na) loses one electron to form a sodium ion (Na⁺): Na→Na++????−Na→Na++e−
• Chlorine (Cl) gains one electron to form a chloride ion (Cl⁻): Cl+????−→Cl−Cl+e−→Cl−
• The Na⁺ and Cl⁻ ions attract each other to form sodium chloride: Na++Cl−→NaClNa++Cl−→NaCl

2. Covalent Bonding

Covalent bonds involve the sharing of electrons between atoms, typically between non-metals. The shared electrons allow each atom to achieve a stable electron configuration, similar to that of noble gases.

Example Reaction: Formation of Water (H₂O)

• Hydrogen (H) atoms share electrons with an oxygen (O) atom to form water molecules: 2H2+O2→2H2O2H2​+O2​→2H2​O In a water molecule, each hydrogen atom shares one electron with the oxygen atom, forming two covalent bonds: H−O−HH−O−H

3. Metallic Bonding

Metallic bonds occur between metal atoms. In metallic bonding, atoms in a metal lattice share a 'sea' of delocalized electrons, which are free to move throughout the structure. This electron sharing gives metals their characteristic properties, such as conductivity, malleability, and ductility.

Example: Metallic Bonding in Copper (Cu)

• Copper atoms in a solid state share a pool of electrons that move freely throughout the metal lattice. This delocalization of electrons binds the metal ions together, but it is not represented by a simple reaction equation like ionic or covalent bonding.

Other Types of Bonding

Hydrogen Bonding

• Hydrogen bonds are weak bonds that occur when a hydrogen atom, covalently bonded to a highly electronegative atom (such as oxygen or nitrogen), is attracted to another electronegative atom. This type of bond is crucial in biological molecules like DNA and proteins.

Example: Hydrogen Bonding in Water (H₂O)

• The hydrogen atoms of one water molecule are attracted to the oxygen atom of another water molecule, forming hydrogen bonds: H2O⋯H2OH2​O⋯H2​O

Van der Waals Forces

• Van der Waals forces are weak intermolecular forces that arise from temporary dipoles in molecules. They include dispersion forces and dipole-dipole interactions.

Example: Interaction Between Noble Gas Atoms

• Inert gases like argon (Ar) exhibit Van der Waals forces due to temporary dipoles: Ar⋯ArAr⋯Ar