Category: Chemistry Notes PPT

Do You Know the Ligand Definition?

A ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex in coordination chemistry. Formal donation of one or more of the ligand’s electron pairs, often via Lewis Bases, is necessary for bonding with the metal. Metal ligand bonding can be either covalent or ionic in nature. The metal-ligand bond order can also vary from one to three. Lewis bases are considered ligands, while Lewis acidic “ligands” have been found in rare cases.

As we already defined ligands, now let’s take a look at examples of neutral ligands (neutral molecule), cationic ligands, and neutral ligands.

What is Ligand in Chemistry?

Ligand Examples

Occasionally ligands can be cations (NO⁺, N₂H₅⁺) and electron-pair acceptors. F^–, Cl^–, Br^–, I^–, S₂^–, CN^–, NCS^–, OH^–, NH₂^– are examples of anionic ligands, while NH₃, H₂O, NO, and CO are examples of neutral ligands. 

Metals and metalloids are almost always bound to ligands, while gaseous “naked” metal ions can be formed in a high vacuum. Ligand substitution rates, ligand reactivity, and redox are all factors that affect the reactivity of the central atom in a complex. In several practical fields, such as bioorganic and medicinal chemistry, homogeneous catalysis, and environmental chemistry, ligand selection is important.

Types of Ligands 

1. Monodentate Ligands

Monodentate ligands are also known as “one-toothed” ligands because they only bite the metal atom in one place.

2. Bidentate Ligands

Bidentate ligands are Lewis bases that donate two lone pairs of electrons to the central metal atom. Chelating ligand is a term used to describe them. Chelates refers to a complex containing chelating ligands.

3. Tridentate Ligands and Polydentate Ligands

To the central metal atom or ion, tridentate ligands have three lone pairs of electrons. Tetradentate molecules have four donor atoms, pentadentate molecules have five donor atoms, and hexadentate molecules have six donor atoms. Polydentate ligands are a common term for them.

4. Trans- Spanning Ligands

Bidentate ligands that can span coordination positions on opposite sides of a coordination complex are known as trans-spanning ligands.

5. Ambidentate Ligand

Ambidentate ligands, unlike polydentate ligands, can bind to the central atom in two separate ways. Thiocyanate, SCN, is a clear example of this, since it may bind to either the sulfur or nitrogen atom. Linkage isomerism is caused by such compounds. Polyfunctional ligands, especially proteins, may form isomers by bonding to a metal centre via different ligand atoms.

6. Bridging ligand

A bridging ligand is a molecule that binds two or more metal centres. Coordination polymers, which are made up of metal ion centres connected by bridging ligands, make up virtually all inorganic solids with simple formulas. Both anhydrous binary metal ion halides and pseudohalides fall into this category. In solution, bridging ligands are also present. Since polyatomic ligands like carbonate are ambidentate, they often bind to two or three metals at the same time. The prefix “” is often used to denote atoms that bridge metals. The presence of many bridging ligands makes most inorganic solids polymers. Because of their possible use as building blocks for the fabrication of practical multimetallic assemblies, bridging ligands, which are capable of coordinating multiple metal ions, have attracted a lot of attention.

7. Binucleating Ligand

Binucleating ligands are molecules that bind two metal ions together. Binucleating ligands usually involve bridging ligands like phenoxide, or pyrazine, as well as other donor groups that only bind to one of the two metal ions.

8. Metal–Ligand Multiple Bonds

Some ligands can bind to a metal centre by using the same atom but a different number of lone pairs. The metal-ligand bond angle (MXR) can be used to determine the bond order of the metal-ligand bond. This bond angle is often referred to as either linear or bent, depending on the degree to which the angle is bent. In its ionic form, an imido ligand, for example, has three lone pairs. One lone pair serves as a sigma X donor, while the other two serve as L-type pi donors. The MNR geometry is linear when both lone pairs are used in pi bonds. If either or both of these lone pairs are nonbonding, the MNR bond is bent, and the degree of the bend is determined.

9. Spectator Ligand

A spectator ligand is a closely coordinating polydentate ligand that eliminates active sites on metal but does not participate in chemical reactions. The reactivity of the metal centre to which they are bound is influenced by spectator ligands.

10. Chiral Ligands 

Chiral ligands can be used to create asymmetry in the coordination sphere. In several cases, the ligand is used as an optically pure group. Asymmetry results from coordination in some situations, such as secondary amines. For homogeneous catalyzes, such as asymmetric hydrogenation, chiral ligands are used.

Did You Know?

The number of times a ligand binds to a metal via noncontiguous donor sites is referred to as denticity (represented by). Many ligands may bind metal ions at multiple sites, which is typically due to the presence of lone pairs on multiple atoms. Chelating ligands are those that bind to more than one molecule. Bidentate ligands bind to two sites, while tridentate ligands bind to three sites. The angle formed by the two bonds of a bidentate chelate is known as the “bite angle.” 

Chelating ligands are frequently made by joining donor groups together with organic linkers. Ethylenediamine, which is made by connecting two ammonia groups with ethylene (CH2CH2) linker, is a classic bidentate ligand. The hexadentate chelating agent EDTA, which can bind through six sites and fully surround certain metals, is an example of a polydentate ligand.

The Lucas Test is the test that is performed by using Lucas reagent with alcohols to distinguish primary, secondary and tertiary alcohols. In this carbocation is formed as intermediate and it follows a unimolecular nucleophilic substitution reaction mechanism. 

As primary, secondary and tertiary alcohols differ in their reactivity with Lucas reagent, so they give different results as well and it forms the base for Lucas Test. A positive test indicates the change in colour of the sample from clear and colourless to turbid signalling formation of a chloroalkane. 

Lucas test is performed to distinguish primary, secondary and tertiary alcohols and which alcohol gives the fastest alkyl halide. Lucas test is based on the difference in reactivity of alcohols with hydrogen halide. Primary secondary and tertiary alcohols react with hydrogen halide (hydrochloric acid) at different rates. It follows the SN1 reaction mechanism. 

The Lucas test was given by Howard Lucas in 1930. After that, it soon became popular in organic chemistry for qualitative analysis. Although with the discovery of spectroscopic and chromatographic methods of qualitative analysis in organic chemistry, this test has taken a back seat and is generally used for teaching purposes in schools and colleges.

 

The Lucas test is an important topic of Class XII Chemistry. Every year many questions are asked about this topic in the final exam. So, you need to give special attention to the preparation of this topic. In this article, we will discuss the Lucas test in detail with its mechanism.

What is Lucas Reagent? 

The solution of concentrated hydrochloric acid with zinc chloride is called Lucas reagent. Thus it can also be defined as a solution of anhydrous zinc chloride present in the concentrated Hydrochloric acid. It is used to classify the alcohols that have a lower molecular weight. It is a substitution reaction where the chloride finally replaces the hydroxyl group. The reaction results from the clear and colourless solution to the turbid that indicates the formation of chloroalkanes. In this reaction, the tertiary alcohols from their respective halides are much faster than the primary or the secondary alcohols because the intermediate tertiary carbocation is much stable in tertiary alcohol. Both concentrated HCl and ZnCl₂ are taken in equimolar quantities to make the reagent. 

Since the test was first conducted in the year 1930, the test became the standard test for identification of primary, secondary and test the test was first conducted in the year 1930 and since then it became the standard method for the identification of primary, secondary and tertiary alcohol. But with the development of new testing methods like spectroscopy and various chromatography, the Luca’s test is becoming less popular.   In the Lucas test, Lucas reagent reacts with alcohols and gives different results on the basis of stability of carbocation intermediate formed during the reaction. The Chloride ion of hydrochloric acid reacts with an alkyl group of alcohol and forms alkyl chloride while zinc chloride is used as a catalyst. The rate of reaction of primary, secondary, and tertiary alcohols with Lucas reagent differ which forms the base of the Lucas Test. The simple reaction involved is represented below –

ROH + HCl →  RCl + H₂O

(In presence of  ZnCl₂)

Lucas Test

Lucus test is performed to differentiate between primary, secondary and tertiary alcohols. This test is based on the difference in the reactivity of the primary, secondary and tertiary alcohol with hydrogen halide by SN1 reaction. 

ROH + HCl →  RCl + H₂O

(In presence of  ZnCl₂)

The difference in the reactivity of the degrees of alcohol provides the differing ease of formation of the corresponding carbocations. The primary carbocation is least stable followed by secondary carbocation and the tertiary alcohols form the most stable tertiary carbocations. 

The reagent for the test is the equimolar mixture of ZnCl₂ and concentrated HCl. the alcohol becomes protonated and the water molecule that is formed leaves, that makes the carbocation, and the nucleophile Cl⁻ (which is present in excess) readily attacks the carbocation, forming the chloroalkane. Due to the low solubility of the organic chloride in the aqueous mixture, the tertiary alcohol reacts immediately within five minutes that becomes evident by turbidity.  

How to Perform Lucas Test? 

Lucas test is performed by following steps –

• Preparation of Lucas Reagent – Take equimolar quantities of zinc chloride and concentrated HCl and make a solution.

• Take a very small quantity of the given sample in a test tube.

• Now add ~2ml of the Lucas reagent in the test tube containing the given sample and mix them.

• Record the time until the solution become turbid or cloudy.

Result of the Lucas Test if Sample Contains 1° Alcohol

If the sample contains primary alcohol, then it will not give a turbid or cloudy solution as a result at room temperature. If we give heat to the solution, then after 30-45mins turbidity comes. General reaction can be represented as follows –

Sample containing primary alcohol + Lucas Reagent 🡪 No turbidity in the solution

After giving heat/ 30-45min –

Sample containing primary alcohol + Lucas Reagent 🡪 Turbidity in the solution

For example, if an ethanol solution reacts with Lucas reagent at room temperature, then it doesn’t give any turbid solution. 

Result of Lucas Test if Sample Contains 2° Alcohol 

If the sample contains secondary alcohol, then the test will give a turbid or cloudy solution as a result at room temperature after 3-5minutes. General reaction can be represented as follows –

Sample containing secondary alcohol + Lucas Reagent 3-5min.→ Turbidity in the solution

For example, if isopropyl alcohol is present in the sample solution then after adding Lucas reagent in it, it will give a turbid solution after 3-5min. The reaction is given below –

(CH₃)₂CHOH + HCl+ZnCl2→ (CH₃)₂CHCl + H₂O + ZnCl₂

Isopropyl alcohol                 2°alkyl chloride (turbid solution)

Result of the Lucas Test if Sample Contains 3° Alcohol 

If the sample contains tertiary alcohol, then the test will instantly give a turbid or cloudy solution as a result at room temperature. General reaction can be represented as follows:-

Sample containing tertiary alcohol + Lucas Reagent Instantly→ Turbidity in the solution

For example, if tertiary butyl alcohol is present in the sample solution then after adding Lucas reagent in it, it will give a turbid solution instantly. The reaction is given below –

(CH₃)₃COH + HCl + ZnCl2 → (CH₃)₃CCl + H₂O + ZnCl₂

Explanation of Difference in Reactivity of 1°,2° & 3° Alcohols with Lucas Reagent 

The reaction of primary, secondary and tertiary alcohols with Lucas reagent takes place through a unimolecular nucleophilic substitution reaction mechanism. Lucas reagent forms carbocation as intermediate with all three alcohols. But the stability of carbocation intermediate differs in all three reactions. Tertiary alcohol gives instant results with Lucas reagent as its carbocation is highly stable. While secondary alcohol gives results with Lucas reagent after 3-5mins as its carbocation intermediate is moderately stable and primary alcohol don’t give any result with Lucas reagent at room temperature because its carbocation is highly unstable. Thus, we can write stability of carbocation intermediate of primary, secondary and tertiary alcohol is –

3°>2°>1°

Mechanism of Lucas Test Reaction

The reaction generally occurs in the SN1 nucleophilic reaction which is a two step reaction. The alcohols that are highly reactive become carbocation intermediates and then exhibit an immediate reaction. The SN1 nucleophilic reaction is exhibited by the primary and the secondary alcohols.

()

The two-step is generally as follows:-

1. In the first step, the H⁺ proton from the hydrogen halide will first protonate the negatively charged hydroxyl ion that is OH⁻ in the alcohol. Thus the water, thus, attached to the carbon atom is a weaker nucleophile than the chlorine ion and thus the chlorine ion replaces the water. This results in the formation of a carbocation. 

2. In the second step the Cl⁻ attacks the carbocation and thus forms alkyl chloride.

Loss of leaving group and formation of carbocation – In this step zinc chloride reacts with alcohol and forms carbocation intermediate and loss of leaving group takes place. ZnCl₂ behaves as lewis acid. Zinc gains electrons from the oxygen atom and gets bonded with it. Thus, zinc gets a negative charge while oxygen atom gets a positive charge. Now the electron-deficient oxygen atom being an electronegative element gains electrons from the alkyl group. It leads to the formation of a carbocation. This is the slowest step of the reaction. So, it is the rate-determining step. Thus, we can say the rate of reaction depends on the formation of carbocation and its stability. The reaction is given below –

Nucleophilic attack – Cl⁻ acts as nucleophile and attacks on carbocation and forms alkyl chloride. Due to the higher entropy of water, H⁺ of HCl reacts with the hydroxyl group and forms water. Catalyst zinc chloride gets removed as it is. 

Applications of Lucas Test

Lucas test has the following applications –

It is used to distinguish primary, secondary and tertiary alcohols in the sample.

• It gives information about which alcohol gives the fastest alkyl halides. By Lucas test, we can write the order of giving alkyl halides by primary, secondary and tertiary alcohols. Tertiary alcohol gives the fastest alkyl halide.

 

3°>2°>1°

Malathion is a broad-spectrum organophosphate acaricide and insecticide also known as mercaptothion, maldison, and carbophos (that is used to kill mites and ticks). Considerably less toxic to humans compared to parathion, malathion can be suited for the control of garden and household insects and is essential in the control of boll weevils, mosquitoes, lice, and fruit flies.

Properties

Malathion is given as a colourless yellow-brown liquid having a characteristic unpleasant odour. Malathion iupac id is 4004. Generally, it can be prepared by combining O and O-dimethyl phosphorothioate with diethyl maleate. It is also soluble in most of the organic solvents except paraffin hydrocarbons. Practically, it insoluble in water and is readily decomposed by alkalies. The malathion chemical acts by binding to the enzyme acetylcholinesterase (AChE) at nerve endings, causing the neurotransmitter acetylcholine (ACh) to be disrupted, eventually leading to death.

Mechanism of Action

Malathion is a compound that inhibits acetylcholinesterase, which is a large family of chemicals. When this is injected into the target organism, it binds irreversibly to the serine residue in the cholinesterase enzyme’s active catalytic site. The phosphoester group is then tightly bound to the cholinesterase and irreversibly deactivates the enzyme, resulting in a rapid build-up of acetylcholine at the synapse.

Production Method

Malathion can be produced by the addition of dimethyl dithiophosphoric acid to either diethyl fumarate or diethyl maleate. The compound is chiral but can be used as a racemate.

Mix Ratio for 55% Malathion for Killing Bedbugs

Generally, while malathion is considered as an exterior pesticide, there are a few formulations, which are labelled interior use, with certain label restrictions. In the U.S.A., the product is not labelled at all for the treatment of bedbugs. The other issue with malathion is the fact that, generally, it can be sold as a malathion oil emulsion and holds a nasty odour. Malathion, that being said, is still considered as an effective treatment of malathion insecticide against bedbugs that have developed a tolerance to the pyrethrin and pyrethroid in a few areas.

In the United States, the last labelled use for bedbug control happened in 1965, a time when lindane, chlordane, D.D.T., and other insecticides were in wide use. At that time, the label suggested a concentration ranging from 0.5% to 1.0%, which would be achieved by mixing a single ounce ranging from 55% with 50–100 ounces of water (also, kerosene was a listed vehicle for malathion, but certainly it should not be used inside the home).

There are many other malathion insecticides, which are considered more appropriate for bedbug control, and they should be readily available for purchases either at hardware or home supply stores. The other suggested treatment is using the ethanol diluted in water is as a topical treatment, but it kills only bedbugs which are sprayed directly with it, and after it dries, it does not have a residual effect.

Most of the websites that cover the bedbug’s information recommend hiring a professional pest control company to treat the premises for bedbugs since the local populations tend to develop some resistance to chemical treatment quickly and fairly.

Malathion Uses

Let us see some of the malathion insecticide uses.

Malathion Pesticide Use

Malathion is given as a pesticide, which may be used widely in residential landscaping, agriculture, in public health pest control programs such as mosquito eradication, and also in the public recreation areas. And, in the U.S., it is known to be the most commonly used organophosphate insecticide.

In the 1980s, a malathion mixture containing corn syrup was used to battle the Mediterranean fruit fly in California and Australia. Whereas, in the United States and Canada, starting in the early 2000s, malathion was sprayed in several cities to combat the West Nile virus.

Malathion was also used over the last two decades regularly during the summer to kill mosquitoes. However, homeowners were allowed to exempt their properties in the case if they chose. Today, Winnipeg is the only major city in Canada with a program of ongoing malathion adult-mosquito-control.

Medical Use

In low doses (0.5% preparations), malathion can be used as a treatment for:

• Body lice and head lice. Malathion is also approved by the United States Food and Drug Administration for the treatment of pediculosis. It is effectively claimed to kill both the adult lice and eggs, but in fact, it has been shown in the United Kingdom, studies to be only 36 percent effective on the head lice and less so on their eggs. This specific low efficiency was observed when malathion was applied to lice found on schoolchildren in the Bristol region of the United Kingdom, and it is believed by some to be caused by the lice developing resistance to malathion.

• Scabies

Some of the preparations of malathion include Prioderm, Derbac-M, Quellada-M, and Ovide.

Toxicity

Malathion is more highly toxic to the bees and other aquatic invertebrates, beneficial insects, and a few species of fish, notably largemouth and bluegill bass. Also, it is of moderate toxicity to the birds.

Melting refers to a change in the phase of a substance from a solid-state to a liquid state. This is a phase transition phenomenon. It means that, in this procedure, a substance is converted from one state of matter (solid) to the other (liquid). Melting and otherwise, fusion is a physical condition that involves the change of a substance from a solid-state to a liquid state. This occurs whenever the internal energy of the solid increases, generally via the application of heat or pressure, which raises the temperature of an object to the melting point.

()

What is Latent Heat?

It is the energy absorbed or released by a substrate throughout a change in its physical state (phase) that takes place without modifying its temperature. The heat which is associated with the melting of a solid or the freezing of a liquid is called the latent heat of fusion, and the heat that is associated with the vaporization of a liquid or solid or even the condensation of vapour is called the latent heat of vaporization.

Latent heat is usually defined as the amount of heat required in units of joules or calories, per mole or unit mass of the substance currently experiencing a change of state of matter.

Latent heat is linked with procedures other than changes in the solid, liquid, and vapour states of a specific element. Almost all solids exist in different crystalline modifications, and the transformations between them usually involve the absorption or evolution of latent heat. 

How Does Melting Occur?

• Almost all solids are assembled or packed in a rigid crystal lattice with strong intermolecular forces of attraction.  When the heat passes, the internal binding energy of the crystal lattice will be overcome by the heat energy, and the intermolecular attraction forces get weakened.

• This weakening of intermolecular forces of attraction leads to instability in the crystal lattice. The molecules of the solids tend to separate from each other and begin moving in different directions. The instability of the crystal lattice triggers the melting of a solid substance.

• According to the accepted melting theory, when the temperature of the substance starts to increase as a result of heat supplied or increased pressure, the molecules of the substance begin to vibrate at their places. When the amplitude (or distance covered) of the vibration surpasses the interatomic distance of the material, it causes vibrational instability and induces the substance to melt.

Melting Point 

The melting point is the temperature of the solid at which it transforms its physical state of matter from solid-state to liquid state at atmospheric pressure. The two phases of the solid and liquid state remain at equilibrium at the melting point. This means that at the melting point both the solid-state and the liquid state exist concurrently.  The melting point of the substance also changes depending on the change in atmospheric pressure.

Example Questions 

Question 1) What is the melting point of metals? What are the melting temperatures of metals found commonly?

Answer) The melting point of a material is the temperature at which it changes its physical state from solid to liquid at atmospheric pressure. At the melting of a substance, it’s solid and liquid states are actually in equilibrium. The melting point of a substance depends on the pressure and is usually specified as the standard pressure. The melting points of all metals also depend on their physical and chemical properties which involve their intermolecular forces of attraction, and hence the values are different for different metals.

The melting points of common metals are: 

Bronze: 913 °C

Brass: 927 °C

Copper: 1083 °C

Iron: 1538 °C

Steel: 1371 °C

Nickel: 1452 °C

Gold: 1064 °C

Silver: 961 °C

Question 2) Why is the heat energy required to melt a solid? 

Answer) Heat energy is needed to melt the solid because the heat energy increases the kinetic energy of the particles, which is sufficient to break the attraction or bond between the particles and to make them move faster. As a result, the state of matter is transformed from solid-state to a liquid state, or we can say that it is a conversion of solid to liquid.

Methyl Ethyl Ketone (MEK), also known as butanone, is an organic chemical with the formula CH3C(O)CH2CH3. This dialkyl ketone is a colourless liquid that has a sharp and sweet odour. It occurs in nature in a small amount. However, due to its various applications, the industrialists produce this chemical on a large scale. The other popular name of methyl ethyl ketone is methyl acetone. This chemical liquid is soluble in water, which makes it an excellent industrial solvent.  It is an isomer of the tetrahydrofuran, which is also a popular solvent. 

Production of Methyl Ethyl Ketone

The oxidation of 2-butanol leads to the production of butanone. The dehydrogenation of 2-butanol takes place using a catalyst like copper, zinc, or bronze. The reaction for the production of MEK from 2-butanol is as follows:

CH3CH(OH)CH2CH3 → CH3C(O)CH2CH3 + H2

The industrialists use this process to manufacture approx 700 million kilograms of MEK annually. The other method that is not much popular but can yield methyl acetone is Wacker oxidation of isobutyl benzene and 2-butene. The modification of the cumene process can lead to the production phenol and a mixture of butanone and acetone. The liquid-phase oxidation of Fischer-Tropsch and heavy Napthla reaction can generate mixed oxygenate streams, which can lead to the extraction of methyl ketone using fractionation. 

MEK is an organic compound that exists in the liquid state at room temperature. It has a unique and sweet odour, which is similar to acetone. This colourless solvent is flammable, and it has a low boiling point that is 79.64oC. It has a fast evaporation rate and has excellent solvent properties. Methyl acetone is miscible with almost all organic solvents. It makes this compound excellent for a variety of resin systems in various industries. It forms an azeotrope with many organic solutions that makes it an excellent solvent. The vapours of MEK are even heavier than ordinary air. In the below table, we have provided the various MEK properties.

Physical Properties of Methyl Ethyl Ketone

The above image shows the structure of Methyl Ethyl Ketone.

Chemical Properties of MEK

Methyl Ethyl Ketone is a highly reactive compound that can undergo various chemical reactions under proper conditions. The reactivity of this compound centres around its adjacent hydrogen atoms and the carbonyl group. 

Two moles of MEK can undergo aldol condensation to yield hydroxyl ketone. Then, it can readily dehydrate to form an unsaturated ketone. This reaction is as follows.

Methyl Ethyl Ketone can also react with aldehydes to give cyclic compounds, ketals, and higher ketones depending upon the conditions. It also condenses with organic oxides and glycols to yield derivatives of dioxolane. Moreover, the reaction of MEK with aqueous ammonia and hydrogen yields sec-butylamine. 

In the above reaction, the use of MEK in excess amounts can give di-sec-butylamine.

The oxidation of MEK with oxygen leads to the production of diacetyl that is a flavouring material. Chlorination of this compound yields a mixture of dichloro and monochloro derivatives in several percentages. MEK can also react with hydrogen peroxide to give a mixture of peroxides. 

The other chemicals that can react with MEK to form resins include phenol, formaldehyde, acetaldehyde, etc. These resins are useful in moulded products as well as electric insulation. Moreover, the reaction of methyl acetone with acrylonitrile will give di-nitrile, which can undergo hydrogen to produce amines. 

Applications and Uses of Methyl Ethyl Ketone 

MEK is an excellent solvent due to which it has numerous applications in many industries. This compound is beneficial in the production of resins, gums, cellulose acetate, etc. The manufacturing of vinyl films also requires the use of this compound. For these reasons, MEK is also helpful in producing textiles, plastics, and paraffin wax. 

The production of various household products like paint remover, lacquer requires MEK due to its solvent properties. It is also used as a denaturing agent for glues and denatured alcohol. Butanone can dissolve many plastics, including polystyrene. Hence, it is also a part of many scale model kids to connect different components. The adhesive property of methyl acetone makes it a welding agent in plastic products. Butanone can also work as a cleaning agent. 

MEK acts as a precursor to methyl ethyl ketone peroxide that is a significant catalyst for many polymerization reactions. It can also lead to the production of dimethylglyoxime by reacting with ethyl nitrile. This reaction first yields diacetyl monoxime, which then converts into di-oxime to give the desired product. MEK is also required for the production of petroleum.

It is indeed true that for many of us the picture of a periodic table is the symbol for chemistry. The periodic table is not just an organised tabular arrangement of elements but the greatest source of information as far as chemistry is concerned. Without a proper classification and arrangement of elements, it would have been really complicated to study and explore the plethora of possibilities in chemistry. It is really fascinating to know how the periodic table was invented.

History of the Periodic Table

The periodic table that we study today is the modern periodic table and was invented by Dmitri Mendeleev. However, Mendeleev was not the first one who grouped and arranged the elements in the form of a periodic table. The following are a few attempts to classify elements that took place prior to the recognition of the modern periodic table.

• In 1789, Antoine Lavoisier was the first to classify elements based on their properties. He grouped the elements into gases, non-metals, metals and earthly elements. 

• After several decades, in 1829 Johann Döbereiner made an attempt to group elements. Based on the similarity in chemical properties, he grouped elements into triads. The middle element’s atomic weight in the triad stands approximately as the average of the first and third elements’ atomic weights. This can also help define the properties of the middle element. Eg: Lithium, sodium and potassium. Since Döbereiner failed to classify all known elements into triads, the law of triads did not find success.

• The attempt to classify elements did not stop here and it was in 1865 that the English Chemist John Alexander Newland arranged the elements in the order of their increasing atomic weights. He discovered a periodic pattern in the arrangement. He showed that the physical and chemical properties of the eighth element is similar to the properties of the first element in that row. This generalization made by Newlands is known as the ‘Law of Octaves’.

• It was only in 1869 that Mendeleev published his paper in the Journal of Russian Chemical Society where he classified the elements based on their atomic masses and arranged them into horizontal rows called periods and vertical columns called groups. The periodic law states that the properties of elements are a periodic function of their relative atomic masses. Mendeleev was successful in arranging all 63 elements that were known in his time into a tabular form which contained eight columns and seven rows. It also contained some gaps which were later filled after the discovery of new elements.

()

Mendeleev was successful in classifying the elements and showing the periodic similarity in elements. However, there were a few anomalies in the arrangement which were later cleared by the English Physicist, Henry Moseley.

Moseley’s Periodic Law

Henry Moseley showed that the chemical and physical properties of elements are determined by the atomic number and not by the atomic mass. He restated the periodic law as-

‘Physical and chemical properties of an element are a periodic function of its atomic number.’ 

Moseley’s periodic law is also known as the Modern Periodic Law and it paved the way to the modern periodic table.

Modern Periodic Table:

The modern periodic table of elements is the one that we use today. It is based on the concept of Mendeleev’s periodic table but differs in the fact that the elements are arranged in the increasing order of atomic number and not atomic mass.

 

Features of Modern Periodic Table

In the study of chemistry, the modern periodic table holds prime importance. Following are some of the main features of the modern periodic table.

• Elements are grouped in ascending order of their respective atomic numbers.

• There are seven horizontal rows called periods and eighteen vertical columns called groups.

• The elements in a group show similar physical and chemical properties since they contain the same number of outer electrons. But they show a gradual change as we move from top to bottom in a group. 

• The elements in a period show a gradual change in properties on moving from left to right. Atomic size gradually decreases as we move from left to right. 

• The modern periodic table consists of more elements when compared to the 63 elements in Mendeleev’s periodic table. Presently it has 118 elements. 

It would not be wrong to say that the study of the chemistry of elements would have been impossible without the existence of the modern periodic table. The classification of elements in the modern periodic table helps in the easier understanding of the properties of elements.

Classification of Elements in the Periodic Table

Following are the major classification of elements in the modern periodic table-

• Alkali and Alkaline Earth metals: The first two groups on the left side of the periodic table consist of highly reactive elements (except hydrogen). The first group elements contain one electron in the valence shell while the second group elements contain two electrons in their valence shell.

• Transition metals: These elements occupy the centre of the periodic table and mostly show the properties of metals. The elements starting from group 3 to 12 fall under transition metals. Some of the transition metals are placed separately in two rows at the bottom of the periodic table. These are known as Lanthanides and Actinides.

• Metalloids and nonmetals: Metalloids generally appear in a diagonal line at the right side of the periodic table. These are basically the elements that separate metals on the left side of the periodic table from the nonmetals on the right side of the periodic table. These elements exhibit the properties of metals and non-metals, thus, they are called metalloids. 

• Noble gases: The extreme right side of the periodic table is occupied by gases. They are placed in the 18th group and have completely filled valence shells. These gases are non-reactive and are called the inert or noble gases.

The study of modern periodic table (groups and periods) is so significant in the world of chemistry that it can be called as the pillar of che

Modern Periodic Table – Features & Significance

A modern periodic table is a table consisting of elements that we still use in the present time. The table is based mainly on the periodic table of Mendeleev but has some differences. The elements here are arranged in the order of increasing atomic number and not atomic mass like the previous tables. Some of the features we should know about this table are:

1. The constituents of the table are arranged in the increasing order of their atomic number.

2. The table has 7 rows that are horizontal and are called the rows. Similarly, there are vertical columns that are 18 in number and called the groups.

3. The properties like physical and chemical are similar for all the elements present in the table. In the outer electrons, they have the same number but gradually change from the top to the bottom of the group of elements.

4. The elements present in the period change gradually when we move from left to right. The atomic size also decreases from the left to right direction.

5. This periodic table has more elements compared to the previous table like Mendeleev’s periodic table. It has about 118 elements.

Without the modern periodic table, it would have been hard to study all the elements that are required today. From the classification of these elements, we can understand the properties of the elements. Thus, the table provides a huge help to us in the present day.