In these chemistry notes, we learn about biomolecules class 12 chemistry. Some state boards books also contain these notes in 11th class chemistry book i.e. Biomolecules class 11. We prepare chemistry notes as per biomolecules class 12 ncert book, that is good for both classes of students.
Cell is the basic unit in all living organisms. Cell has very complicated structure and contains a large number of lifeless molecules.
Biomolecules Definition
The Biomolecules are lifeless molecules which combine in specific manner to produce life or control biological reactions. We can also say that, Bio-molecule is a molecule that is produced by a living organism.
Biomolecules Chemistry
Biomolecules in chemistry may be simple molecule like water, minerals, salts, vitamins, hormones etc. or macro-molecules like carbohydrates, proteins lipids (fats and oils) etc. They play an important role in function of organisms. The chief constituents of our diet are carbohydrates and proteins.
Biomolecules Notes
Carbohydrates are major source of energy. Proteins are nitrogenous substances present in hair, skin, muscle, nails etc. which plays variety of role in proper functioning of the living organisms. Enzymes which catalyze chemical reaction that take place in cell are proteins. Lipids are a source of stored energy. RNA (Ribonucleic Acid) and DNA (Deoxyribonucleic Acid) present in the nucleus of the cell are responsible for the storing genetic characteristics and synthesis of proteins.
Biomolecules Class 12th
Biomolecules are optically active polyhydroxy aldehydes or polyhydroxy ketones or the compounds that can be hydrolysed to polyhydroxy aldehydes or polyhydroxy ketones.
CARBOHYDRATES
Early chemist noticed that carbohydrates had molecular formula which could be represented as Cx(H2O). They were considered to be hydrates of carbon hence the name was given as carbohydrates.
Later studies revealed that these compounds were not hydrates of carbon because they did not contain free water molecules.
Similarly the compounds which fit into this formula are not classified as carbohydrates. e.g. formaldehyde HCHO, acetic acid CH3COOH, fit into this formula but are not carbohydrates.
Rhamnose C6H12O5 does not fit into this formula but are carbohydrate. You read these Biomolecules class 12 notes of NCERT chemistry on ChemistryNotesInfo.com
Classification of carbohydrates
On the basis of their hydrolysis products, carbohydrates are classified as:
Simple carbohydrates
Simple carbohydrates are also known as monosaccharides cannot be hydrolyzed further to any simpler compound. They are basic unit of all carbohydrates.
Monosaccharides are further classified as (1) aldoses, containing aldehyde (CHO) group (2) ketoses, containing keto group.
Depending upon number of carbon atoms and functional group, monosaccharides are further classified as triose having three carbon atoms, tetrose having four carbon atoms, pentose having five carbon atoms etc.
Complex carbohydrates
It is further classified as oligosaccharides and polysachharides
Oligosaccharides
An oligosachharide is carbohydrate (sugar) which on hydrolysis gives two to ten monosachharides units.
Depending upon number of monosachharides units produced on hydrolysis oligosaccharides are further classified as:(On hydrolysis)
A carbohydrate that on hydrolysis yields large number of monosaccharide units is called polysaccharide. It is non-sugar. Polysaccharides are tasteless, amorphous, insoluble in water and natural biopolymers of monosaccharides e.g. starch, cellulose, glycogen, gums etc.
Fischer Projection Formula
A tetrahedral carbon atom is represented in a fischer projection by two crossed lines. The horizontal line represents bonds coming out of the page and vertical lines represent bonds going into the page.
D- and L- Sugars
The dextorotatory enantiomer of glyceraldehyde occurs naturally and represented as (+) – glyceraldehyde. It is referred as D – glyceraldehyde.
The leavorotatory enantiomer of glyceraldehyde is represented as (-) – glyceraldehyde and referred as L – glyceraldehyde.
In fischer projection, if the hydroxyl group at lowest chirality center points to the right, the monosaccharide is referred to be D – sugar.
Whereas monosaccharide having the hydroxyl group at lowest chirality center pointing to the left is referred to be L – sugar.
GLUCOSE
Glucose is present in most of the sweet fruits, grapes, cane sugar, honey and in polysaccharides like starch and cellulose. It is also an essential component of human blood. Naturally occurring glucose is dextrorotatory and known as dextrose.
Preparation of glucose
From sucrose (laboratory method)
Glucose is prepared in the laboratory by hydrolysis of sucrose by boiling it with dilute hydrochloric acid or dilute sulphuric acid for about two hours.
C12H22O11 + H2O —–> C6H12O6 +C12H12O6
Glucose Fructose
From starch (commercial method)
When starch is boiled with dilute sulphuric acid at 393K under pressure it undergoes hydrolysis to give glucose. Excess of sulphuric acid is neutralized by adding chalk powder.
(C6H10O5)n + nH2O —–> nC6H12O6
Open chain structure of glucose
Glucose was assigned the open chain structure given below, on the basis of following evidences.
The molecular formula of glucose was found to be C6H12O6 from the elemental analysis and molecular weight determination experiments.
Action of HI: glucose prolonged heating with HI gives n-hexane, indicates that all the six carbon atoms are linked in straight carbon chain.
Action of hydroxyl amine: Glucose reacts with hydroxyl amine in an aqueous solution to form glucose oxime. This indicates the presence of CHO group in glucose.
Action of hydrogen cyanide: Glucose reacts with hydrogen cyanide to form glucose cynohydrin.
Action of bromine water: Glucose on oxidation with bromine water gives gluconic acid, which shows that the carbonyl group in glucose is aldehyde group.
Action of acetic anhydride: When glucose is heated with acetic anhydride in the presence of catalyst pyridine, glucose penta acetate is formed. It indicates that glucose is stable compound Action of dilute nitric acid: glucose as well as gluconic acid on oxidation with dilute nitric acid forms saccharic acid. This indicates the presence of a primary alcoholic group (-CH2OH) in glucose.
Spatial arrangement of different -OH groups in glucose as follows-
Cyclic structure of glucose
Open chain structure does not explain following reaction and facts.
Glucose in spite having aldehyde group does not give condensation reaction with 2,4 dinitro- phenyl hydrazine, addition reaction with sodium bisulphite and gives negative test with schiff base.
Glucose penta- acetatedoes not condense with hydroxyl amine indicating the absence of aldehyde group.
Glucose is found to exist in two different crystalline form ฮฑ and ฮฒ, called anomers. On crystallization from hot and aqueous solution, ฮฑ- glucose is obtained at 303K while ฮฒ-glucose is obtained at 371K.
Cyclic six-membered structure of glucose are called pyranose structures, in analogy with pyran. Pyran is a heterocyclic compound with one oxygen atom and five carbon atoms in the ring.
Haworth Projection Formula of Glucose Biomolecules
The substituents that are to right in a fischer projection formula. Orient the haworth projection formula with the ring oxygen at the back and the anomeric carbon at the right.
Haworth projection formula of glucose
FRUCTOSE
Fructose is made by isomerization of glucose. Fructose is the sweetest. On the basis of different reaction it is shown that, fructose is penta-hydroxyl ketone. The open structure of fructose is as follows.
The cyclic five-membered structures of fructose are called furanose structures in analogy with furan in a heterocyclic compound with one oxygen atom and four carbon atoms in the ring.
Haworth Projection Formula of Fructose Biomolecules
Disaccharides
A carbohydrate that on hydrolysis gives two same or different monosaccharide units is called disaccharide. In disaccharides, anomeric carbon of one monosaccharide molecule is bounded to a carbon of another monosaccharide molecule through an oxygen atom. Such a linkage between two monosaccharide molecules through oxygen is called glycosidic linkage. e.g. – sucrose, maltose, cellobiose, lactose.
Sucrose
Sucrose is known as cane sugar or common table sugar. In sucrose, C-1 of ฮฑ-D-glucopyranose is linked to C-2 of ฮฒ-D-fructofuranose by glycosidic linkage. On hydrolysis sucrose give equimolar mixture of dextrorotatory glucose and laevorotatory fructose. It is invert sugar.
Maltose
Maltose is obtained by partial hydrolysis of starch. In maltose C-1 of one ฮฑ-D-glucopyranose is linked to C-4 of another ฮฑ-D-glucopyranose molecule by glycosidic linkage. Thus maltose contains 1—>4ฮฑ glycoside bond. Maltose is a reducing sugar.
Cellobiose
Cellobiose is obtained by partial hydrolysis of cellulose. In cellobiose, C-1 of one ฮฒ-D-glucopyranose is linked to C-4 of another ฮฒ-D-glucopyranose molecule by glycosidic linkage. Thus cellobiose contains 1—>4ฮฒ glycoside bond. Cellobiose is also reducing sugar.
Lactose
Lactose is known as milk sugar. In lactose, C-1 of ฮฒ-D-glycopyranose is linked to C-4 of ฮฒ-D-glucopyranose by glycoside linkage. Thus lactose also contains 1—->4ฮฒ glycoside bond. Lactose is also reducing sugar.
Polysaccharides (C6H10O5)n
A carbohydrate that on hydrolysis yields large number of monosaccharide unit is called polysaccharides. In polysaccharides, a large number of same or different monosaccharaides are linked together by glycosidic linkages.
Starch
The Starch is mainly found in cereal grains, roots, tubers, potatoes etc. it is an important part of our diet. Starch is polymer of ฮฑ-D-glucopyranose and consists of two fractions, amylose and amylopectin.
Amylose is water soluble component and constitutes about 20% of starch. It is long chain unbranched polymer Amylopectin is insoluble in water and constitutes about 80% of starch. It is a branched chain polymer.
Cellulose
Cellulose is the principal component of plant structure viz. wood, cotton to support the weight of the plant. Cell wall of the plant is made up of cellulose. It is long chain unbranched polymer.
Cellulose contains 1—>4ฮฒ glycosidic linkages like cellobiose. Humans cannot hydrolyze cellulose due to lack of enzyme required for hydrolysis, hence they cannot use it directly as a food.
Glycogen
The glycogen is stored in animal body in the form of glycogen. It also known as animal starch, because its structure is similar to amylopectin Which is fraction of starch. Glycogen is highly branched.
Importance of carbohydrates bio molecules
Carbohydrates are essential for life in both plants and animals.
They form major part of our food and store chemical energy in plants.
They act as the major source of energy for all the animals.
In the plants they are important constituent of supporting tissues e.g. cellulose in wood, cotton in clothes.
The main source of energy of animals is glucose which is in the form of glycogen.
PROTIENS
Proteins are the biopolymers of a large number of ฮฑ-amino acid and they are naturally occurring polymeric nitrogenous organic compound containing 16% nitrogen and peptide linkages (-Co-NH-)
Successive amino acids are joined together to form protein.
It is present in hair, skin, muscle.
Common source of proteins is egg, fish, cereals, milk etc.
Proteins on hydrolysis give ฮฑ amino acid.
Peptide bond and polypeptide
The amide linkage (-Co-NH-) by which ฮฑ-amino acids are joined in linear fashion in proteins is called peptide linkage.
Peptide linkage is an amide formed between –COOH and NH3 group by elimination of water.
When many molecules of amino acid joined together by peptide linkages they form polypeptide which is proteins.
Classification (on the basis of molecular shape)
Fibrous proteins: The proteins in which the polypeptide chains lie side by side to form fiber like structure. The polypeptide chain held together by hydrogen bond. Example: keratin ( in hair, skin, nails)
Globular proteins: The proteins have intramolecular hydrogen bonding and are folded to form spherical structures are called globular proteins. Example: haemoglobin (in blood)
Structure of proteins
Each structure of protein is more complex than previous one.
Primary structure
In primary structure, proteins contain polypeptide chain. Amino acids linked with each other in specific sequence in protein this sequence is known as primary structure.
Secondary structure
In a protein, long amino acid chain is coiled, looped, or folded then this Structure of protein is known as secondary structure. It has two different types
A polypeptide chain gets coiled by twisting into right handed spiral is known as ฮฑ-helix.
The polypeptide chains are converted into flat sheet by stretched out. The contraction results in a pleated called ฮฒ-pleated sheet.
Tertiary structure
ฮฑ-helix bend on twist and folding gives three dimensional structure is known as tertiary structure.
Quaternary structure
The spatial arrangement of polypeptide chain with respect to each other is known as quaternary structure.
Amino acids
ฮ-amino acids are derivatives of carboxylic acids obtained by replacing ฮฑ-H by amino group.
Amino acids contain two functional groups –COOH and -NH2. On hydrolysis protein gives about 30 amino acids. All amino acids have trivial names. E.g. Gly, Val, Glu, Met etc.
Amino acids are crystalline, colourless, non volatile, high melting solid. In aqueous solutions it behaves like salts.
Amino Acids Classification
Amino acids are classified into –
Acidic amino acid
If numbers of carboxyl groups are more than amino groups the amino acids are acidic. Example-glutamic acid.
Basic amino acid
If numbers of amino groups are more than carboxyl groups, then amino acids are basic in nature. Example-lysine.
Neutral amino acids
The amino acid having equal number of carboxyl and amino groups are called neutral amino acids. Example-valine
ENZYMES
All biological or bio-catalysts which catalyze the reaction in living organisms is called enzymes. All enzymes are protein. Enzymes help in the digestion of food and absorption of certain molecule producing energy. Enzyme maltase converts maltose to glucose.
C12H22O11 ( Maltose ) —–> C6H12O6 ( Glucose )
LIPIDS
Now we learn about lipids. Lipids form naturally occurring biomolecules of a diverse group having limited solubility in water and can be isolated from organism by extraction with non polar solvents. Also, Lipids have variety of function in living organisms.
Lipids Classification for Biomolecules chapter class 12
The Lipids are classified as
Complex lipids
Complex lipids are esters of long chain fatty acid and can be hydrolyzed. They include triglycerides (animal fats and vegetable oils), glycolipids, phospholipids and waxes.
Simple lipids
Simple lipids do not have ester linkages and they cannot be hydrolyzed. They include steroids, tarpenes and prostaglandins.
HORMONES
Hormones are the secretion of endocrine glands (ductless glands). They regulate vital functions of body and produced in very small quantity like enzymes. They have low molecular weight and are easily diffusible.
Functions of hormones
The hormone thyroxine is secreted by thyroid gland in the neck to increase the rate of energy exchange and consumption of oxygen.
Insulin is peptide hormone, secretes in pancreas and controls carbohydrate metabolism by increasing glycogen in muscles and oxidation of glucose in tissue and also lowers the blood sugar.
VITAMINS
Vitamins are organic compounds which are required in the diet in small quantity to perform biological function and proportionate growth in living beings. Enough supply of vitamins is obtained from a well balanced diet.
Vitamins Classification
Depending upon the solubility
Water soluble vitamins (B and C)
Vitamins B and C are soluble in water. They are readily excreted in urine, have low toxicity and cannot be stored in body.
Fat soluble vitamins (A, D, E, K and P)
Vitamins A, D, E, K and P are soluble in oils and fats and are stored in the body in liver and in tissue.
Depending upon chemical structure
First, Vitamins of aliphatic series e.g. vitamin C
Second, Vitamins of aromatic series e.g. vitamin K
Third, Vitamins of alicyclic series e.g. Vitamin A
Fourth, Vitamins of heterocyclic series e.g. vitamin B
NUCLIEC ACID
Nucleus is present in every cell of living organisms. Nucleus has acidic properties and hence it was named as nucleic acid. It is the substance of heredity. Nucleotide is the structural unit of nucleic acid. It has three components they are as follows.
Sugar: It has deoxyribose sugar in DNA and ribose sugar in RNA. It is pentagonal ring having five carbon atoms.
Phosphate group: it has three active –OH groups of which two groups are involved in the formation of strand.
Nitrogen base: These are cyclic compound named as adenine (A) and guanine (G) called purines and are double ring compounds, whereas thymine (T), cytosine (C), and uracil (U) are called as pyrimidines and are single ring compounds.
DNA (Deoxyribonucleic Acid)
It has deoxyribo sugar. DNA stores the genetic information of an organism as sequence of base A, G, T, and C. uracil base is absent in DNA.
RNA (Ribonucleic Acid)
RNA is produced on DNA template by the process called transcription. It has ribose sugar. RNA is involved in protein synthesis in most of the organisms. mRNA, rRNA and tRNA are three types of RNA. The uracil base is present in RNA molecule.
These chemistry notes of d and f block elements are provided by Chemistry Notes Info website to students. So in this chemistry notes article we learn about d and f block elements class 12 notes of chemistry.
d-Block Elements
Introduction to d-Block Elements
The d-block elements are defined as the elements from periodic table in which, last electron enters into the ‘d-orbital’ of the penultimate shell i.e. (n-1)d orbital. Where ‘n’ is the last shell.
The d-block elements are also called transition elements, because their properties are intermediate between the properties of highly electropositive s-block elements and highly electronegative p-block elements. Transition elements have partly or incompletely filled (n-1)d orbital in their elementary states.
Position in Periodic Table of d-Block Elements
d-block elements are placed between s-block elements and p-block elements. The elements are placed between group 3 to 12 belonging to the period 4 to 7.
There are four series of transition elements i.e. 3d,4d,5d and 6d series
The 3d series starts from Sc(z=21) to Zn(z=30) and belongs to 4th period.
The 4d series includes all the elements from Y(z=39) to Cd(z=48) and belongs to 5th period.
The 5th series starts with element La(z=57) and then includes all the elements from Hf(z=72) to Hg(z=80) and belongs to 6th period.
Thus 6d series starts from Ac(z=89) and then includes all the elements from Rf (z=104) to Unb(z=112) which belong to 7th period.
Electronic Configuration d-Block Elements
General electronic configuration of four series of d-block elements are given below.
3d series:[Ar] 3d1-10 4S1-2
4d series :[Kr] 4d1-10 5S0-2
5d series :[Xe] 5d1-10 6S2
6d series :[Rn] 6d1-10 7S2
Abnormal Electronic Configuration of Chromium And Copper among d Block Elements
Chromium(24Cr) has electronic configuration
(Expected):[Ar] 3d4 4S2
(Observed):[Ar] 3d5 4s1
Copper (29Cu) has electronic configuration
(Expected):[Ar] 3d9 4s1
(Observed):[Ar] 3d10 4s1
Explanation: due to more stability of half –filled and completely filled orbital.
Occurrence of d Block Elements
The soft elements of d-block or transition occur as sulphide minerals. Hard and electropositive elements occur as oxide.
General Characteristics of d-Block Elements
All the transition elements are metal having high melting as well as boiling point. They are good conductors of electricity and heat. They form complexes and colored compound.
Due to unpaired electrons most of the elements are paramagnetic while some exhibit diamagnetic character. They are good catalyst and they form alloys with different metals.
General Trend in Properties Of First Row Elements of d Block
Ionization Enthalpy
The ionization enthalpies of transition elements are quite high and lie between those of s-block and p-block elements. This is because the nuclear charge and atomic radii of transition elements lie between those of s-block and p-block elements.
If IE1, IE2 and IE3 are the first, second and third ionization enthalpies of transition elements then IE1 <IE2<IE3.
The ionization enthalpies value of transition metal can be used to predict thermodynamic stability of their compound.
Metallic Character
All the transition metals have high thermal as well as electrical conductivity and having very high melting and boiling points compared to those of representative elements due to their closed-packed structure.
On going across the period melting points first increase, attain maximum value and then steadily decreases as atomic number.
The strength of metallic bond depends upon the number of unpaired electrons, it increases up to middle i.e., up to (n-1)d5 hence accordingly melting and boiling points also increases.
After (n-1) d5 configuration, the electrons starts pairing ,hence metallic strength, melting and boiling points decreases with increase in atomic number.
Oxidation States
The transition metal exhibit variable oxidation states and form compounds showing more than one (variable) oxidation states because energies levels of ns and (n-1)d are nearly similar. The oxidation state of 3d series is from +2 to +7 (except Cr and Cu).
The lowest oxidation states of these elements are +1 or +2 which is due to their 4s electrons.
As the number of unpaired electrons in 3d orbital increases the number of oxidation state also increases form Sc to Mn and after that electrons starts pairing in 3d orbital hence oxidation states decreases form Fe to Zn.
The highest oxidation state of transition elements is +8 which shown by Os and Ru.
Atomic and Ionic Radii
Atomic radii of the elements of 3d series gradually decreases up to Cr and then remain almost same for a few more elements and then increases slightly towards the end of end of the series.
In 3d series the decreases in atomic radii is small from Sc to Cr because increase in nuclear charge at the centre of atom and added electrons filled up in vacant penultimate d orbital, atomic
radii from Cr to Cu are almost similar and then slightly increases to Zn
Ionic radii also follow similar trend as observed in the atomic radii values.
Color
Most of the compounds of transition metals are coloured in their solid or solution form. Colour of the compound of the transition metals may be attributed to the presence of incomplete (n-1)d orbital and the number of unpaired electrons.
Transition metal ions with no unpaired electrons are colourless because there is possibility of d-d transition. e.g. Sc+3(3d0), Ti+4(3d0), Cu+(3d10). Ions with unpaired electrons are coloured i.e. ions with electronic configuration 3d1 to 3d9 are coloured.
In general, the colour of transition ions can be related to –
Presence of unpaired d electrons.
d-d transition.
Nature of groups i.e. ligands linked to metal ions.
Geometry of the complexes formed by the metal ion.
Catalytic Properties
Many transition metals are used as catalysts which influence the rate of chemical reaction.
A catalytic substance is capable of forming an unstable intermediate compound which readily decomposes yielding the products and regenerating the catalyst.
The commonly used transition metals as a catalyst are Fe, Co, Pt, Cr, Mn etc.
Examples-
Mno2 acts as a catalyst for decomposition of KClo3 to O2.
In manufacture of ammonia, Fe with Mo is used as catalyst.
Nickel acts as a catalyst in hydrogenation of oils to fats.
Magnetic Properties
Due to presence of unpaired electrons in the (n-1)d orbital, most of the transition metal ions and their compounds are paramagnetic i.e. they are attracted by magnetic field.
The transition element and their ions having all electrons paired are diamagnetic i.e. they are repelled by magnetic field.
Metals like Fe, Co and Ni posses very high paramagnetic and acquire permanent magnetic moment hence, they are ferromagnetic.
Alnico is an alloy of Al(12%), Ni(20%),Co(50%) and remaining Fe(18%). It is used to make permanent magnets.
Bohr magneton (B.M) is unit of magnetic moment:
1B.M = eh/4ฯmec
The effective magnetic moment ยตeff, of a paramagnetic substance is given by ‘spin only’ formula.
Where n is the number of unpaired electrons.
Alloy Formation
The transition metals can form large number of alloys among themselves having high melting points.
In the molten state, transition metals are miscible with one another, which forms solid alloy on cooling.
Transition metals can form alloy with non transition metals such as brass (Cu-Zn) and bronze (Cu-Sn).
POTASSIUM DICHROMATE (K2Cr2O7)
Properties of Potassium Dichromate
Potassium Dichromate exists as orange red crystals and is soluble in water.
Action of Alkali
Alkali (KoH) reacts with potassium dichromate to give yellow coloured solution of potassium chromate.
K2Cr2O7 + 2KOH ——> 2K2CrO4 + H2O.
Oxidizing Properties
Potassium Dichromate is good oxidizing agent in acidic medium. Potassium dichromate (oxidation number of Cr = +6) is reduced to chromium sulphate (oxidation number of Cr = +3)
Cr2 O72- + 14H+ + 6e– ——> 2cr3+ + 7H2O.
By gaining six electrons dichromate ions acts as an oxidizing agent.
Structure of Chromate Ion and Dichromate Ion
Uses of Potassium Dichromate
It is used as an oxidizing agent.
Used in a dyeing.
In manufacture of lead chromate and chrome alum.
Used in the detect ion of chloride ion.
In the tanning of leather.
In manufacture of pigments and inks.
POTASSIUM PERMANGANATE (KMnO4)
Properties of KMnO4
It is crystalline solid having deep purple colour. It is soluble in water at room temperature.
When heated it decomposes giving oxygen at 473K.
2KMnO4 —–> K2MnO4 + MnO2 + O2
At red heat, it further decomposes to K2MnO3 and oxygen.
Heated solid KMnO4 gives KOH, MnO and water vapour in current of H2.
Structure of Manganate Ion and Permangate Ion
Uses of KMnO4
It is used as –
Disinfectant.
An oxidizing agent.
Baeyer’s reagent.
For detecting halides in qualitative analysis.
f-Block Elements
Introduction to f-Block Elements
The elements in which differentiating electrons (last electrons) enters into pre-penultimate shell i.e. (n-2)f orbital are known as f block elements. f block elements are also known as inner transition elements.
There are two series of inner transition elements i.e. lanthanoid series (4f block elements) and actinoid series (5f block elements).
Lanthanoid Series
This includes all the elements from cerium (z=58) to lutetium (z=71) which are group of 14 elements with differentiating electron occupying 4f sub shell.
The name lanthanoid has been derived from lanthanum which is prototype of lanthanoid.
Lanthanoid series are also called 4f series because last electron enters pre-penultimate in 4f orbital.
Position of Lanthanoids
Lanthanoids belongs to group 3 of periodic table and are placed in the sixth period.
Lanthanoids are shown at the bottom of the periodic table because it interrupts third transition series of d-block elements.
Actual position of lanthanoids is in between (z=57) and hafnium (z=72).
Electronic Configuration of Lanthanoids
The general electronic configuration of 4f block elements is
[xe] 4f1-14 5d0-1 6s2
Elements La, Gd, Lu posses single electrons in 5d sub shell. In case of other lanthanoids 5d orbital is empty.
Due to empty, half filled and completely filled orbital f0, f7 and f14 electronic configuration elements achieve extra stability.
Oxidation States of Lanthanoids
The common oxidation state of all lanthanoids is +3. It is characteristics of series.
Lanthanum, gadolinium and lutetium shows only +3 oxidation state by losing two 6s and one 5d electrons giving stable outer electronic configuration 4f0,4f7 and 4f14 respectively.
In +4 oxidation state cerium, terbium attains 4f0 and 4f7 respectively.
Europium (Eu) as well as ytterbium (Yb) attains 4f7 and 4f14 respectively in +2 oxidation state.
Chemical Reactivity of Lanthanoids
Earlier members of lanthanoids are quite reactive, behaves like calcium but as atomic number increases, they behaves more like aluminium.
Lanthanoids forms carbides, hydrides, oxides, nitrides, halides, etc.
Carbides-
Ln + C —-> Lanthanoid Carbide
Hydrides-
2Ln + 3H2 —-> 2LnH3
Oxides-
2Ln + 3O2 —-> 2Ln2O3
Nitrides-
2Ln + N2 —-> 2LnN
Halides-
2Ln + 6HCl —-> 2LnCl3 + 3H2
Sulphides-
2Ln + 3S —–> Ln2S3
Lanthanoid Contraction
The gradual decrease in atomic and ionic size of lanthanoids with increases in atomic number is called lanthanoid contraction. You read these notes under 12 class chemistry notes of chapter d and f block elements.
Causes of Lanthanoid Contraction
Increases in atomic number of member of lanthanoids series, the positive charge on nucleus increases by +1 unit and one more electron enters in the pre-penultimate 4f sub shell. There is imperfect shielding of one electron by another electron in the same 4f sub shell.
With increase in nuclear charge the valence shell is pulled slightly towards nucleus. As a result of the pull, the size of M3+ ions goes on decreasing with increasing in atomic number.
In complete lanthanoid series atomic radii and ionic radii decreases with 10 pm and 21 pm respectively. Atomic radii shows some irregularities but ionic radii decreases steadily.
Effects of Lanthanoid Contraction
Decrease in Basicity
Due to lanthanoid contraction the size of the tri-positive lanthanoid ion (M3+) regularly decreases with increase in atomic number i.e. from La3+ to La3+. It results into decrease of basic from La3+ to Lu3+.
Ionic Radii of the Post Lanthanoids
The elements which follow the lanthanoids in the third transition series are known as post lanthanoid. As a result of lanthanoid contraction the atomic radii (size) of the elements which follow lanthanum (Hf, Ta, W, etc) are similar to that of the elements of 4d series. Since Zr-Hf, Nb-Ta, Mo-W and Tc-Re have almost identical sizes, similar number of valence electrons and similar properties these pairs of elements are called chemical twins.
Actinoid Series
The series of fourteen elements 90Th to 103Lr which follows actinium (89Ac) and in which last electrons are progressively filled in 5f orbital in pre-penultimate shell are called actinoid series or 5f series.
The elements which are synthetically or artificially prepared by man having atomic number higher than uranium (z=92) are called as trans-uranic elements (Np-93 to Uno-118).
Position of Actinoids
The actinoids belongs to third group of the periodic table in the seventh period.
Actinoids are placed at the bottom of the periodic table below the lanthanoid series because it interrupts fourth transition series of d-block elements.
Electronic Configuration
The general electronic configuration of 5f elements is represented as 5f1-14 6d0-1 7s2.
The electronic configuration of actinoids is not definite.
Actinium and thorium does not contain any 4f electron.
Oxidation States of Actinoids
Actinoid show varied oxidation states like lanthanoids. The common oxidation states of actinoids is 3+. Ac, Th and Am shows +2 oxidation states. Th, Pa, U, Np, Pu, Am and Cm shows +4 oxidation states.
+5 oxidation state is shown by element Th to Am. U, Np, Pu and Am show oxidation state +6.The highest oxidation state is +7 shown by Np and Pu.
How To Actually Learn More D And F Block Elements
In these notes you understand, How To Actually Learn More about D And F Block Elements. You just completed learning of d block elements and f block elements of 12 class chapter 8 d and f block elements. Also learn about p Block Elements on www.ChemistryNotesInfo.com
