Periodic Classification of Elements

There are total of 118 element6 known so far and the discovery of new elements still continues. Every element has its own importance in the environment , some of them are useful while other are harmful. The harmful elements may also be useful in other way.

It becomes a necessity to study the chemical and physical properties of each of the elements present around us because every element is related to our life in some or other way. As the number is very large and there is possibility that more elements will be discovered, so it becomes very difficult to study each of them separately. We need to classify them in some groups according to their properties so that we can study them in group more easily and effectively. Periodic table is the tool which is used to classify the known elements in groups.



It helps us to undertake a systematic study of the various elements found in nature without which it would have been impossible for us to study all the elements in the table. With the help of periodic table a comparative study of the elements and their compounds can be done.

Periodic table also helps us to analyse the periodic trend in various properties such as ionisation potential, electron affinity, electronegativity etc.
Dobereiner's Triads, 1829

Dobereiner was the first scientist to classify the elements in some groups.

He tried to classify the elements with similar properties in groups of three elements (Triads). He could succeed in making only a few triads.

In the triads of elements the atomic weight of the middle element was the arithmetic mean of the atomic weights of the other two. Some of the triads are as under



Li

Na

K

Ca

Sr

Ba

P

As

Sb

Cl

Br

I


7

23

39

40

88

137

31

75

120

35.5

80

123

Newland's Law of Octaves, 1864

If the elements are arranged in order of their increasing atomic weights, every eighth element had similar properties to the first one like the first and eighth note in music. For example



sa

re

ga

ma

pa

dha

ni

sa


Li

Be

B

C

N

O

F

Na


Na

Mg

Al

Si

P

S

Cl

K


The main problem with this classification was that inert gases were not discovered at that time and also all the elements could not be classified on this basis.
Mendeleef’s Periodic Table

Mendeleef’s Periodic law : The physical and chemical properties of elements are a periodic function of their atomic weights.

If the elements are arranged in order of their increasing atomic weights, after a regular interval elements with similar properties are repeated

On the basis of his law, Mendeleef proposed a periodic table for classicication of elements which is known as Mendeleef’s Periodic Table

The table is divided into nine vertical columns called groups and seven horizontal rows called periods.



Merits or Advantages of Mendeleev’s Periodic Table:


Study of Elements: First time all known elements were classified in a group according to their similar properties. So, study of the properties of elements become easier.


Predicting new Elements: It gave encouragement to the discovery of new elements as some gaps were left in it. Sc, Ge, Tc were the elements for whom position and properties were defined by Mendeleev even before their discoveries and he left the blank spaces for them in hit table.
Blank space at atomic weight 72 in Si group was called Eka silicon and element discovered later was named Germanium.


Eka Aluminium: Gallium


Eka Boron: Scandium


Eka Manganese: Technetium

Demerits of Mendeleev’s Periodic Table:


Position of Hydrogen: Hydrogen resembles both, the alkali metals (1A) and halogens(VII A) in properties so Mendeleev could not decide where to place it.


Position of Isotopes: As atomic weight of isotopes differ, they should have been placed in different position in Mendeleev’s periodic table. But there were no such positions for isotopes in Mendeleev’s periodic table


Anomalous Pairs of Elements: There were some pair of elements which did not follow the increasing order of atomic weights. E.g: Ar and Co were placed before K and Ni respectively in the table but having higher atomic weights.


Like Elements were Placed in Different Groups: Pt and Ag which have similar properties were placed in group VIII and group IB respectively.


Position of Isotopes: As atomic weight of isotopes differ, they should have been placed in different position in Mendeleev’s periodic table. But there were no such positions for isotopes in Mendeleev’s periodic table


Anomalous Pairs of Elements: There were some pair of elements which did not follow the increasing order of atomic weights. E.g.: Ar and Co were placed before K and Ni respectively in the table but having higher atomic weights.

Refer to the following video for Modern Periodic Table



We will gain in depth knowledge of this chapter under following subtopics:


Tips and Tricks to Learn Periodic Table


Periodicity in Properties


Solved Examples
Modern Periodic Table
Modern Periodic Law: Properties of elements are the periodic function to their atomic numbers.


The periodicity in properties is due to repetition of similar outer shell electronic configuration at a certain regular intervals.


In modern periodic table is based on modern periodic law in which elements are arranged in increasing order of their atomic numbers.


In the modern periodic table, the elements are arranged in rows and columns. These rows and columns are known as periods and groups respectively.


The table consists of 7 periods and 18 groups


Period indicates the value of ‘n’ (principal quantum number) for the outermost or valence shell.


Same number of electrons is present in the outer orbitals (that is, similar valence shell electronic configuration


Characteristics of Periods


First period is called shortest period and contains only two elements. Second and third periods are called short periods containing eight elements each. Fourth and fifth periods are long periods containing eighteen elements each. Sixth period is the longest period with thirty-two elements. Seventh period is an incomplete period containing nineteen elements. Numbers 2, 8,8,18,18, 32 are called magic numbers.


Lanthanide and actinide series containing 14 elements each are placed separately under the main periodic table. These are related to sixth and seventh periods of III group respectively.


Elements of third period from sodium (Na) to Chlorine (Cl) are called representative or typical elements.


Valency of an element in a period increases from 1 to 7 with respect to oxygen.



Na2O

MgO

Al2O3

SiO2

P2O5

SO3

Cl2O7


1

2

3

4

5

6

7



From left to right in a period generally


Atomic weight, effective nuclear charge, ionisation potential, electronegativity and electron affinity of an element increases.


Atomic radius, electropositive character and metallic character of an element decreases.


Diagonal relationship — Elements of second period Li, Be and B resemble closely with the elements Mg, Al and Si of third period in the next higher group.

Second Period Li Be B C

Third Period Na Mg Al Si
Elements of second period are called bridge elements.
Characteristic of Groups


There are nine groups in modern periodic table and they are represented by roman numerals as I, II, III, IV, V, Vi, VII, VIII and zero.


Groups I to VII are further divided into two subgroups A and B, Group VIII consists of three sets, each one containing three elements.


Inert gases or noble gases are placed in zero group.


The valency of an element in a group is equal to the group number.


The elements of the groups which resemble the typical elements are called normal elements. For example IA, IIA, IIIA, IVA, VA, VIA, VIIA group elements are normal elements.


Those elements of the groups which do not resemble the typical elements are called transition elements. For example- IB, IIB, IIIB, IVB, VB, VIB, VIIB, and VIII group elements are transition elements.Hydrogen is placed in both IA and VIIA groups.


Atomic weight, atomic size, electropositive character and metallic character of elements increases down the group.


Ionisation potential, electron affinity and electronegativity of elements decreases down the group.
Classification of Elements



On the basis of electronic configuration, the elements may be divided into four groups.
s-block elements
These are the elements in which last electron enters s subshell.


These are present in the left part of the periodic table.


These include 1 and 2 group elements.


All the s- block elements are metals.


The general electronic configuration of valence shell for s block elements is ns1-2 ( n = 1 to 7).
p – block elements


These are the elements in which last electron enters p subshell.


These are present in the right part of the periodic table.


These include 13 to 18 of the periodic table.


Most of the p block elements are metalloids and non metals but some of them are metals also.


The general electronic configuration of the valence shell is ns2np1-6 ( n = 2 to 7).


ns2 np6 is stable noble gas configuration. The electronic configuration of He is Is2.
d-Block Elements


These are the elements in which last electron enters d-subshell.


These are present in the middle part of the periodic table (between s & p block elements)


d block elements include group 3 to 12 groups of the periodic table.


All are d block elements are metals.


The last electrons fills in ( n – 1)d orbital.


The outermost electronic configuration of d block elements is (n-1)d1-10 ns1-2 (n = 4 to 7).


There are three series of d-block elements as under
3d series – Sc(21) to Zn (30)
4d series – Y (39) to Cd (48)
5d series – La (57), Hf (72) to Hg (80)
f -Block Elements


These are placed separately below the main periodic table.


These are mainly related to IIIB i.e. group 3 of the periodic table


There are two series of f-block elements as under


4f series – Lanthanides – 14 Elements i.e. Ce (58) to Lu (71)


5 f series – Actinides – 14 Elements i.e. Th (90) to Lw (103)


the last electron fills in ( n – 2) ¦-orbital


Their general outermost electronic configuration is (n-2)¦1-14 (n-1)s2 (n-1)p6 (n-1)d0-1ns2 (n = 6 and 7).

Revision Notes on Carbon and its Compounds

Two or more elements combine to form compound. There are two types of compounds- Organic Compound and Inorganic Compounds. Organic compounds are the one which are made up of carbon and hydrogen.
Covalent Bond

The bond formed by sharing a pair of electrons between two atoms are known as Covalent Bond. Carbon forms covalent bond. Carbon exists in two forms- as free state and as combined state. Free form of carbon is found in graphite, diamond and fullerene. In combined state, carbon exists as Carbon-dioxide, Glucose, Sugar etc.
Allotropes of Carbon

Different forms of an element that has same chemical properties but different physical properties are known as Allotropes. There are three allotropes of carbon- diamond, graphite and fullerene.
Diamond

Diamond exits as three-dimensional network with strong carbon-carbon covalent bonds. Diamond is hard in nature with high melting point. It shines in presence of light and it is a bad conductor of electricity. The most common use of diamond is in making jewellery. It is also used in cutting and drilling tools.
Graphite

Graphite is made from weak van der wall forces. Each carbon atom is bonded with other three carbon atoms in order to form hexagonal rings. It serves as good conductor of heat and electricity. It is used as dry lubricant for machine parts as well as it is used in lead pencils.
Fullerene

It is a hollow cage which exits in the form of sphere. Its structure is similar to fullerene. But along with hexagonal rings, sometimes pentagonal or heptagonal rings are also present.



Fig.1 Structure of fullerene
Two Important Properties of Carbon

Catenation and tetravalency are the two important properties of carbon. Catenation is a property of carbon by which carbon atoms can link one another via covalent bond and can form long chains, closed ring or branched chains etc. Carbon atoms can be linked by single, double or triple bonds. Carbon has a valency of 4 due to which it is known to have tetravalency. Due to this one carbon atom can bond with other 4 carbon atoms, with other atoms also such as Oxygen, Nitrogen etc.
Hydrocarbons

Compounds which are made up of carbon and hydrogen they are known as Hydrocarbons. There are two types of hydrocarbons found - Saturated Hydrocarbons and Unsaturated Hydrocarbons. Saturated Hydrocarbons consist of single bonds between the carbon atoms. For Example, Alkanes. Alkanes are saturated hydrocarbons represented by a formula, CnH2n+2.

Unsaturated Hydrocarbons are the one with double or triple bonds between the carbon atoms. For Example, Alkenes and Alkynes. Alkenes are represented as CnH2n whereas alkynes are represented as CnH2n-2. Some saturated hydrocarbons and unsaturated hydrocarbons are represented as -



Fig.2. Saturated hydrocarbons



Fig. 3. Unsaturated hydrocarbons

Structure of hydrocarbons can be represented in the form of electron dot structure as well as open structures as shown below-



Fig.4. Electron dot structure and open structure of ethane



Fig.5. Electron dot structure and open structure of ethyne


Carbons Compounds based on the basis of structure

Carbon Compounds can be classified as straight chain compounds, branched chain compounds and cyclic compounds.They are represented as -



Fig.6. Straight chain carbon compound



Fig.7. Branched chain compounds



Fig.8. Cyclic carbon compounds
Functional Groups

One of the hydrogen atoms in hydrocarbon can be replaced by other atoms according to their valencies. The atoms which decides the properties of the carbon atoms, are known as Functional Groups. For Example, Cl, Br, -OH, Aldehyde, Ketone, Carboxylic Acid etc.
Homologous Series

Series of compounds in which same functional group substitutes for the hydrogen atom in a chain of carbon.



Fig.9. Homologous series
Nomenclature of Carbon Compounds


First of all, identify the number of carbon atoms in compounds. And in it identify the longest chain


Then functional group can be indicated by suffix or prefix.


Cyclic hydrocarbon is designated by prefix cyclo.


If there are two or more different substituents they are listed in alphabetical order


If the same substituent occurs more than once, the location of each point on which the substituent occurs is given



Fig.10. Different functional groups
Chemical Properties of Carbon Compounds

Combustion

Carbon along with its compound is used as a fuel as it burns in presence of oxygen to release energy. Saturated hydrocarbons produce blue and non-sooty flame whereas unsaturated hydrocarbons produce yellow sooty flame.

CH4 + 2O2 → CO2 + 2H2O

Oxidation

Alcohol can be oxidized to aldehydes whereas aldehydes in turn can be oxidized to carboxylic acid. Oxidizing agent such as potassium permanganate can be used for oxidation.


Addition Reaction

Hydrogenation of vegetable oil is an example of addition reaction. Addition of hydrogen in presence of catalyst such as nickel or palladium. This converts oil into ghee.



Substitution Reaction

When one atom in hydrocarbon is replaced by chlorine, bromine, etc. this is known as Substitution Reaction.


Important Carbon Compounds: Ethanol and Ethanoic Acid

Ethanol is a volatile liquid with low melting point. It reacts with sodium to form sodium ethoxide.



This above reaction is used to test the presence of ethanol by the evolution of hydrogen gas.

Dehydration of ethanol in presence of hot sulphuric acid forms alkene.



Ethanoic acid is a colourless liquid. When pure ethanoic acid freeze like ice, it is known as Glacial Acetic Acid. It is formed at a temperature of about 16.6 degree centigrade

Ethanoic Acid/Acetic acid when reacts with ethanol it forms an ester. Ester can be identified by its sweet smell.



Reaction of ester with strong base is used to form soap. This is known as Saponification. Acetic acid also reacts with strong base to form sodium acetate and water.

NaOH + CH3­COOH + CH3COONa + H2O
Soaps and Detergents

Sodium or potassium salt of carboxylic acid is known as Soap. They work most effectively in soap water. Detergents are sulphonate or ammonium salt of long chain of carboxylic acid. They can work effectively on soft as well as hard water.
Cleansing Action of Soaps and Detergents

Cleansing action of soaps and detergents is due to ability to minimize the surface tension of water, to emulsify oil or grease and to hold them in a suspension of water. When soap dissolves in water, it forms soap anions and soap cations. The hydrophobic part of soaps and detergents are soluble in grease and hydrophilic part is soluble in water.
Soap and Micelle Formation

When dirt and grease are mixed with soap water, soap molecules arrange them in tiny clusters known as Micelle. The hydrophilic part sticks to the water and form outer surface of the micelle and hydrophobic part binds to oil and grease.