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### Introduction

Before the beginning of the eighteenth century when there are only 30 elements known, it was easier to study and remember their properties. In later years when number of elements discovered were increased then it became difficult to study them. So scientist fell the need of simple method to facilitale the study of the properties of various elements and their compounds. After numerous attempts they got success & elements were arranged in such manner that similar elements were grouped together and different elements were separated. This arrangement of elements is known as classification of elements which led to the formation of periodic table. Periodic table may be defined as “the arrangement of all the known elements according to their properties in such way that the elements of similar properties are grouped together in a tabular form.”
Earlier attempts of classification of elements (development of periodic table):
Earlier attempts to classify the elements resulted in grouping as metals and non-metals. Later on they were classified on the basis of their atomic masses.

In 1817, Johann walfgang dobereiner. A German chemist, arranged the elements as group of three elements and in a manner that the atomic mass of middle element was roughly the average of the atomic masses of the other two elements of the triad.
Example:Element:                   Lithium,                Sodium       and                Potassium.
Atomic mass                                     7                                  23                                                39
Average of the atomic masses of Lithium and Potassium is   ${{7 + 39} \over 2} = 23$ only three triads could be arranged in this manner at that time. They were :

##### Short coming of Dobereiner’s triads rule

This classification was not found satisfactory as it could be applied to the limited number of elements. Now a days some more triads have been made they are
(i)         Potassium            Rubidium               Cesium
K                                    Rb                       Cs

(ii)      Phosphorus           Arsenic                Antimony
P                                 As                         Sb

(iii)        Sulphur               Selenium               Tellurium
S                                 Se                               Te

(iv)      Hydrogen         Fluorine                       Chlorine
H                                 F                                  Cl

(v)        Scandium              Yttrium                  Lanthanum
Sc                                Y                                  La

For a Debereiner’s triad all the three elements should belong to the same group and the difference in atomic number should be 8 or 18.

##### New lands law of octave

In 1866, J.A.R. Newlands correlated the chemical properties of the elements with the increasing order of atomic masses. i.e. to arrange the element having lowest atomic mass (H) firstly and ended to at secondly the element having highest atomic mass. (Thorium which was 56th known element at that time).
Definition: When the elements are arranged in order of their increasing atomic masses, every eighth element has the properties similar to those of the first elements like the eighth note of an octave in music. Thus according to this law, the physical & chemical properties are repeated after an interval of eight elements and this is similar to eight notes of an octave on a musical scale shown below:

The properties of Lithium are similar to that of 8th element i.e. Na, Be is similar to Mg and so on.

Limitations:

1. Law of octaves was applicable only up to calcium. It worked well with lighter elements only.
2. At that time only 56 elements were existed in nature, but later several elements were discovered which can not be kept in periodic table as per this law. Their properties were not in accordance with the law of octaves.
3. (i) In order to fit element in to his table Newlands adjusted two elements in the same column. For example cobalt and nickel were placed in the same position and in the same column as fluorine, chlorine and bromine.
(ii) Iron which resembles cobalt and nickel in properties were placed far away from these elements.
4. After the discovery of inert gases & included in the periodic table it becomes the eighth element from alkali so this law has to be dropped out.
##### Mendeleev’s Periodic table

In the year 1861, D Mitri Ivanovich Mendeleev arranged all the known elements (63 elements) in the form of a table in which elements were arranged in the increasing order of their atomic mass and also on the similarities of chemical properties.

♦ The arrangement of element was based on the physical and chemical properties of the elements and also the formulae of the compounds they formed with oxygen and hydrogen. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements.r The arrangement of element was based on the physical and chemical properties of the elements and also the formulae of the compounds they formed with oxygen and hydrogen. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements. The table which classifies the elements in such a way that elements having similar properties are placed in same vertical column or group is known as periodic table. The term periodic means repetition of elements having similar properties after a certain regular interval. The periodic table consists of vertical columns which are called as groups and horizontal rows called as periods. Mendeleev’s periodic table had six periods and eight groups as shown in the table, he arranged all the elements horizontally in the order of their increasing atomic masses and vertically according to their similarities in properties.
Each group was further sub divided into two sub groups A & B.
Achievements of the Mendeleev’s periodic table:

1. Systematic study of the elements : All the elements in general were arranged systematically in increasing order of their atomic masses. This arrangement helped to study the properties of various elements. If the nature of the element present in a group is known, it become easier to predict or guess the expected properties of other elements.
2. Prediction of new elements : Mendeleev predicted the properties of some unknown elements and left gaps for these elements to be filled as and when discovered. For eg. Scandium, Gallium and Germanium were not known at that time but Mendeleev already named these elements as eka-boron, eka-aluminium and eka-silicon. When these elements were later on discovered, they were found to have more or less similar properties as predicted by Mendeleev.
3. Position of Noble gases : When noble gases were discovered they were placed in a new group without disturbing the existing order.
4. Correction of atomic masses : Atomic masses of several elements were corrected on the basis of periodic table. eg. Atomic mass of Beryllium was corrected from 135 to 9. Mendeleev predicted that atomic mass of gold is incorrect. Later on it was found to be so. Similarly atomic masses of Indium, Uranium and Platinum were also corrected.

Drawbacks of Mandeleev’s periodic table:

Position of Hydrogen is uncertain becomes it resemble with IA group alkali metals elements and VII A (halogens) group elements.
(i) Isotopes: Isotopes of an element have similar chemical properties but different atomic masses.
(ii) Position of isotopes: Since basis of periodic table was increasing atomic mass. So isotopes should be placed separately but no separate place was given to isotopes.
(iii) Anamolus pairs of certain elements: Certain elements were not arranged according to their increasing atomic mass eg.
(a) Argon (Atomic mass 39.9) was placed before potassium (atomic mass 39.0)
(b) Cobalt (58.95) before Nickel (58.70)
(c) Tellurium (127.6) before Nickel (126.9) (d) Thorium (232) before Protactimum (231)

(iv) Similar elements were placed in different groups. eg.
(a) Silver and thallium
(c) Copper and mercury
(d) Platinum and gold.

(v) Dissimilar elements were placed in same group eg. silver and gold were placed in a same group while there is little similarity in physical and chemical properties.
(vi) Cause of periodicity : Mendeleev did not explain the cause of periodicity in the physical and chemical properties of the elements.(vii) Metals have not been separated from non-metals.(viii) Position for elements of group
(VIII): There is no proper position for the elements of group (VIII) consisting of elements in three triads. These elements are placed out side the main structure of the periodic table.

The modern periodic table:
In 1913 Henry Moseley showed that properties of the elements are determined by atomic numbers instead of the atomic mass. It formed the basis of modern periodic law. The law is – “The physical and chemical properties of the elements are periodic function of their atomic numbers”. Since atomic mass is a nuclear property where as atomic number implies for the no. of electrons in neutral atom or no. of protons in nucleus. Nucleus is deep seated in the atoms and does not take part in chemical reactions. Therefore the physical and chemical properties depends upon the no. of electrons and their electronic configuration which in turn depends upon atomic number (Z). So when elements are arranged in the increasing order of atomic numbers, after an regular interval elements have similar no. of valence electrons therefore chemical properties are repeated i.e. periodicity in the chemical properties of the elements occurs.

Modern periodic table or long form of the periodic table:

• It is also called as Bohr, Bury & Rang, Werner periodic table(1) It is based on the Bohr-Bury electronic configuration concept and atomic number.
(2) This model is proposed by Rang and Werner. This table is based on modern periodic law, the elements are arranged in the increasing order of atomic numbers in such a way that elements having the same number of valence electrons are placed in the same vertical coloumn.
It consists of 18 vertical colums and seven horizontal rows. Vertical columns of periodic lable are known as groups while horizontal rows are known as periods.The corelation between the groups in long form of periodic table and in modern form of periodic table are given below:-
IA           IIA        IIIB         IVB       VB        VIB       VIIB        VIII        IB          IIB
1               2            3              4             5            6              7        8,9,10      11         12
III A      IV A      VA           VIA      VIIA        0
13        14          15             16        17           18Elements belonging to same group having same number of electrons in the outer most shell so their properties are similar.Description of periods:-

Description of periods :

Description of Groups:

1st/IA/Alkali metals
H = 1s1
Li = 1s2 , 2s1
Na = 1s2 , 2s2 2p6 , 3s1
K = 1s2 , 2s2 2p6 , 3s2 3p6 , 4s1
General electronic configuration = ns1(n = Number of shell)
Number of valence shell e– = 1

2nd/IIA/Alkali earth metals :
Be = 1s2, 2s2
Mg = 1s2, 2s2, 2p6, 3s2
Ca = 1s2, 2s2, 2p6, 3s2, 3p6, 4s2
General electronic configuration = ns2
Number of valence shell e– = 2

13th/IIIA/Boron Family :
B = 1s2, 2s2, 2p1
Al = 1s2, 2s2, 2p6, 3s2, 3p1
Ga = 1s2, 2s2, 2p6, 3s2, 3p6, 3d10,4s2,4p1
General electronic configuration = ns2 np1
Number of valence shell e– s = 3

14th/IVA/Carbon Family:
C = 1s2, 2s2, 2p2
Si = 1s2, 2s2, 2p6, 3s2, 3p2
Ge = 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p2
General electronic configuration = ns2 np2
Number of valence e– s= 4

15th/VA/Nitrogen family/Pnicogen: (Used in fertilizer as urea)
N = 1s2, 2s2, 2p3
P = 1s2, 2s2, 2p6, 3s2, 3p3
As = 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p3
General electronic configuration = ns2 np3
Number of valence shell e– = 5

16th/VIA/Oxygen family/Chalcogen: (Ore forming)
O = 1s2, 2s2, 2p4
S = 1s2, 2s2, 2p6, 3s2, 3p4
Se = 1s2, 2s2, 2p6, 3s2, 3p6,3d10, 4s2, 4p4
General electronic configuration : ns2 np4
Number of valence shell e– s= 6

17th/VIIA/Fluorine family/Halogens: (Salt forming)
F = 1s2, 2s2, 2p5
Cl = 1s2, 2s2, 2p6, 3s2, 3p5
Br = 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p5
General electronic configuration = ns2 np5
Number of valence shell e– s= 7

18th/Zero group/Inert gases / Noble gases:
Ne = 1s2, 2s2, 2p6
Ar = 1s2, 2s2, 2p6, 3s2, 3p6
Kr = 1s2, 2s2, 3p6, 3s2, 3p6, 3d10,4s2, 4p6
General electronic configuration = ns2 np6 ( except. He)
Number of valence shell e– = 8
Elements of group 16 are known as chalocogens Elements of group 17 are known as halogens. Classification on the basis of subshell in which last electron (e–) enters

Characteristics of the elements present in groups are listed.
(i) The elements present in a group are separated by definite gaps of atomic numbers (8, 8, 18, 18, 32).
(ii) There are eighteen (18) independent groups in the Long from of Periodic Table. These are numbered from 1 to 18.
(iii) The elements present in a group have the same number of electrons in the valence shell of their atoms.(iv) The elements present in a group have the same valence.
(v) The elements present in group have identical chemical properties.
(vi) The physical properties of the elements in group such as melting point, boiling point, density vary gradually.
(vii) Atomic radii of the elements present in a group increase downwards.

Characteristics of the element present in period
(i) In all the elements present in a period the electrons are filled in the valence shell.
(ii) As the number of electrons in the valence shell change, the chemical properties of the elements present in a period also change.
(iii) Atomic radii of the elements in a period decrease from left to the right.
(iv) Along a period, the metallic character of the elements decreases and the non-metallic character increases.
(v) Along a period, the reducing character of the elements decreases and their oxidizing character increases.

Advantages of Long form over Mendeleev’s Periodic Table

There are several advantages of long form of periodic table over Mendellev’s periodic table. Some of these are as follows:
(i) It is based upon atomic number which is a fundamental property instead of atomic mass.
(ii) The elements have been grouped as s, p, d and f-block elements. Which helps us to understand the electronic configuaration in a bettery way.
(iii) In the long form of periodic table, the elements are arranged in the increasing order of their atomic number,s therefore, no separate place is required for isotopes.
(iv) The position of some of the elements which were a misfit on the basis of atomic mass is not because argon has atomic number 18 which is less than that of potassium which is 19.
(v) Metals, non-metals, metalloids, transition elements, lanthanoids and actinoids and actinoids are now better classified.

##### Periodicity in properties

The electronic configurations of atoms display a periodic variation with increase in atomic number. Since the properties of elements depends upon the electronic configurations. So the elements exhibits periodic variation of physical & chemical properties. Some properties of elements are :-

(A) Valency:- It is defined as the combining capacity of the element. Valency is determined by the number of electrons present in outer most shell. These electrons are known as valence electrons.

Variation of valency across a period:- The number of valency electrons increases from 1 to 8 on moving across a period. The valency of an element with respect to hydrogen and halogen increases from 1 to 4 and then decreases from 4 to zero. With respect to oxygen valency increases from 1 to 7.

Variation of valency along a group:- On moving down a group. The no. of valence electrons remains same so the valence of all the elements of a group is same.
Group (1) elements have valency – 1
Group (2) elements have valence – 2
Atomic size:– Atomic size means atomic radius of an atom which is defined as the distance between the centre of the nucleus of an atom and the valence shell containing electrons in an isolated atom since it is very difficult to measure the atomic radius because –

(i) The isolation of single atom is very difficult.
(ii) There is no well defined boundary for the atom.
So the more accurate definition of atomic radius is –
(Half the internuclear distance between the two atoms in a homatomic molecule is known as atomic radius) This internuclear distance is also known as bond length. It depends upon the type of bond by which two atoms combine. Based on chemical bonds, atomic radius is divided in to four categories.

(a) Covalent radius (Single bonded covalent radius) For homoatoms It is half of the internuclear distance between two singly bonded homoatoms.
(b) Covalent radius for hetero atoms.
(i) In case of hetero atomic molecule (A – B), if the electronegativity difference is less. Then covalent
radius of oxygen, nitrogen and carbon is taken from the compound H2O2, N2H4 and C2H6 respectively.
This radii is subtratied from the bond length of A–B molecules.
eg. C–I (electronegativity is almost same 2.5)
Internuclear distance C–I is 2.13Å, covalent radius of carbon in compound C2H6 is 0.77Å
covalent radius of I will be.
dC–I = rC + rI (covalent radius of iodine)
i.e. 2.13 = 0.77 + rI
rI = 2.13 – 0.77 = 1.36Å

(ii) When electronegativity difference is more. Then bond length is determined by the schole maker and Stevenson law –
dA–B = rA + rB – 0.09(XA – XB)
where dA–B = Bond length of dA–B molecule
XA = Electronegativity of A
XB = Electronegativity of B

Example –
Bond length of F2 = 1.44 Å

i,e. dF–F = 1.44Å ${r_F} = {{1.44} \over 2} = 0.72$Å

dH–H = 0.74Å      ${r_H} = {{0.74} \over 2} = 37$Å

Electronegativity of Fluorine is 4.0 and Electronegativity of Hydrogen is 2.1
dH–F = rF + rH + 0.09(XF – XH)
= 0.72 + 0.37 – 0.09(4 – 2.1)
= 1.09 – (0.09 × 1.9)
= 1.09 – 0.171 = 0.919Å

Size of cation  ∝  ${1 \over {MagnitudeOfTheCh\arg eOr{Z_{eff}}}}$

eq. Fe > Fe+2 > Fe+3

(ii) Anionic radius – Anionic radius is always greater than atomic radius because in an anion electrons are more than the protons so effective nuclear charge reduces and inter electronic repulsion increases so size of anion also increases.

If we use the term electropositive in place of metallic character, we can say that electropositive character  goes on increasing as we move from top to bottom in the periodic table. If we consider the electronegative character, it goes on decreasing as we move down in a group of the periodic table.

⇒ Ionization Enthalpy: The minimum amount of energy required to remove the most losely bounded electron from an isolated gaseous neutral atom to form gaseous electropositive ion called Ionization enthalpy. Its unit is kilo joules per mole (kJ/mol)
M(g) + Energy   →    M+(g) + e
It is a measure of tendency to lose electrons by atoms. The tendency to lose electron increases from top to bottom in a group and it decreases on moving left to right in a period.

⇒ Electron gain enthalpy: It is defined as the amount of energy released when an isolated gaseous atom in the ground state accepts an electron to form gaseous negative ion i.e. and anion. It is a measure of tendency of an atom to accept an extra electron to form an anion. Its unit is kilo joule mole (kJ/mole). Electron gain enthalpy of elements goes on increasing as we move from left to right in a period. In group it decreases from top to bottom.

#### DIVISION OF THE PERIODIC TABLE IN s, p, d AND f BLOCKS

⇒ s-Blocks Elements: The elements in which the last electron enters the s-sub-shell of their outermost energy level and electronic configuration is ns1 or ns2 (I or II group) are called s-block elements are:
(i) They are soft metals.
(ii) They have low ionisation energies.
(iii) They are very reactive and form ionic compounds.
(iv) They show oxidation states of +1 group and +2 group.
(v) They are good reducing agents.

⇒ p-Block Elements: The elements in which the last electron enters the p-sub-shell of their outermost energy level are called p-block elements. The exception is helium (1s2)..?
The general configuaration of their outermost shell is ns2np1-6. These elements are kept in group 13 to 18. Some of the general characteristics of p-block elements are:
(i) They show variable oxidation states.
(ii) They form ionics as well as covalent compounds.
(iii) Most of them are non-metals.
(iv) Most of them form acidic oxides.

⇒ d-Block Elements:
(i) They are hard and having high melting point.
(ii) They show variable oxidation states.
(iii) They form coloured compexes.
(iv) They form ionic as well as covalent compounds.
(v) Most of them exhivit paramagnetism.
(vi) Most of them possess catalytic properties.

f-Block elements: The elements in which the last electron enters the f-block elements.
Their general configuaration is (n-2) f1-14 (n-1)d0-1,ns2. They consist of two series of 28 elements (14 in each) placed at the bottom of the periodic table.

The elements of first series followed by lanthenum (57La) are called Lanthenides.
The elements of second series followed by actinium (89Ac) are called actinides.
The general Charactertics of f-block elements are:
(i) They show variable oxidation states.
(ii) They have high melting points.
(iii) They have high densities.
(iv) They form coloured compounds.
(v) Most of the elements of actinide series are radioactive.

It may be noted that:
1. The elements of group zero are called inert gases, noble gases, rare gases or aerogens.
2. The elements of p-block (except noble gases) are called representative or main group elements. The members of this group of elements have all their occupied subshells filled except their outermost electron shell.

##### REVISION
• Element are classified on the basis of similarities in their physical and chemical properties.
• Dobereiner grouped elements into triads.
• Newland grouped elements on the basis of law of octaves.
• Mendeleev grouped elements in the increasing order of their atomic masses and the similarity in chemical properties.
• Mendeleev was able to predict the existence of some elements on the basis of gaps in the periodic table.
• Moseley discovered that fundamental property of an element is its atomic number, rather than atomic mass. He revised Mendeleev Periodic Table on the basis of atomic numbers of elements and removed some of its anomalies.
• Elements in the long form of Modern Periodic Table are arranged in 18 vertical columns called groups and 7 horizontal rows called periods.
• The elements arranged in the long form of periodic table show
(i) periodicity of properties (ii) atomic size (iii) valency (iv) metallic and non-metallic character.