INTRODUCTION
In the universe 115 elements are discovered till today.
Each of those elements possesses different properties.
It is difficult to know and use the properties of every element at a time.
Hence attempts were made to get ways to find out the properties of elements in systematic order.


Dobereiner’s triads

In 1829, 30 plus elements were known. Dobereiner, a German scientist made some groups of three elements each and called them triads.



Characteristics:

Atomic mass of the second element of a triad is rarely equal to average atomice mass of 1st and 3rd element 
Elements in triad have similar properties.

Triad 1

Li-7
Na-39+7/2=23
K -39


Mean of the masses of the calcium (Ca) and barium (Ba) iS  rarely equal to strontium (Sr).

Arithmetic mean of calcium (Ca) and barium (Ba) =(40+137)2=88.5

Actual mass of the strontium (Sr) = 87.6

Triad 3


Arithmetic mean of Chlorine (Cl) and Iodine (I) = (35.5+127)2=81.5

Actual mass of the second element = 80


Limitations:

Dobereneir’s idea of classification of elements into triads failed to receive wide acceptance as he could arrange only 9 elements in triad form.

Newland’s law of Octaves

Newland an English chemist in 1866 gave Law of Octaves.
Till then 56 elements were known.
Law of Octaves says that “If elements are arranged by the increasing order of their atomic masses, property of each eighth element (starting from first element) repeats”.
Characteristics of Law of octaves:

It contained the weather ranging from hydrogen and ends at thorium.
Properties of each eighth element follow of that of first element.




Limitations of Newlands law of octaves:

Similarity in properties of elements as per the law was seen up to calcium only.
Only 56 elements known that point were talked about. At that point around 1 element was discovered per annum. the weather to be discovered weren't considered.
At many places, 2 elements were placed in a very single slot (ex Co &Ni)
Placing of iron far-flung from cobalt and nickel, which have similar properties as iron, could also not be explained.
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Mendeleev’s tabular array

Dmitry Mendeleev a Russian chemist in 1869 gave Mendeleev’s tabular array.
Till then 63 elements were known.
Mendeleev arranged elements in increasing order of their mass.
He tried to place elements with similar properties in a very group.
Due to this we discover empty boxes in his table.
Properties of groups studied by Mendeleev:

(a) Formation of Oxides: Oxides are compounds of elements with oxygen.

For example:

Li2O , Na2O and K2O resembles to R2
MgO, CaO, ZnO resembles to RO.


(b) Formation of Hydrides: Hydrides are compounds of elements with hydrogen.

For example:


The horizontal rows present within the tabular array are called periods.
The vertical columns present in it are called groups.
There have been total eight groups in Mendeleev’s tabular array, I to VIII.
Properties of elements in a very particular period show regular gradation (i.e. increase or decrease) from left to right.
Groups I to VII are subdivided into A and B subgroups. Groups VIII don’t have any subgroups.
All the weather in a very particular group have similar properties. They show regular gradation in their physical properties and chemical reactivities.
Limitations of Mendeleev’s Periodic Table:

1. Position of Isotopes

Isotopes are atoms of same element having different atomic masses but have similar chemical properties.
Isotopes are placed together by Mendeleev as they need similar properties. then again this violated the arrangement scheme of accelerating atomic masses.
Mendeleev couldn't explain that problem.

2. Anomalous pairs of elements

At some locations, elements were put so as of decreasing mass.

For example; Co, Ni and Te, I.

This wasn't explained by Mendeleev.

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3. Position of hydrogen

Properties of H are kind of like group 1 moreover as group 7. But Mendeleev placed it in group 1 with none proper explanation.

Merits of Mendeleev’s periodic classification:

Earlier 63 elements were known.
Mendeleev discovered Prediction of latest elements.
Mendeleev’s tabular array had some blank spaces in it. These vacant spaces were for elements that were yet to be discovered.
For example, he proposed the existence of some unknown elements
1. Eka – boron → Scandium

2. Eka – aluminium → Gallium

3. Eka – silicon → Germanium

Scandium, Gallium and Germanium were discovered later and their properties matched very closely with the anticipated properties of Eka - boron, Eka – aluminium and Eka – silicon respectively.


Atomic Number

Atomic number is defined because the total number of protons present within the nucleus of an atom. it's denoted by ‘Z’.
Atoms of two different elements will always have different number of protons.
Atoms of same element have same number of protons and thus they need same number ‘Z’.
In fact, elements are defined by the quantity of protons they possess. For hydrogen, Z = 1, because in atom, just one proton is present within the nucleus.


Electron Distribution in Orbits

It is arrangement of electrons in atomic orbitals.


Rules for Electron Distribution:

There are major rules for e- distribution:

1. An orbit can have a maximum of 2n2 e-.

2. Orbits are filled from inside to outside. First, n = 1 shell is filled, then n = 2 shell, and so on.

3. The outermost shell of an atom cannot accommodate over 8 electrons, whether or not it's a capacity to accommodate more electrons.

This is a really important rule and is additionally called the Octet rule. The presence of 8 electrons within the outermost shell makes the atom very stable.

Electronic configuration of some elements :


Valency

Valence Electrons: Valence electrons are the electrons within the outermost orbit of an atom. Outermost orbit is additionally called valence shell.


Stable and Unstable Electronic Configuration :

If K shell is outermost shell of an atom and if the atom has 2e- in outermost shell,

Or

if K shell isn't the outermost shell of an atom and if the atom has 8e- in outermost shell, the arrangement of electrons is named stable electronic configuration.

Atoms do chemical reactions with one another to gain stable electronic configuration.
Noble gases (He, Ne and Ar) are inert as they have already got stable electronic configuration.
Valency of a component is that the number of electrons that its atom should reveal or want attain stable electronic configuration.
The number of electrons present within the valence shell/outermost shell determines the valency
Silver has 1 electron in its outermost shell. Silver donates one electron to finish its octet so valency of silver is 1.







Modern table

In 1913, Moseley showed or proved that number may be a important property of a element.
After that, Neil Bohr made the fashionable table using number.
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Basic concept of contemporary Periodic Table:

Most of the properties of a part rely upon number of valence electrons.
Elements having same number of valance electrons are grouped together.
Thus elements in a very group have similar properties.


Exception: In 18th group, element have 8 valence e- except Helium. But still helium may be a appropriately placed in 8th group because it also has stable electronic configuration therein group. Also its properties are very just like other elements of that group.

Characteristics of contemporary Periodic Table:

In table, elements are arranged by increasing number.
Horizontal rows on the periodic chart are called periods.
There are seven rows within the table. Each row is termed a period. The periods are numbered from 1 to 7.
The first period is that the shortest period of all and contains only 2 elements, H and He.
The second and third periods are called short periods and contain 8 elements each.
Fourth and fifth periods are long periods and contain 18 elements each.
Sixth period is extremely long period containing 32 elements.
Vertical columns are called groups. There are 18 groups within the table.
Group 1 on extreme left position contains alkali metals (Li, Na, K, Rb, Cs and Fr).
Group 18 on extreme right side position contains noble gases (He, Ne, Ar, Kr, Xe and Rn).
Inner Transition Elements:

14 elements with atomic numbers 58 to 71 (Ce to Lu) are called lanthanides.
14 elements with atomic numbers 90 to103 (Th to Lr) are called actinides.

Trends in Modern Periodic Table:

1. Valence e- and valence shell

a) Across a period:

Valence e- increases from left to right.
Valence shell is constant.
b) Down the group:

Valence e- remains constant.
Valence shell increases.
2. Valency

a) Across the period:

Valency increases till group 14 and so decreases till 18.
b) Down the group:

Valency remains constant.
3. Size of atom

a) Across the period:

As we move to right, charge on nucleus increases, so attraction of outer electron increases. Therefore, electron comes near nucleus. Thus size of atom decreases from left to right.
b) Down the group:

As we go down, number of shells increases, so size of atom also increases.
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4. Metallic character

a) Across the period:

Decreases from left to right.
b) Down the group:

Increases down the group.


5. Electro positivity

It is the flexibility of an atom to loose electron.

If electropositivity is high, it's easy to loose electron.
If electropositivity is low, it's difficult to loose electron.
a) Across the period:

As we move to right, size of atom decreases and so more attraction on electrons. So it's difficult to require e- Thus electropositivity decreases from left to right.
b) Down the group:

As we move down in a very group, size of atom increases so less attraction on electrons. So it's easy to require e- Thus electropositivity increases down the group.
6. Nature of oxides

a) Across the period:

Acidic nature of oxides increases from left to right.
b) Down the group:

Acidic nature of oxides decreases down the group.


Metals normally form basic oxides and are electropositive.
Non – metals normally form acidic oxides and are electronegative.

7. Chemical reactivity

a) Across the period:

First decreases because it is tougher to loose more e- and so increases from left to right because it is less complicated to realize lesser number of e-.
b) Down the group:

As we move down in a  group, chemical reactivity increases for metals and decreases for non metal
Explanation of Limitations of Mendeleev’s Periodic Table:

1. Position of Isotopes:

As we all know isotopes are atoms of same element having different atomic masses but have similar chemical properties.
They are placed together by Mendeleev as they need similar properties. on the other hand this violated the arrangement scheme of skyrocketing atomic masses.
Modern table use number for arrangement of element.
Atomic number of isotopes is same in order that they should be at same location in per table.
3. Position of hydrogen

Electronic configuration of hydrogen matches with electronic configuration of other elements of group 1. So hydrogen should be placed in group1