The process of grouping the elements on the basis ofthe similarities and differences for theeasyrecognition and study of the element and theirproperties and characteristics is called the classification ofelements.

The contribution of fohn Dalton in the classification ofelements is that he profounded the atomic theory andsaid that the atoms of different elements havedifferent atomic weights.

The contribution of Prout is that he said the atomic weight of an element is a simple multiple of the atomic weight of hydrogen.

The table with 7 periods and 18 groups that are designed in such a waythat it can arrange all elementsaccording to their properties, similarities die dissimilarities,etc which helps in the study of elements is called aperiodic table.

Dobereiner arranged elements in increasing order oftheir atomic weight in well-marked groups of three-elementcalled trides.

So, the pattern of arranging the dements in eachtriads, that the atomic weight of middle elements wasfoundto be the arithmetic mean of the atomic weights ofother two elements, called Dobereiner's law of trials.

For example

Elements: Li Na K
Atomic weight: 7 23 39

So, 23 = (7+39) / 2

Newlands law of octaves stated, 'Elements whenarranged increasing order of their atomic weightsshow resemblance in physical and chemical propertiesbetween the eight and first element is similar to theeight and first notes on a musical scale.

For example, potassium is the eight element after sodium.
These elements have similar physical and chemical properties.

Increasing order of atomic weight was the basis ofMendeleev's periodic table.

Mendeleev's periodic table states that 'The physical &chemical properties of elements are the periodic functionof their atomic weight'.

Any three characteristics of Mendeleev's periodic tableare:

1. It was the first scientist and systematic study of known elements.
2. It containsmany groups for undiscovered elements.
3. Properties of several elements can be studied.
   

Any 4 defects in Mendeleev's periodic table are:

1. Inert gas or noble gases were absent.
2. It couldn't arrange lanthanides and actinides.
3. Its eight groups weren't divided into sub-group.
4. It couldn't arrange isotopes.
   

The increasing order of an atomic number of the elementswas the basis of the modern periodic table.

Modern periodic rule states that, 'Thephysical and chemical properties of the elements are theperiodic function of their atomic number'.

In the modern periodic table, the physical & chemical propertiesof the elements are changed to the periodic function of theiratomic numbers instead of atomic weight which solvesthe problem with isotopes. All isotopes of some elementrepresented by thesame atomic number. All thegaps ofMendeleev's periodic table are filled by discovering newelements. The column for inert gases was developedand all groups have sub-groups. In thisway, the modernperiodic table removes all the demerits of...

The 14 elements from cerium te lawrencium after lutetiumare called lanthanides.
The elements from thorium to lawrencium afteractinium are called actinides.

These elements are placed at the bottom of the periodictable because of their similar properties. Lanthanides arerare metals whereas actinides are artificial radioactivemetals. They show similar properties to the transitionalelements. So, they are placed separatelybelow thetransitional elements in blocks of the modem periodictable.

The horizontal rows of the periodic table where theelements are arranged according to their similarities anddifferences for their easy study are called periods.

The vertical columns where elements are arrangedfor the recognization of the properties and otherstudies are called groups.

The elements where the last electron enters in s-orbital arecalled s-block elements.
The elements where the last electron enters thep-orbital of the outermost shell are called p-blockelements.

The position of s-block elements is IA and IIA groups.
The position of p-block elements are from IIIB to IIBgroup in the periodic table

The electrons present in the outermost shell or valenceshell of an atom are called valence electrons.

In a period,valency increases up to the 4thgroupand decreases up to the 8th group and again increases.

a. Mendeleev's and the modern periodic table.

We know that sodium has positive valency but that of chlorine is negative. The last electron of sodium enters into the s-orbital but that of chlorine enters the p-orbital of the outermost shell. So, sodium is kept in s-block but chlorine is kept in p-block.

We know, both potassium and sodium are metals andlose the valence electrons. The atomic size of potassiummore than that of sodium. So, the valence electron of potassium canbe moreeasily taken by other reacting atoms ascompared to that of sodium. Hence, potassium is morereactive than sodium.

We know, the atomic size of fluorine is smaller than that ofchlorine. So, the nuclear attraction is more on the valenceshell of the Fluorine atom. As a result, fluorine can getone electron more easily during the chemical reaction. Hence,fluorine is more reactive than chlorine.

We know, the properties of f-block elements are similarto the transition metals of d-block, However, theirlast electron enters count f-orbital. So, f-block elementsare kept separately below d-block elements or transitionalelements. Hence, these elements are called innertransitional elements.

We know, alkaline earth metals are foundon the surfaceof the earth in form of oxide which showsproperties similar to that of alkali metals. So, theseelements are called alkaline earth metals.

We know,every element of the zero group has achievedstable electronic configuration being duplet stable oroctet stable They have the valencyzero and do not takepart in anychemical reaction. So, elements of thezero group are called inert gas.

When we move from top to doon of periodic groups ofmetals, the atomic size increases gradually. Due to this,nuclear attraction to the valence electron decreases.As a result, the tendency of metal atoms to loseelectrons increases. Hence chemical reactivityof metalsincreases when moving from top to bottom in a group.

When we move from top to bottom in a group of non-metal,the atomic size increases gradually. Hence, nuclearattraction for incoming electrons decreases. As a result,the tendency of non-metallic atoms to gain electronsdecreases. Therefore, chemical reactivity of non-metals decreaseson moving down in a group.

We know, lanthanides are rare earth metals, and actinidesare radioactive metals. The properties of lanthanidesresemble each othervery closely but are differentfrom the rest elements due to the preferential fillingof f-orbitals. So, they are placed separately to avoidunnecessarysidewise expansion of the periodic table.