a. Mendeleev's and the modern periodic table.
|It supported the first scientific study of elements.||It is the latest and standard table of elements.|
|Elements were arranged with the increasing order of their atomic weight.||Elements were arranged with the increasing order of their atomic number.|
|It had several gaps for undiscovered elements.||Most of the elements are discovered and arranged.|
|It was unable to arrange isotopes.||It is able to arrange isotopes.|
|It didn't have lanthanides and actinides series.||It consists of lanthanides and actinides series.|
b. Periods and groups.
|The horizontal rows in the periodic table are periods of the periodic table.||The vertical column in the periodic table are groups of the periodic table.|
|The change in valency is found in a period.||The valency of elements remains the same in a group.|
|A period contains the elements of different blocks.||A group contains the element of the same block.|
c. s-block elements and p-block elements.
|The last electron of these elements enters the s-orbital of the outermost shell.||The last electron of these elements enters the p-orbital of the outermost shell.|
|These elements are alkali metals.||These elements are non-metals.|
|These elements are electronically positive. i.e. they have positive valency.||These elements have negative valency.|
d. Alkali metals and halogens.
|These elements are highly reactive electro-positive metals.||These elements are highly reactive electro-negative metals.|
|These elements are good conductors of electricity.||These elements are insulators of electricty.|
|These elements have positive valency.||These elements have negative valency.|
e. Alkali metals and alkaline earth metals.
|Alkali metals||Alkaline earth metals|
|These elements are placed in group IA of the periodic table.||These elements are placed in group IIA of the periodic table.|
|These elements are more reactive.||These elements are comparatively less reactive.|
|These elements have valency +1.||These elements have valency +2.|
f. Duplet stable state and octet stable state.
|Duplet stable||Octet stable|
|The atom which has a stable electronic configuration having 2 electrons in its valance shell is duplet stable.||The atom which has a stable electronic configuration having 8 electrons in its valance shell is octet stable.|
|This is possible only in 1st or 2nd shell.||This is possible in any shell of an atom.|