Sixth period is the longest period in the modern periodic table.

Eight elements are present in the 3^rd period of the modern periodic table.

Electron affinity decreases down a group because due to increase in atomic size, the force of attraction between the nucleus and the electrons to be added becomes less.

Ionisation energy goes on decreasing down a group. It is due to increase in the distance between the valence electrons and the nucleus. As the atomic size increases down a group, the force of attraction between the nucleus and the valence electrons decreases. Therefore, the energy required to remove the electrons, i.e. ionisation energy, goes on decreasing.

Metallic character increases down a group due to decrease in ionisation energy.

Be would show the maximum metallic character as it is present on the extreme left of the periodic table.

Elements were first classified into metals and non metals

Johan Newland discovered the law of octaves

oxygen and hydrogen are very reactive in nature and formed compounds with almost every element

mendeleevs' periodic table contained 8 groups and 6 periods

when we go from left to right in a period the effective nuclear charge of an atom increases and therefore the attraction between the nuclear charge and the shells(electrons) increases hence the atomic size of an atom decreases

dobereiner studied chemistry from Germany

criteria used by mendeleev in creating his periodic table is atomic masses

as per dobereiners law ,  the middle element should be the average of the other two elements

therefore Ca, Sr , Ba is the proper set of elements which satisfy dobereiners law

there are 18 groups in the long form of the periodic table

28 elements are present in the long form of the periodic table

32 elements are present in the 6th period of modern periodic table

63 elements were known when mendeleev classified the elements

Li & Be are metals among the first ten elements

AS ELEMENT X BELONGS TO GROUP 2 , ITS VALENCY IS 2

each new period in the periodic tabl represents +1 shell

AS IT BELONGS TO GROUP 15 , IT VALENCY WOULD BE 3 (8-5) .

18 elements are present in the 5th period of the periodic table

7 periods are present in the modern periodic table