Periodic Properties - Mendeleef's Periodic Table
The table is divided into nine vertical columns called groups and seven horizontal rows called periods.
Characteristics of Periods:
(i) First period is called shortest period and contains only two elements. Second and third periods are called short periods containing eight elements each. Fourth and fifth periods are long periods containing eighteen elements each. Sixth period is longest period with thirty-two elements. Seventh period is an incomplete period containing nineteen elements. Numbers 2, 8, 18, 32 are called magic numbers.
ii) Lanthanide and actinide series containing 14 elements each are placed separately under the main periodic table. These are related to sixth and seventh periods of III group respectively.
iii) Elements of third period from sodium (Na) to Chlorine (Cl) are called representative or typical elements.
iv) Valency of an element in a period increases from 1 to 7 with respect to oxygen.
Na2O MgO Al2O3 SiO2 P2O5 SO3 Cl2O
1 2 3 4 5 6 7
v) From left to right in a period generally
a) Atomic weight, effective nuclear charge, ionisation potential, electronegativity and electron affinity of an element increase.
b) Atomic radius, electropositive character and metallic character of an element decrease.
vi) Diagonal relationship — Elements of second period Li, Be and B resemble closely with the elements Mg, Al and Si of third period in the next higher group.
Second Period Li Be B C
Third Period Na Mg Al Si
viii) Elements of second period are called bridge elements.
Characteristics of groups
i) Mendeleef's periodic table contains nine groups. These are represented by Roman numerals I, II, III, IV, V, VI, VII, VIII and zero,. Groups I to VII are divided into two subgroups A and B, Group VIII consists of three sets, each one containing three elements.
ii) Inert gases are present in zero group. These were not discovered till that time.
iii) The valency of an element in a group is equal to the group number.
iv) There is no resemblance in the elements of subgroups A and B of same group, except valencyv) The elements of the groups which resemble with typical elements are called normal elements. For example IA, IIA, IIIA, IVA, VA, VIA, VIIA group elements are normal elements