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 valency
(v) 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
(vi) Those elements of the groups which do not resemble with typical elments are called transition elements. For example- IB, IIB, IIIB, IVB, VB, VIB, VIIB, and VIII group elements are transition elements.
(vii) Hydrogen can be placed in both IA and VIIA groups.
(viii) In a group, from top to bottom in general
(a) Atomic weight, atomic size, electropositive character and metallic character of an element increase.
(b) Ionisation potential, electron affinity and electronegativity of an element decrease.
Uses of Mendeleef’s periodic table
(i) Atomic weights of elements were determined with the help of periodic table Atomic weight = Valency x Equivalent weight = Group number x Equivalent weight.
(ii) Atomic weight of elements were corrected. Atomic weight of Be was calculated to be 3 x 4.5 = 13.5 by considering its valency 3. Mendeleef calculated it 2 x 4.5 = 9
(iii) Discovery of new elements − In Mendeleef’s periodic table two consecutive members differ by two or three units in the atomic weight. Where this gap was more, the gaps were left in the periodic table.
Defects of Mendeleef’s periodic table
(i) Position of hydrogen is uncertain. It has been placed in IA and VIIA groups because of its resemblance with both the groups.
(ii) No separate positions were given to isotopes.
(iii) It is not clear whether the lanthanides and actinides are related to IIA or IIIB group.
(iv) Although there is no resemblance except valency of subgroups A and B, they have been put in the same group.
(v) Order of increasing atomic weights is not strictly followed in the arrangement of elements in the periodic table. For e.g. − Co (At.wt. 58.9) is placed before I (127) and Ar (39.9) before K (39).