Mendeleev’s Periodic Table

Question 1 State Mendeleev’s Periodic Law ?

Question 2 What was the Mendeleev’s basis for the classification of elements?

Question 3 Which group elements was missing from Mendeleev’s original periodic table?

Question 4 State the merits of Mendeleev’s classification of elements?

Question 5 Why did Mendeleev’s leave gaps in his periodic table of elements?

Question 6 Why noble gases are placed in a separate group?

Question 7 What are the characteristics of Mendeleev’s periodic table?

Question 8 What are the defects in the Mendeleev’s periodic table?

Mendeleev’s Periodic Table

Dmitri Ivanovich Mendeleev tried to co-relate the atomic masses of the elements with their physical and chemical properties.

Mendeleev’s Periodic Law states that the physical and the chemical properties of the elements are a periodic function of their atomic masses.

 

Mendeleev’s Periodic Table

Mendeleev arranged the elements known at that time in order of increasing atomic masses and this arrangement was called periodic table. Elements with similar characteristics were present in vertical columns called groups. The horizontal rows were known as periods.

Characteristics of the Periodic Table

The main characteristics of the periodic table are:

(1) In the periodic table, the elements are arranged in vertical columns called groups and horizontal rows known as periods.

(2) There are eight groups indicated by Roman Numerals as I, II, III, IV, V, VI, VII, VIII and the elements belonging to the first seven groups have been divided into sub-groups designated as A and B on the basis of similarities in properties.

(3) The elements that are present on the left hand side in each group constitute sub-group A while those on the right hand side form sub-group B. Group VIII consists of nine elements which are arranged in three triads.

(4) There are six periods (numbered from 1 to 6) or horizontal rows in the Mendeleev’s periodic table. The periods 4, 5 and 6 are divided into two halves. The first half of the elements are placed in the upper left corners and the second half occupy lower right corners in each box.

5) Many gaps were left in the periodic table for the undiscovered elements which were identified later on and were placed at their respective positions.

6) Noble gases could be easily placed in a separate group called zero group (noble gas elements have zero valency) without disturbing the main periodic table.

Achievements of Mendeleev’s Periodic Table

Systematic study of the elements

Mendeleev for the first time arranged a very large number of the elements into groups and periods. This made the study of the elements quite systematic.

Prediction of new elements and their properties

Mendeleev laid more stress on similarity in properties rather on increasing atomic masses of the elements. So whenever a particular element did not fit in the arrangement, he left a gap in the periodic table. Thus, many gaps for the undiscovered elements were left in the periodic table by Mendeleev.

Correction of doubtful atomic masses

Mendeleev also corrected the atomic masses of certain elements with the help of their expected positions and properties.

 

Defects in the Mendeleev’s Periodic Table

Position of hydrogen

Hydrogen (Z = 1) is placed at the top of the alkali metal family because it resembles alkali metals in its properties.

For example: Both hydrogen and alkali metals form similar compounds with elements like oxygen, chlorine and sulphur etc.

Compounds of sodium:  Na₂O, NaCl, Na₂S

Compounds of hydrogen: H₂O, HCl, H₂S

But at the same time, hydrogen also resembles halogens present in group VII A in many of its properties. Both are non-metals and also diatomic in nature. The compounds of both hydrogen and halogens with certain non-metals are of covalent nature.

Covalent compounds of hydrogen : CH₄, SiH₄, GeH₄

Covalent compounds of chlorine: CCl₄ , SiCl₄, GeCl₄

Anomalous positions of some elements

The elements in the Mendeleev’s Periodic table have been arranged in order of increasing atomic masses, but in some cases, the element with higher atomic mass precedes the element with lower atomic mass.

For example: Ar (Atomic mass =39.9) precedes K (Atomic mass= 39.1) and similarly Co (Atomic mass 58.9) has been placed ahead of Ni (Atomic mass=58.7). No justification has been given for such anomalous positions.

Position of isotopes

Isotopes of an element have different atomic masses but the same atomic number. Since the periodic table has been framed on the basis of increasing atomic masses of the elements, different positions must have been allotted to all the isotopes of a particular element.

For example: Three different positions should have been allotted to the isotopes of hydrogen with atomic masses 1, 2 and 3. But they have been assigned only one position.

No co-relation of elements in sub-groups

According to Mendeleev, the elements placed in the same group must resemble in their properties. But there is no similarity among the elements in the two sub-groups of a particular group.

For example: Li, Na and K present in group IA are quite different from Cu, Ag and Au which belong to group IB.

Different groups for similar elements

Elements with similar properties have been placed in different groups.

For example: Both copper and mercury resemble in many characteristics. But copper has been placed in group IB while mercury has been assigned position in group IlB

Cause of periodicity

No proper explanation has been offered to why the properties of the elements get repeated after gaps of atomic masses in a particular group.