Question

(a) State the merits of Mendeleev's classification of elements.
(b) Describe two anomalies of Mendeleev's periodic classification of elements.

Answer 1

Answer:

Merits

(i)mendeleev left proper to the undiscovered w

elements

(ii)mendeleev had pointed out of the properties of certain elements just before the search some elements

(iii) mendeleev had already kept a separate period leave for noble gases to not disturb the table

Anomalies

(i) the position of isotopes could not be explained

(ii) wrong order of atomic masses of some elements could not be explained

(iii) A correct position could not be assigned to hydrogen in the periodic table

Answer 2

        Mendeleev's Classification of Elements

Statement: "The properties of the elements are the periodic function of their atomic weights"

1. Also known as short form of periodic table.

2. He relied on the similarities in the empirical formulas and properties of the compounds formed by the elements.

3. He not only followed the order of atomic weights but also placed elements with similar properties.

 (a) Almost same atomic weights.

          Ex: Fe(56), Co(59) & Ni(59).

 (b) Atomic weights increasing constantly.

          Ex: K(39), Rb(85) & Cs(133).

4. Vertical columns - groups ( 9 groups ).

   Horizontal rows - periods ( 7 periods ).

5. First three periods are short periods & remaining are the long periods.

6. Leaving O & VIII, each group is subdivided into sub-groups known as A and B groups.

7. Group VIII of Mendeleev's table consists of 3 triads known as transition triads.

   (i) Iron, Cobalt, Nickel.

   (ii) Ruthenium, Rhodium, Palladium.

   (iii) Osmium, Iridium, Platinum.

8. Zero group elements were not known at the time of Mendeleev and later introduced by Ramsey.

Merits:

1. Gave an elaborate system of classification based on physical and chemical properties.

2. Broadly left some gaps for undiscovered elements like Ge, Sc.

3. He corrected the weights of Beryllium, Iridium & Osmium.

Anomaly:

1. Some elements with higher atomic weights were placed before lower atomic weight elements.

2. Position of hydrogen couldn't be specified.

3. No place for noble gas.

4. Isotopes are not included.

5. Cause of periodicity is not shown.