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Classification of Elements and Periodicity in Properties
Question
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What are the defects or drawbacks of Mendeleev’s table? 

Answer

Drawbacks of Mendeleev's table:
(i) Position of hydrogen: Hydrogen is placed in group I-A of the periodic table. However, it resembles the elements of both group I A (alkali metals) and group VII A (halogens). Therefore, the position of hydrogen in the periodic table is not clear.
(ii) Anomalous pairs of elements: In some cases, elements of higher atomic masses are placed before those having lower atomic masses. Examples:
  Ar        and         K;
39.95                     39.1
Co          and        Ni;         Te       and    I
58.93                  58.71    127.6           126.9
(iii) Metals and non-metals: No attempt has been made to place metals and non-metals separately in it.
(iv) Lanthanoides and Actinodes: Fifteen elements are placed in one position.
Group III B
6th period:  La + 14 lathanoide elements. (58Ce to 71Lu).
7th period:   Ac + 14 actinoide elements. (90Th to 103Lr)
Thus, lanthanoids and actinoides have not been provided separate and proper places in the Mendeleev's periodic table.
(v) Position of isotopes:  Isotopes of elements are placed in the same position in the table though according to their atomic masses they should have been placed in different positions.
(vi) Similar elements separated in the table: Certain elements such as copper and mercury or gold and platinum which possess similar chemical properties are placed in different groups.
(vii) Dissimilar elements placed together in the same group: Alkali metals such as Li, Na, K etc. (group IA) are grouped together with coinage metals such as Cu, Ag and Au (group IB) though their properties are quite different.
(viii) Number of elements in the periods. The presence of only 2 elements in the first period, 8 in the second and third etc. can not be explained.

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Some More Questions From Classification of Elements and Periodicity in Properties Chapter

How many elements are present in:
(i) second period    
(ii) fourth period    
(iii) sixth period?

What is the name given to the s-block elements?

To which block the element with outer electronic configuration 4s23d10 belongs?

Why are there 10 elements in each series of d-block?

In terms of electronic configuration,
what is common in a given period and group?

Give the general electronic configuration of
(i) Transition elements     (ii) Inner transition elements.

Name the group and period to which an element with Z = 15 belongs ?

What are the advantages of periodic classification of elements?

In the modern periodic table, the period indicates the value of:
(a) atomic number
(b) atomic mass
(c) principal quantum number
(d) azimuthal quantum number.

Which of the following statements related to the modern periodic table is incorrect?
(a) The p-block has 6 columns, because a maximum of 6 electrons can occupy all the orbitals in a p-shell.

(b) The d-block has 8 columns, because a maximum of 8 electrons can occupy all the orbitals in a d-subshell.

(c) Each block contains a number of columns equal to the number of electrons that can occupy that subshell.

(d) The block indicates value of azimuthal quantum number (l) for the last subshell that received electrons in building up the electronic configuration.