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periodic classification of elements — Science STD 10 — Question

Gujarat BoardEnglish MediumSTD 10Scienceperiodic classification of elements4 Marks
Question
Explain the contribution of Mendeleev in the classification of elements.
✓

Answer

  • After the rejection of Newlands’ law of octaves, attempts were made to search for a pattern that correlated the properties of elements with their atomic masses.
  • The main credit for classifying elements goes to Dmitri Ivanovich Mendeleev, a Russian chemist.
  • Mendeleev arranged the elements on the basis of their fundamental property like the atomic mass and also on the similarity in chemical properties.
  • When Mendeleev started his work on classification of elements, only $63$ elements were known.
  • Mendeleev examined the relationship between the atomic masses of the elements and their physical and chemical properties.
  • Mendeleev concentrated on the compounds formed by the elements with hydrogen and oxygen, as hydrogen and oxygen were very reactive and formed compounds with most of the elements.
  • The formulae of the hydrides and oxides of the various elements were treated as one of the basic properties of an element for its classification.
  • Thereafter, Mendeleev took $63$ cards and on each card, he wrote down the properties of each element.
  • He sorted out the elements with similar properties and pinned the cards together on a wall one after the other in a vertical column.
  • Mendeleev observed that most of the elements accommodated in a periodic table and elements were arranged in the order of their increasing atomic masses.
  • Mendeleev also observed that that there occurs recurrence of elements with similar physical and chemical properties after certain regular intervals.
  • On the basis of these similarities, Mendeleev proposed his periodic law.
  • Mendeleev’s periodic law:
  • “The properties of elements are the periodic function of their atomic masses.”
  • Mendeleev’s periodic table consists of vertical columns called ‘groups’ and horizontal rows called ‘periods’.
Table 3 : Mendeleev’s Periodic Table
Group I II III IV V VI VII VIII
Oxide
Hydride		 $\mathrm{R}_2 \mathrm{O}$
RH		 Ro
$\mathrm{RH}_2$		 $\mathrm{R}_2 \mathrm{O}_3$
$\mathrm{RH}_3$		 $\mathrm{RO}_2$
$\mathrm{RH}_4$		 $\mathrm{R}_2 \mathrm{O}_5$
$\mathrm{RH}_34$		 $\mathrm{RO}_3$
$\mathrm{RH}_2$		 $\mathrm{R}_2 \mathrm{O}_7$
RH		 $RO_`$
Periods
↓		 A B A B A B A B A B A B A B Transition
Series
1 H
1.008		                            
2 Li
6.939		   Be
9.012		   B
10.81		   C
12.011		   N
14.007		   O
15.999		   F
18.998		    
3 Na
22.99		   Mg
24.31		   Al
29.98		   Si
28.09		   P
30.974		   S
32.06		   Cl
35.453		    
4 First
series:		 K
39.102		   Ca
40.08		     Sc
44.96		   Ti
47.90		   V
50.94		   Cr
50.20		   Mn
54.94		 Fe Co Ni 55.85 58.93 58.71
Second
series:		   Cu
63.54		   Zn
65.37		 Ga
69.72		   Ge
72.59		   As
74.92		   Se
78.96		   Br
79.909		    
5 First
series:		 Rb
85.47		   Sr
87.62		     Y
88.91		   Zr
91.22		   Nb
92.91		   Mo
95.94		   Tc
99		 Ru Rh Pd 101.07 102.91 106.4
Second
series:		   Ag
107.87		   Cd
112.40		 In
114.82		   Sn
118.69		   Sb
121.75		   Te
127.60		   I
126.90		    
6 First
series:		 Cs
132.90		   Ba
137.34		     La
138.91		   Hf
178.49		   Ta
180.95		   W
183.85		     Os Ir Pt 190.2 192.2 195.09
Second
series:		   Au
196.97		   Hg
200.59		 Tl
204.37		   Pb
207.19		   Bi
208.98		            

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