RBSE Solutions
           10th Science Chapter 5
     Periodic Classification of Elements
  Intext Questions
  Intext Questions Page No. 81
  Question 1.
  Did Dobereiner’s triads also exist in the columns of Newland’s
  Octaves? Compare and find out.
  Answer:
  Yes, Dobereiner’s triads also exist in the columns of Newland’s
  octaves.
  Example: lithium (Li), sodium, (Na) and potassium (K) which are
  present in the second column (‘re’ column) of Newland’s octave and
  this is also the example of Dobereiner’s triads. If we consider lithium
  as the first element, then the eighth element from it is sodium and if
  sodium is considered as first element then the eighth element from it
  is potassium.
  Question 2.
  What were the limitations of Dobereiner’s classification?
  Answer:
  Dobereiner’s classification of triads was applicable only for few
  elements. Dobereiner could identify only three triads from the
  elements known at that time so, this system of classification into triads
  was not found to be succesful.
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  Question 3.
  What were the limitations of Newland’s Law of Octaves?
  Answer:
  Newlands Law of Octave’s had the following limitations.
  (1) The Law of Octaves was applicable only upto calcium, as after
  calcium every eighth’ element did not possess properties similar to
  that of the first.
  (2) It was assumed by Newlands that only 56 elements existed in
  nature and no more elements would be discovered in the future. But,
  later on, several new elements were discovered, whose properties did
  not fit into the Law of Octaves.
  (3) In order to fit elements into his Table. Newlands put two elements
  in the same slot, but also put some unlike elements under the same
  note. For example cobalt and nickel are placed in the same slot and
  these are placed in the same column (Sa, column) with fluorine,
  chlorine and bromine which have very different properties than cobalt
  and nickel. Iron, which resembles cobalt and nickel in properties, has
  been placed far away from these elements.
  (4) Newlands’Law of Octaves worked well with lighter elements only. 
  (5) With the discovery of noble gases, the Law of Octaves become
  irrelevant.
  Intext Questions Page No. 85
  Question 1.
  Use Mendeleev’s Periodic Table to predict the formulae for the oxides of
  the following elements : K, C, Al, Si, Ba.
  Answer:
  (i) K (Potassium) belongs to group IA of Mendeleev’s periodic table. Hence,
  its valency is 1. Similarly oxygen belongs to group VI A, Hence its valency is
  2. Thus, the formula of its oxide is K2O.
  (ii) C (Carbon) belongs to group IV A of Mendeleev’s periodic table.
  Hence, its valency is 4. Thus, the formula of its oxide is CO2
  (iii) Al (Aluminium) belongs to group III A of Mendeleev’s periodic table.
  Hence, its valency is 3. Thus, the formula of its oxide is A2O3.
  (iv) Si (Silicon) belongs to group IV A of Mendeleev’s periodic table.
  Hence, its valency is 4. Thus, the formula of its oxide is SiO2.
  (v) Ba (Barium) belongs to group II A of Mendeleev’s periodic table. Hence,
  its valency is 2. Thus, the formula of its oxide is BaO.
  Question 2.
  Besides gallium, which other elements have since been discovered that
  were left by Mendeleev in his Periodic Table (any two)?
  Answer:
  Besides gallium, scandium and germanium have since been discovered
  that were left by Mendeleev in his periodic table. The properties of
  scandium, gallium and germanium were similar to Ma – boron, Eka –
  aluminium and silicon respectively.
  Question 3.
  What were the criteria used by Mendeleev in creating his periodic table?
  Answer:
  The criteria were used by Mendeleev increating his periodic table was the
  relationship between the atomic masses of the elements and their physical
  properties and chemical properties. Among chemical properties he
  concentrated on the compounds formed by elements with oxygen and
  hydrogen.
  Question 4.
  Why do you think the noble gases are placed in a separate group?
  Answer:
  The noble gases like helium (He), neon (Ne), argon (Ar), krypton (Kr) and
  xenon (Xe) were placed in a separate group called zero group because
  these are most unreactive so they do not react with other elements. These
  gases were discovered very late because they are very inert and present in
  extremely low concentrations in our atmosphere.