Periodic Classification of Elements-NCERT Solutions

Q1.   Did DobereinerA's triads also exist in the colurrxrts of Ne lord's Octaves? Compare and find out.

Ans. Yes, it exists in the Iewland's octave. It is Li, Na and

Q2.   What were the limitations of Dobereiner's classification?

         (a) All the existing elements were not classified.

         (b) D bereiner could identify only three triads from the elements.

         Hence this system wa4s not useful.

Q3.   What were the limitations of Newlands� Law of Octaves?

Ans. • Newlands’ law was applicable only till calcium, after Ca, every eighth element did not possess properties similar to first.

           • New elements discovered did not fit into the law.

           • Wrong order of arrangement of elements was done, e.g. Co and Ni do not resemble halogen were found together in same slot, Fe being similar to Co and Ni was separated and kept in different slot.

Q4.   Use Mendeleev's Periodic Table to predict the formulae for the oxides of the following elements:

         K, C, Al, 1Si, Ba

         

Q5.   Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (any two)

Ans. Besides gallium, germanium and scandium have been discovered.

Q6.   What were the criteria used by Mendeleev in creating his Periodic Table?

Ans. (i) Increasing order of atomic mass of the elements.

           (ii) Al elements in a group with similar properties.

           (iii) The formula of oxides and hydrides formed by an element.

Q7.   Why do you think noble gases are placed in a separate group?

Ans. Noble gases are inactive, do not resemble other elements and all of them show same properties, hence they are grouped separately.

Q8.   How could the Modern Periodic Table remove various anomalies of Mendeleev's Periodic Table?

Ans. Modern Periodic Table is based on the atomic number of elements, therefore

           (i) problem of isotopes was solved because isotopes have same atomic number

           (ii) wrong order of r, K, Go, Ni was removed.

Q9.   Name two elements you would erect to show chemical reactions similar to magnesium. What is the basis for your choice?

Ans. Calcium and barium.

           Reason : (i) Both of them belong to same group as magnesium.

           (ii) Ba and Ca has same valence electrons as Mg, and will show same properties as of magnesium.

Q10.   Name:

           (a) Three elements that have a single electron in their outermost shells.

           (b) vo elements that have two electrons in their outermost shells.

           (c) Three elements with filled outermost shells.

Q11.   (a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?

           (b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What,. if anythin, do their atoms have in common?

Ans. (a) All these metals are highly reactive, they have same valence electrons i.e. 1 and can readily loose electrons to become positive ions.

           (b) Helium and neon have completely filled outermost shell.

Q12.   In the Modem Periodic Table, which are the metals among the first ten elements.

Ans. Lithium and beryllium are metals.

Q13.   By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristics?

           Ga, Ge, s, Se, Be

Ans. Among the given elements Be will show maximum metallic characteristics as it belongs to extreme left of the Periodic Table.

Q1.   Which of the following statements is not a correct statement about the trends when going from, left to right across the periods of the Periodic Table?

           (a) The eiements become less metallic in nature.

           (b) The number of valence electrons increases.

           (c) The atoms lose their electrons more easily,

           (d) The aides become more acidic.

Ans. (c) The atoms lose their electrons more easily.

Q2.   Element forms a chloride with the formula XCl2, which is a solid with a high melting point. X would most likely be in the same gaup of the Periodic Table as

           (a) Na           (b) Mg           (c) Al           (d) Si

Ans. (b) Mg.

Q3.   Which element has

           (a) two shells, both of which are completely filled with electrons?

           (b) the electronic configuration 2, 8, 2?

           (c) a total of three shells, with four electrons in its valence shell?

           (d) a total of two shells, with three electrons in its valence shell?

           (e) twice as many electrons in its second shell as in its first shell?

Ans. (a) Ne (2, 8) (b) Mg (2, 8, 2)

           (c) Si (2, 8, 4) (d) B (2, 3)

           (e) C (2, 4)

Q4.   (a) What property do all elements in the same column of the Periodic Table as boron have in common?

           (b) What property do all elements in the same column of the Periodic Table as fluorine have in common?

Ans. (a) All other elements have same valence electrons and their valency is 3.

           (b) All are non-metals, they have same valence electrons i.e., 7 and their valency is 1, all of them gain electrons to form negative ions.

Q5.   An atom has electronic configuration 2, 8, 7.

           (a) What is the atomic number of this element?

           (b) To which of the following elements would it be chemically similar?

                 (Atomic numbers are given in parentheses)

                 N(7) F(9) P(15) Ar(18)

Ans. (a) The atomic number of the element is 17.

           (b) F(9) (2, 7) will be chemically similar to the given element.

Q6.   The positkm of three elements A, B and C in the Per iodic Table are shown below:

         

         (a) State whether A is a metal or non-metal.

         (b) State whether Cis more reactive or less reactive than A.

         (c) Will C be larger or smaller in size than B?

         (d) Which type of ion, cation or anion, will be formed by element A?

Ans. (a) ‘A’ is non-metal.

         (b) 'C' is less reactive than ‘A’

         (c) 'C' is smaller in size than ‘B'

         (d) ‘A’ will form negatively charged ion �V�V Anion

Q7.   Nitrogen (atomic number 7) and phosphorous (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of the two elements. Which of the will be more electronegative? Why?

Ans. Nitrogen atomic number 7 → 2, 5

         Phosphorus atomic number 15 → 2, 8, 5

         Nitrogen with two shells will be more electronegative because it can easily gain electron due to its smaller size of atom, the nuclear charge attracts the electron easily to become negative ion.

Q8.   How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?

Ans. The position of element depends upon its electronic configuration. The number of shells is equal to the period number. The valence electrons decides the group number in which it will be, elements with 1 valence electrons belong to group 1.

         Elements with 2 valence electrons belong to group 2.

Q9.   In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium.

Ans. Ca atomic number — 20

         Electronic configuration — 2, 8, 8, 2

         Elements with atomic number 12 → 2, 8, 2

         and atomic number 38 → 2, 8, 18, 8, 2

         will resemble calcium as they all have same valence electrons and their chemical properties are also same.

Q10.   Compare and contrast the arrangement of elements in Mendeleev's ,Periodic Table and the Modern Periodic fable.