NCERT Solutions

Class 10 Science

periodic classification of elements

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

  • (a) The elements become less metallic in nature.
  • (b) The number of valence electrons increases.
  • (c)The atoms lose their electrons more easily.
  • (d)The oxides become more acidic.
Answer :

(C) the atoms lose their electrons more easily because their size increases from left to right

Q2)Element X forms a chloride with the formula XCl2, which is solid with a high melting point. X would most likely to be in the same group of the periodic table as

  • (a) Na
  • (b) Mg
  • (c) Al
  • (d) Si
Answer :

(B) Since elementX forms a chloride XCl2,so valency of X is 2.Now out of all, the valency 2is 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 ?
Answer :

  • (a) Neon(2,8
  • (b) Magnesium
  • (c) Silicon(2,8,4)
  • (d) Boron(2,3)
  • (e)Carbon(2,4)

(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 ?

Answer :

The property that all the elements in the same column of the periodic table as boron have is: “the same number of valence electrons (3). Hence, they all have valency equal to 3.”

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)

Answer :

? The atomic number of the given element is 2 + 8 + 7(= 17).
? It would be chemically similar to fluorine [F(9)] because its electronic configuration is 2, 7.

Q6) The positions of three elements A, B and C in the periodic table are shown below :
(a) State whether A is a metal or non-metal.

Group 16 Group 17
- -
- -
(b) State whether C is 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 ?

Answer :

  • (a)? Since the valency of group 17 elements is 1 and all these elements accept electrons, thus A is a non-metal.
  • (b) C is less reactive than A because as we move down in a group, the reactivity of non-metals increases.
  • (c) C is smaller in size than B because B and C both are related to the same period and the size decreases as one moves from left to right in a period.
  • (d) A will form anion because it is a non-metal

Q7) Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the periodic table. Write the electronic configuration of these two elements. Which of these will be more electronegative ? Why ?

Answer :

Electronic configuration of nitrogen -2,5
Electronic configuration of phosphorus = 2, 8, 5
Nitrogen will be more electronegative because outermost shell is nearer to nucleus and therefore nucleus will attract electrons more strongly. In a group of the periodic table, electron attracting tendency decreases as we move from top to bottom.

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

Answer :

The position of an atom/element in the periodic table is related to the number of electrons in the outermost shell. In a given column or group, all elements have the same number of electrons in their outermost shells. For example, in group 2, all elements have two electrons in their outermost shell. In group 15, all elements have 5 electrons in their outermost shells.

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

Answer :

The electronic configuration of elements with :
Atomic number 12 = 2, 8, 2
Atomic number 19 = 2, 8, 8, 1
Atomic number 20 = 2, 8, 8, 2
Atomic number 21 = 2, 8, 9, 2
Atomic number 38 = 2, 8, 18, 8, 2
Elements with atomic number 12 i.e., magnesium (Mg) and 38 i.e., strontium (Sr) will have similar physical and chemical properties as element with atomic numbers 20 i.e., calcium (Ca).

Q10) Compare and contrast the arrangement of elements in Mendeleev’s Periodic Table and the Modern Periodic Table.

Answer :

Mendeleev's Periodic Table
Elements are arranged in the increasing order of their atomic masses.
> There are 8 Groups.
> Each groups are subdivided into sub group 'a' and 'b'
> Groups for Noble gas was not present as noble gases were not discovered by that time.
> There was no place for isotopes.

Modern periodic table
> Elements are arranged in the increasing order of their atomic numbers.
> There are 18 Groups.
> Groups are not subdivided into sub-groups.
> A separate group is meant for noble gases.
> This problem has been rectified as slots are determined according to atomic number.