NCERT solutions for class 10 science-Exercise Solutions

Reviewed By:
Krishna Kant Majee
M.Sc., B.Ed.

Chapter 05-Periodic Classification of Elements-Exercise Solutions

NCERT Exercise Page Number 91-92

Q.1. Which of the following statements is not a correct statement about the trends when 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.

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

Q.2. Element X forms a chloride with the formula $XCl_{2}$, which is a solid with 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.

Ans. (b) Mg.

Q.3. 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 the valence shell?
(d) a total of two shells, with three electrons in the valence shell?
(e) twice as many electrons in the second shell as in its first shell?

Ans. (a) Neon (Ne) (b) Magnesium (Mg) (c) Silicon (Si) (d) Boron (B) (e) Carbon(C).

Q.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?

Ans. (a) Boron belongs to group number 13 of the modern periodic table. All the elements of this group has similar valency as boron has. The valency of boron is 3, because it has three electrons in its valence shell. So all the elements of Group-13 have valency 3.
(b) Valency of all the elements of the same group will be same. So all the elements of this group shows valency one like fluorine (F). They are very reactive non-metals.

Q.5. 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 parenthesis)
N(7), F(9), P(15), Ar(18)

Ans. (a) Atomic number = 17
(b) F(9) (Number of valence electrons are same).

Q.6. The position of three elements A, B and C in the Periodic table are shown below :

(a) State whether A is a metal or non-metal.
(b) State whether C is more 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 because in case of non-metals reactivity decreases down the group.
(c) C is smaller than B because along the period nuclear charge increases thats why atomic size decreases.
(d) A will form anions because it has 7 electrons in valence shell that is why acquire one electron to complete its octet (element A will form A–).

Q.7. Nitrogen (atomic number 7) and phosphorous (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?
Ans. N(7) = 2, 5
P(15) = 2, 8, 5
Nitrogen is more electronegative than phosphorous because the size of nitrogen is smaller than phosphorous. (Smaller the size, high nuclear charge and due to this high electro-negativity).

Q.8. How does the electronic configuration of an atom relate to its position in the Modern periodic table?
Ans. Modern periodic table is based on the atomic number or electronic configuration of elements. The elements of a group have same electronic configuration of the outermost valence shell. After a certain interval repetition of similar outer electronic configuration of elements of atoms causes periodicity.
Group I Group 17
e.g. Li(3) = 2, 1 F(9) = 2, 7
Na(11) = 2, 8, 1 Cl(17) = 2, 8, 7
K(19) = 2, 8, 8, 1
So the electronic configuration of an atom justify its position in the periodic table.
Atoms of different elements with the same number of occupied shells are placed in the same period. Eg. Na, Mg, Al, Si, P, S, Cl and Ar belong to the third period of the modern Periodic Table, since the electrons in the atoms of these elements are filled in K, L and M shells.

Q.9. In the Modern periodic table calcium (atomic no. 20) is surrounded by elements with atomic number 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?
Ans. Elements in a group have similar properties. Elements with atomic number 12 and 38 lie in the same group as calcium. Therefore, they will have properties resembling calcium.

Q.10. Compare and contrast the arrangement of elements in Mendeleev’s periodic table and the modern periodic table.