NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements

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.

Answer: (c) The atoms lose their electrons more easily.

While moving from left to right across periodic table, ability of atoms to lose electrons decreases.

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

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

Answer: (b) Mg

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 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

(b) Magnesium

(c) Silicon

(d) Boron

(e) Carbon

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: (a) Number of electrons in valence shell of all elements in column of periodic table is 3. Therefore, their valency is 3.

(b) All elements have seven electrons in valence shell as fluorine has. Thus the valency of all elements is one.

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

Answer:

(a) Atomic number of given element is 17 therefore, the element is chlorine.

(b) Given element will be chemically similar to F(9) which has electronic configuration 2, 7.

6. The position of three elements A, B and C in the Periodic Table are shown below –

Group 16	Group 17
–	–
–	A
–	–
B	C

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

(b) Reactivity decreases as we move down along the column. Therefore, C is less reactive that A.

(c) C will be larger in size, as size of atom increases while moving down the group.

(d) A will accept one electron to complete its octet therefore it will be anion.

7. 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

As we move down the group number of shells in atom of an element decreases, therefore, electronegativity decreases. Thus, nitrogen is more electronegative than phosphorus.

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

Answer: Elements with similar electronic configuration is placed in same column called group. Here, number of valence electron in an atom of an element remains same. While moving along the period from left to right valence electrons of the atom of n element increases.

9. 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?

Answer: Element having same physical and chemical properties resembling to calcium is element having atomic number 12. This is because, both have 2 electrons in their outermost shell.

10. Compare and contrast the arrangement of elements in Mendeléev’s Periodic Table and the Modern Periodic Table.

Answer:

Mendeléev’s Periodic Table	Modern Periodic Table
Elements are arranged in increasing order of the atomic masses	Elements are arranged in increasing order of atomic numbers
Have 7 groups and 8 periods	Have 18 groups and 7 periods
Positions of metals and nonmetals cannot be distinguished	Positions of metals and nonmetals can be distinguished
Groups were divided into subgroup a, b	No subgroups formed

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