NCERT Class X Science Solutions: Chapter 5 – Periodic Classification of Elements Part 1

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Question 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.

  1. The elements become less metallic in nature.

  2. The number of valence electrons increases.

  3. The atoms lose their electrons more easily.

  4. The oxides become more acidic.

Answer:

C. The atoms lose their electrons more easily.

(On moving from left to right across the periods of the periodic table, the non-metallic character increases. Hence, the tendency to lose electrons decreases.)

Image result for atoms lose electrons easily

Atoms Lose Electrons Easily

Question 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

  1. Na

  2. Mg

  3. Al

  4. Si

Answer: A

X would most likely be in the same group of the Periodic Table as magnesium (Mg). Because Mg has the valency 2, and valency in a group remains the same. Also, when it combines with Clorine (having valency) it forms

Na has valency 1

Al has valency 3

Si has valency 4

Question 3:

Which element has?

  1. Two shells, both of which are completely filled with electrons?

  2. The electronic configuration 2, 8, 2?

  3. A total of three shells, with four electrons in its valence shell?

  4. A total of two shells, with three electrons in its valence shell?

  5. Twice as many electrons in its second shell as in its first shell?

Answer:

  1. Neon has two shells, both of which are completely filled with electrons (2 electrons in K shell and 8 electrons in L shell).

  2. Magnesium has the electronic configuration 2, 8, 2

  3. Silicon has a total of three shells, with four electrons in its valence shell (2 electrons in K shell, 8 electrons in L shell and 4 electrons in M shell).

  4. Boron has a total of two shells, with three electrons in its valence shell (2 electrons in K shell and 3 electrons in L shell).

  5. Carbon has twice as many electrons in its second shell as in its first shell (2 electrons in K shell and 4 electrons in L shell).

Question 4:

  1. What property do all elements in the same column of the Periodic Table as boron have in common?

  2. What property do all elements in the same column of the Periodic Table as fluorine have in common?

Answer:

  1. All the elements in the same column as boron have the same number of valence electrons (3). Hence, they all have valency equal to 3.

  2. All the elements in the same column as fluorine have the same number of valence electrons (7). Hence, they all have valency equal to 1.

Question 5:

An atom has electronic configuration 2, 8, 7.

  1. What is the atomic number of this element?

  2. To which of the following elements would it be chemically similar? (Atomic numbers are given in parentheses.)

  3. N(7)

  4. F(9)

  5. P(15)

  6. Ar(18)

Answer:

  1. The atomic number of this element is 17.

  2. It would be chemically similar to F (9) with configuration as 2, 7.

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