Chapter1: Textual Exercises

1. Why is there a need to classify the elements? What difficulties are likely to arise if the classification is not made?

Ans:- NEED FOR CLASSIFICATION OF ELEMENTS:

Knowing the properties of all the elements is a great burden. So, it is required to study least but vast knowledge. For this reason classification of elements is needed.

   If classification of elements is not made, it will give a great burden to chemist.

2. Name the period and group to which the element with atomic number 15 belongs.

Ans:- The element with atomic number 15 belongs to 3rd period (as it has 3-shells) and in group 5 (as it has 5 valence electrons).

3. State one reason for keeping boron and aluminium in the same group of the periodic table.

Ans:- This is because both Boron and Aluminium have same 3-valence electrons.

4. Write the electronic configurations of the elements Na, Al, Ca, O and Si and justify their positions in the periodic table.

Ans:- Electronic configuration of Na is 2, 8, 1.

The element is in group 1 in 3rd period.

The Electronic configuration of Al is 2, 8, 3

The element is in group 3 in 3rd period.

Electronic configuration of Ca is 2, 8, 8, 2

The element is in group 2 in 4th period.

Electronic configuration of O is 2, 6

The element is in group 16 in 2nd period.

Electronic configuration of Si is 2, 8, 4

The element is in group 14 in 3rd period.

5. Why is atomic number more important than atomic mass to know the properties of the elements?

Ans:- An element has only one atomic number but may have more than one atomic mass. Atomic number is the number of protons or electrons present in an atom. Hence, atomic number is more important than atomic mass as the atomic number can give physical and chemical properties of the elements.

6. State how atomic size varies from left to right in a period and from top to the bottom in a group.

Ans:- In period, on moving from left to right the electrons are added to the same shells and hence, the attraction between nucleus valence electrons increases gradually. As a result, the atomic size decreases from left to right in a period.

   But in a group, the electrons are added to new and new shells from the top to the bottom. As a result the atomic size increases gradually from the top to the bottom in a group due to increases in the distance between the nucleus and the valence electrons.

Left to right in a period - decreases.

Top to bottom in a group - increases.

7. The atomic radii of the elements of second period are given below:

2nd period elements:

Atomic radii (pm) 80 90

2nd period elements: B Be O N Li C

Atomic radii (pm): 80 90 74 75 123 77

(a). Arrange the elements in the decreasing order of their atomic radii, keeping the element with the largest atomic radius first.

Ans:- Li, Be, B, C, N and O.

(b). Find out whether the arrangement is in the pattern of a period or not?

Ans:- Yes, the above arrangement is in a pattern of a period. Because the atomic radius decreases in moving from left to right in a period.

(c). From this data, infer how the atomic size of the elements change as one goes from left to right in a period.

Ans:- Atomic size decreases from left to right.

8. By considering their position in the periodic table, which one of the following elements would you expect to have the maximum metallic character? Ga, Ge, As, Se, Be.

Ans:- The given elements belong to 4th period with group as given

4th Period:- Ga Ge As Se Br

Groups:- 13 14 15 16 17

Metallic character: Decreases

  Since metallic character decreases from left to right in a period, Ga has the maximum metallic character among the given elements.

9. 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 non-metal.

                Ans:- As A is in group 17 (halogen group) it is non-metal.

(b). State whether C will be more reactive or less reactive than A.

              Ans:- Element C is less reactive than element A as the reactivity decreases downward in a group in case of non-metals.

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

              Ans:- The size of the element C is less than that of element B as the size of atoms in a period decreases from left to right.

(d). What type of ion (cation or anion) will be formed by A ?

             Ans:- As A is in group 17, it is non-metal. Therefore, it can accept one electron forming anion, A- ion.

10. Element X forms a chloride with the formula XCl2. With which of the following elements, X would most likely be in the same group of the periodic table? Na, Mg, Al, C.

Ans:- Element X would most likely be in the same group of the periodic table with Mg. Since, X is bivalent and belongs to group 2 as does Mg.

11. Name two elements you would expect to show chemical properties similar to Li. What is the basis of your choice?

Ans:- Thetwo elements that would expect to show chemical properties similar to Li are sodium and potassium.

   This is because all these elements in the group 1 have 1 valence electrons.

12. What properties do all elements in the same group of boron in the periodic table have in common?

Ans:- Boron is inthe group 13 of periodic table along with Al, Ga, In and Ti.

Common properties:

a). All of them have 5 valence electrons.

 b). All of them are electropositive as they have tendency to loss electrons.

13. An atom has electronic configuration 2, 8, 6.

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

Ans:- The atomic number of this element is 2+8+6=16.

(b). Which of the following elements would it be similar in chemical properties? N(7), F(9), P(15), O(8), Ar(18).

Ans:- Electronic configuration of N(7)= 2, 5

Electronic configuration of F(9)= 2, 7

Electronic configuration of P(15)= 2, 8, 5

Electronic configuration of O(8)= 2, 6

Electronic configuration of Ar (18)= 2, 8, 8

 The given element would have similar properties with O(8) because they have same valence electrons each.

14. Nitrogen (at. no. 7) and P (at. no. 15) belong to group 15 of the periodic table. Which of them will be more electronegative and why ?

Ans:- Electronic configurationof N(7) = 2, 5

Electronic configuration of P(15) = 2, 8, 5

Since N has less number of shells than that of P, N has a greater tendency to take up electrons resulting more electronegative.

15. In the modern Periodic table, Ca (at. no. 20) is surrounded by elements with atomic number 12, 19, 21 and 38. Which of them have physical and chemical resembling calcium?

Ans:- Given

                    Mg(12)

         K(19) Ca(20) Sc(21)

                    Sr(38)

Elements with atomic number 12 and 38 will have similar physical and chemical properties with Ca as they are same group in the modern periodic table.

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