CHEMISTRY ADDA: X Periodic Classification of Elements solved questions

VERY SHORT ANSWER TYPE QUESTION (1 MARKS)

1. In the modern periodic table which are the metals among the first ten elements.

Ans. Only lithium, beryllium.

2. Write two reasons responsible for late discovery of noble gases? 1
Ans. Two reasons for late discoveries of noble gases are:-
(a) They are very inert.
(b) They are present in extremely low concentrations in our environment.

3. Which of the following statement is not correct statement about the trends when going from left to right across the periods of periodic table.

A. The element becomes less metallic in nature.

B. The number of valence electron increase.

C. The atoms lose their electron more easily

D. The oxide becomes more acidic

Ans. The atoms lose their electron more easily.

4. Element X forms a chloride with the formula XCl2 which is solid with a high melting point. X would most likely be in the same group of periodic table as
(a). Na (b). Mg (c). Al (d). Si?
Ans. (b). Mg

5. The elements Li, Na and K, each having one valence electron, are in period 2, 3 and 4
respectively of modern periodic table. 3
(i) In which group of the periodic table should they be?
(ii) Which one of them is least reactive?
(iii) Which one of them has the largest atomic radius? Give reason to justify your
answer in each case.
Ans. Electronic configuration of Li, Na and K can be written as:
Li → 2, 1
Na → 2, 8, 1
K → 2, 8, 8, 1
(a) As all of them have only on electron in their valence shell they belong to Group 1.
(b) Li is the least reactive among them due to its smallest size as it has only 2 shells.
(c) K has the largest atomic radius among the three because it has maximum number
of shells.
Higher the number of shells, larger is the atomic radius

6. Given below are some elements of the modern periodic table: 3

4Be, 9Fe, 14Si, 19K, 20Ca

(i) Select the element that has one electron in the outermost shell and write its electronic configuration.

(ii) Select two elements that belong to the same group. Give reason for your answer.

(iii) Select two elements that belong to the same period. Which one of the two has bigger atomic size?

Ans. (i) 19K has one electron in the outermost shell and its electronic configuration is 2, 8, 8, 1

(ii) 4Be and 20Ca belongs to same group i.e. Group-2. Electronic configuration:

4Be – 2, 2

20Ca- 2, 8, 8, 2

The number of electrons in the outermost shell of 4Be and 20Ca is same hence they belong to the same shell.

(iii) 9F and 4Be belongs to the same period, Period 2. Electronic configuration:

9F – 2, 7

4Be – 2, 2

4Be has bigger atomic size then 9F because the atomic radius decreases as we move from left to right due to increase in nuclear charge which tends to pull the electrons closer to the nucleus and hence size of F reduces.

7. State the modern periodic law for classification of elements. How many (i) groups and

(ii) periods are there in the modern periodic table?

Ans. Modern periodic law:

“Properties of elements are a periodic function of their atomic number”.

(i) There are 18 groups (group 1 – group 18) and

(ii) 7 periods in the modern periodic table.

8. An element 'M' has atomic number 11. (a) Write its electronic configuration.

(b) State the group to which 'M' belongs. (c) Is 'M' a metal or a non-metal.

(d) Write the formula of its chloride.

Ans. Atomic number of „M‟ = 11

(i) Electronic configuration : 2, 8, 1

(ii) Since, the valence shell contains 1 electron, it belongs to group 1.

(iii) M is a metal.

(iv) The formula of its chloride is M Cl.

9. F, Cl and Br are the elements each having seven valence electrons. Which of these (i) has the
largest atomic radius, (ii) is most reactive? Justify your answer stating reason for each. (3)
Ans. (i) F, Cl, Br belong to same group 17. Atomic size increase down the group. This is because
new shells are added. Hence, Br has largest atomic radius because it has a shells compared
to 3 and 2 in Cl and F respectively.
(ii) F is most reactive due to smaller size, high effective nuclear charge and high electro
negativity.

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

Ans. Electronic configuration of N (7) = 2, 5 Electronic configuration of P (15) = 2, 8,5 Nitrogen will be more electronegative due to electronegativity decreases from top to bottom because atomic size increases.

11. In the modern periodic table Ca (At no. = 20) is surrounded by elements with atomic number 12, 19, 21, and 38. Which of these have physical and chemical properties resembling Ca.

Ans. 38,12

VERY SHORT ANSWER TYPE QUESTION (2-marks each)

Q.1. Did Doberenier‘s triads also exist in the column of Newland octaves. Compare and find out.

Ans. Yes, Li, Na, K

Q.2. What are the limitation of Dobernier‘s classification?

Ans. Only few elements are classified into triads. So, this classification is rejected.

Q.3. What are the limitations of Newlands law of Octaves

Ans. 1. It was applicable only upto atomic mass 40 of Ca.

2. Two elements Ni and Co are placed in column as F, Cl, Br which have different properties.

Q.4. Use Mendeleev‘s periodic table to predict the formula for the oxide of following element K, C, Ba, Al

Ans. K –I A –K2O C -IV A –CO2 Al –III A-Al2O3 Ba –II A –BaO

Q.5.Besides Ga, which other elements have been discovered that were left by Mendeleev in his periodic table (Any two)?

Ans. Eka-Boron-Scandium Eka-Silicon-Germanium

Q.6. What were the criteria used by Mendeleev in creating his periodic table?

Ans. Two criteria used by Mendeleev

(i) Atomic mass (ii) Similar chemical properties.

Q.7. Why do you think the noble gases are placed in a separate group?

Ans. These gases were discovered very late and placed in a separate group because they were inert.

Q.8. Name two elements you would expect to show chemical reaction, similar to magnesium. What is the basis for your choice?

Ans. Ca, Be On the basis of the elements belong to same group-2. So valance electrons are same.

Q.9. (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) All elements in the same column as Boron have 3e- in valence shell and form oxide X2O3.

(b) All elements in the same column as Fluorine have 7e- in valence shell so their valency is one.

Q.10. An atom has electronic configuration 2,8,7

(a) What is the atomic no. of this element?

(b) To which of the following elements would it be chemically similar N(7) F(9) P(15) Ar(18)

Ans. (a) Atomic no=17 (b) F(9)

Q.11. How does the electronic configuration of an atom relate to its position in the Modern periodic table?

Ans. The outermost electron in electronic configuration give idea of group in Modern periodic table and also period.

Short answer type question (3-marks each)

Q.1-How could the modern periodic table remove various anomalies of Mendleev‘s periodic table?

Ans-Various anomalies are removed in following ways-

(1) The position for all isotopes of an element justified since they have same atomic number.

(2) The position of certain elements which are earlier misfit like Co-58.9 is placed before Ni-58.7 are now justified because Co has lower atomic number than Ni.

(3) Cause of periodicity explained due to same electronic configuration repeated after certain gap.

Q.2- Name

(1) three elements that have a single electron in their outermost shells.

(2) two elements that have two electrons in their outermost shells.

(3) three elements with filled outermost shell.

Ans- (1) Li ,Na ,K(Group-1) (2) Be ,Mg (Group-2) (3) Ne ,Ar, Kr (Group-18)

Q.3- (a)Li,Na,K are all metals that react with water to liberate H2 gas .Is there any similarity in the atoms of these elements.

(b)Helium is an unreactive gas and Neon gas is a gas of extremely low reactivity, what do their atoms have in common.

Ans-(a) there are following similarities.

(1) these atoms have same number of electrons in outermost shell 3Li -2,1 11Na-2,8,1 19K-2,8,8,1

(2) Li,Na,K react with oxygen to form oxide which are basic in nature.

(b) both He and Ne have completely filled shell