Chapter 6 Periodic Classification of Elements Textbook Solutions

NIOS Class 10 Science Chapter 6 Question Answers

INTEXT QUESTIONS 6.1

1. Elements A, B and C constitute a Dobereiner’s triad. The atomic mass of A is 20, and that of C is 40. Predict the atomic mass of B.

Ans:

2. Which property of atoms was used by Mendeleev to classify the elements?

Ans: Atomic mass

3. In Mendeleev’s periodic classification, whether chemically similar elements are placed in a group or in a period?

Ans: Group

4. Mendeleev’s periodic table had some blank spaces. What did they signify?

Ans: These were the positions of elements which were yet to be discovered.

5. Explain any three defects of Mendeleev’s periodic table.

Ans: Any three of the following:

(i) position of hydrogen
(ii) position of isotopes
(iii) anomalous pairs of elements
(iv) grouping of chemically dissimilar elements
(v) separation of chemically similar elements
(vi) no explanation for electronic configuration

INTEXT QUESTIONS 6.2

1. Give any two defects of Mendeleev’s periodic table which has been removed in modern periodic table. How were they removed?

Ans: Anomalous pairs, when elements are arranged in the order of their increasing atomic numbers, these anomalies are automatically removed, since the atomic number of the first element is less than that of the second, although their atomic masses show reverse trends.

2. Metalloids are present along the diagonal line starting from group 13 and going down to group 16. Do they justify their position in the modern periodic table?

Ans: Position of isotopes. Since all the isotopes of an element have the same atomic number, they all will occupy the same position in the periodic table.

INTEXT QUESTIONS 6.3

1. Fill in the blanks with appropriate words

(a) The force of attraction between nucleus and valence electrons …………….. in a period from left to right.

(b) Atomic radii of elements …………….. in a period from left to right.

(c) Atomic radii of elements …………….. in a group from top to bottom.

(d) Metallic character of elements …………….. from top to bottom in a group.

Ans:

(a) increases
(b) decreases
(c) increases
(d) increases

2. In the following crossword puzzle, elements are present horizontally, vertically downwards and diagonally downwards. Let us find out how many elements you are able to get within 5 minutes.

Ans:

Hydrogen, Carbon, Barium, Sodium, Boron, Chlorine (horizontally)
Magnesium, Iodine, Helium, Neon, Silicon, (vertically downwards)
Nitrogen, Oxygen(diagonally downwards)

3. Let us find how many riddles you can solve.

(i) I am the only noble gas whose outermost shell has 2 electrons. Who am I?

(ii) I am placed in group 16 of the modern periodic table, and essential for your respiration. Who am I?

(iii) I combine with chlorine to form your table salt. Who am I?

(Hint: Answers are present in the grid]

Ans:

(i) Helium
(ii) Oxygen
(iii) Sodium

TERMINAL EXERCISE

A. Objective questions

I. Mark the correct choice:

1. Which one of the following was the earliest attempt of classification of elements?

(a) Classification of elements into metals and non-metals

(b) Newlands’ Law of Octaves

(d) Mendeleev’s Periodic Table

Ans: (a) Classification of elements into metals and non-metals

2. The ‘law of octaves’ was given by

(a) Mendeleev

(b) Newlands

(d) Dobereiner

Ans: (b) Newlands

3. According to the periodic law given by Mendeleev, the properties of an element are a periodic function of its

(i) atomic volume

(ii) atomic size

(iii) atomic number

(iv) atomic mass

Ans: (iv) atomic mass

4. The particle which is universally present in the nuclei of all elements is

(a) neutron

(b) proton

(d) α-particle

Ans: (a) neutron

5. Potassium is more metallic than sodium because

(a) Both have 1 electron in their outermost shell.

(b) Both are highly electropositive.

(d) Potassium is larger in size than sodium.

Ans: (d) Potassium is larger in size than sodium.

7. Which one of the following elements in its chloride does not show the valence equal to its valence electrons?

(a) NaCl

(b) MgCl₂

(d) PCl₃

Ans: (b) MgCl₂

8. Which one of the following elements has the least tendency to form cation?

(a) Na

(b) Ca

(d) Al

Ans: (c) B

9. Which one of the following does not belong to the family of the alkali metals?

(a) Li

(b) Na

(d) K

Ans: (c) Be

10. The number of elements in the 5th period of the periodic table is

(a) 2

(b) 8

(d) 18

Ans: (d) 18

11. The elements with atomic number 9 resembles with the element having atomic number

(a) 35

(b) 27

(d) 8

Ans: (c) 17

12. In which period of the periodic table, an element with atomic number 20 is placed?

(a) 4

(b) 3

(d) 1

Ans: (a) 4

II. Mark the following statements True (T) or False (F) :

1. The properties of the middle element in a Dobereiner’s triads are intermediate between those of the other two.

2. The vertical columns in the periodic table are called periods.

3. Mendeleev depended only on the atomic mass of elements for his classification.

4. All elements present in a group are chemically similar.

5. The modern periodic law is based upon atomic mass.

6. The importance of atomic number as the fundamental property was realised by Henry Mosely.

7. There are 18 groups in the modern periodic table.

8. Non-metals are present in the middle portion of the periodic table.

9. Each period in modern periodic classification begins with filling of electrons in a new shell.

Ans:

1. True.
2. False
3. True
4. True
5. False
6. True
7. True
8. False
9. True

III. Fill in the blanks:

1. According to the modern periodic law, the properties of elements are periodic function of their ………..

Ans: Atomic number

2. The ………………… number is same as the number of shell which in gradually filled up in the elements of this period.

Ans: Period

3. In normal elements of a particular period the electrons are gradually filled in ……………… shell.

Ans: Same

4. All elements of a particular group have ………………… electronic configurations.

Ans: Similar

5. In the modern periodic table, groups are numbered from ………………… to ………………

Ans: 1 to 18

6. The second and third periods of the periodic table are called ………………… periods.

Ans: Short

7. The main group elements are present in group 1 and 2 on the left side and ………………. to ………………… on the right side of the periodic table.

Ans: 13 to 18

8. All the group eighteen elements (except the first one) contain ………………… valence electrons.

Ans: Eight

9. All transition elements are metals with ………………… melting and boiling points.

Ans: High

10. The group of 14 rare-earth elements belonging to the group 3 and 7th period are called …………..

Ans: Actonoids

11. All elements present in a given ………………… have the same valency.

Ans: Group

12. Atomic size ………………… in a period from left to right.

Ans: Decreases

13. Magnesium is ………………… metallic than calcium.

Ans: Less

14. Carbon belongs to group ………………… of the Periodic table.

Ans: 14

15. All the elements of group 15 have ………………… valence electrons.

Ans: Five

B. Subjective Questions

I. Very short Answer Questions (Answer in one word or one sentence).

1. What was the earliest classification of elements?

Ans: The earliest classification of elements was into two groups, metals and non-metals. After William Prout found that the atomic mass of an element remains constant, there were two more classifications by other scientists.

In 1829, J.W. Dobereiner, a German chemist, made groups of three elements each and called them triads.
In 1864, an English chemist, John Alexander Newlands, arranged the elements in the increasing order of their atomic masses (then called atomic weight).
D’mitri Mendeleev, a Russian chemist, arranged the elements in increasing order of atomic mass.

2. State Newlands’ law of octaves.

Ans: Chemist John Alexander Newlands arranged the elements in the increasing order of their atomic masses (then called atomic weight). He observed that every eighth element had properties similar to the first element. Newlands called it the Law of Octaves.

3. Which classification of elements failed after the discovery of noble gases?

Ans: Newlands’ law of Octaves failed after the discovery of noble gases. It was found that it was the ninth element which had properties similar to the first one and not the eighth element.

4. State Mendeleev’s Periodic Law.

Ans: According to Mendeleev’s Periodic Law, the chemical and physical properties of elements are a periodic function of their atomic masses. A periodic function is one, that repeats itself after a certain interval.

5. How were the groups numbered in Mendeleev’s periodic table?

Ans: The vertical columns present in it are called groups. There are eight groups numbered from I to VIII (Roman numerals).

6. Name the fundamental properties of element on which the modern periodic law is based.

Ans: Atomic number

7. How many groups are there in the modern periodic table?

Ans: There are 18 groups in the modern periodic table.

8. How have groups been numbered in the modern periodic table?

Ans: The groups have been numbered from 1 to 18 (in Arabic numerals).

9. What are normal elements?

Ans: The elements belonging to the groups 1, 3, and then 13-17 are known as normal elements.

10. What are the elements present in the middle portion of the modern periodic table called?

Ans: Transition elements

11. What is atomic size?

Ans: Atomic size is the distance between the centre of the nucleus and the outermost shell of an isolated atom. It is also known as the atomic radius. It is measured in picometres, pm (1 pm = 10^–12 m).

12. How does atomic size vary in a period and in a group?

Ans: The size of atoms decreases from left to right in a period but increases from top to bottom in a group.

13. Where would the element with largest atomic size be placed in any group?

Ans: At the bottom as the atomic size in a group from top to bottom.

14. Give the number of a group in which metallic, metalloid and non-metallic, all three types of elements, are present.

Ans: Group 14 contains metallic, metalloid, and non-metallic elements. Carbon is a non-metal, Silicon and Germanium are metalloids, Tin and Lead are metals.

II. Short Answer Questions (Answer in 30-40 words).

1. State Dobereiner’s law of triads.

Ans: According to this law, when elements are arranged in order of increasing atomic mass, the atomic mass of the middle element is nearly equal to the arithmetic mean of the other two, and its properties are intermediate between those of the other two.

2. Show that chlorine, bromine and iodine (atomic masses 35·5, 80, and 127, respectively) constitute a triad.

Ans: According to Dobereiner’s law of triads, the middle element of the group possesses the same atomic mass as the average of the other two.

Atomic mass of Chlorine = 35.5

Atomic mass of Bromine = 80

Atomic mass of Iodine = 127

The average atomic mass of 81.25 is approximately equal to the atomic mass of Bromine, which is 80.

3. What were the reasons for the failure of Newlands’ law of octaves?

Ans: The demerits of Newlands’ law of Octaves are:

(i) It did not apply to elements of atomic masses higher than 40 u. Hence, all the 60 elements known at that time could not be classified according to this criterion.
(ii) With the discovery of noble gases, it was found that it was the ninth element that had properties similar to the first one, and not the eighth element. This resulted in the rejection of the very idea of octaves.

4. Describe Mendeleev’s periodic table briefly in terms of rows and columns and their raw being.

Ans: The following are the main features of this periodic table :

1. The elements are arranged in rows and columns in the periodic table.
2. The horizontal rows are called periods. There are six periods in the periodic table. These are numbered from 1 to 6 (Arabic numerals). Each one of the 4th, 5th, and 6th periods has two series of elements.
3. Properties of elements in a given period show regular gradation (i.e. increase or decrease) from left to right.
4. The vertical columns present in it are called groups. There are eight groups numbered from I to VIII (Roman numerals).
5. Groups I to VII are further divided into A and B subgroups. However, group VIII contains three elements in each of the three periods.
6. All the elements present in a particular group are chemically similar in nature. They also show a regular gradation in their physical and chemical properties from top to bottom.

5. Give any two achievements of the Mendeleev’s Periodic classification.

Ans:

1. Classification of all elements: Mendeleev’s classification included all the 63 elements known at that time based on their atomic mass and facilitated the systematic study of elements.

2. Correction of atomic masses: Atomic masses of some elements, like Be (beryllium), Au (gold), and In (indium), were corrected based on their positions in the table.

6. What were the defects in Mendeleev’s periodic classification?

Ans: Defects of Mendeleev’s Periodic Table

1. Position of Hydrogen: The position of hydrogen, which is placed in group IA along with alkali metals, is ambiguous as it resembles alkali metals as well as halogens (group VII A).
2. Position of Isotopes: All the isotopes of an element have different atomic masses; therefore, each one of them should have been assigned a separate position. On the other hand, they are all chemically similar; hence, they should all be placed at the same position.
3. Anomalous* Pairs of Elements: At some places, an element with a greater atomic mass had been placed before an element with a lower atomic mass due to their properties.
4. Grouping of chemically dissimilar elements: Elements such as copper and silver have no resemblance to alkali metals (lithium, sodium, etc.), but have been grouped in the first group.
5. Separation of chemically similar elements: Chemically similar Elements, such as gold and platinum, have been placed in separate groups.

7. State modern periodic law.

Ans: The Modern Periodic Law states that the chemical and physical properties of elements are periodic functions of their atomic numbers i.e., if elements are arranged in the order of their increasing atomic number, the elements with similar properties are repeated after certain regular intervals.

8. Briefly describe the modern periodic table in term of groups and period.

Ans:

Groups: There are 18 vertical columns in the periodic table. Each vertical column is called a group. The groups have been numbered from 1 to 18 (in Arabic numerals).
Periods: There are seven horizontal rows in the periodic table. Each row is called a period. The elements in a period have consecutive atomic numbers. The periods have been numbered from 1 to 7 (in Arabic numerals).

9. Give names of four classes into which the elements have been classified and mention to which groups of the modern period table they belong.

Ans:

Alkali group- Group 1
Alkaline earth group- group 2
Transition metal group – 3 to 12
Noble gases – Group 18

10. List the merits of the long form of the modern periodic table and explain any two of them.

Ans:

1. Position of isotopes: All isotopes of an element have the same atomic number and therefore, occupy the same position in the modern periodic table.
2. Anomalous pairs: The anomaly regarding all these pairs disappears when the atomic number is taken as the basis for classification. For example, cobalt (at. no. 27) would naturally come before nickel (at. no. 28) even though its atomic mass is a little more than that of nickel.
3. Electronic configuration: This classification is according to the electronic configuration of elements, i.e., the elements having a certain pattern of electronic configuration are placed in the same group of the periodic table. It relates the properties of elements to their electronic configurations. This point will be further elaborated in the next section.
4. Separation of metals and non-metals: The position of metals, non-metals and metalloids is clearly established in the modern periodic table.
5. Position of transition metals: It makes the position of the transition metals quite clear.
6. Properties of elements: It reflects the differences, the trends and the variations in the properties of the elements in the periodic table.
7. This table is a simple, systematic and easy way of remembering the properties of different metals.

11. How are the electronic configurations of all the elements belonging to a particular group related? Explain with the help of group 17 elements.

Ans: In group 17 elements (halogens), as one moves down a group, more and more shells are added. All the elements of group 17 have seven valence electrons; however, the number of shells is increasing from two in fluorine to five in iodine.

F: 2, 7

Cl: 2, 8, 7

Br: 2, 8, 8, 7

I: 2, 8, 18, 18, 7

12. How does the electronic configuration of elements belonging to a particular period vary? Explain with the example of second period elements.

Ans: The elements in a period have consecutive atomic numbers. The periods have been numbered from 1 to 7 (in Arabic numerals). The period number represents the number of shells for the elements. For example, the electronic configuration of Oxygen is 2,6.

13. Define atomic radius.

Ans: Atomic size is the distance between the centre of the nucleus and the outermost shell of an isolated atom. It is also known as the atomic radius.

14. How and why does metallic character vary in a group from top to bottom?

Ans: The tendency of an element to lose electrons to form cations is called the electropositive or metallic character of an element. Metallic character increases from top to bottom in a group as the tendency to lose electrons increases. This increases the electropositive character and metallic nature.

III. Long Answer Questions (Answer in 60–70 words).

1. State Mendeleev’s Periodic Law and describe the periodic table constructed on this basis.

Ans: A periodic function repeats itself after a certain interval. Mendeleev arranged the elements in the form of a table, which is known as Mendeleev’s Periodic Table.

The following are the main features of this periodic table :

1. The elements are arranged in rows and columns in the periodic table.
2. The horizontal rows are called periods. There are six periods in the periodic table. These are numbered from 1 to 6 (Arabic numerals). Each one of the 4th, 5th, and 6th periods has two series of elements.
3. Properties of elements in a given period show regular gradation (i.e. increase or decrease) from left to right.
4. The vertical columns present in it are called groups. There are eight groups numbered from I to VIII (Roman numerals).
5. Groups I to VII are further divided into A and B subgroups. However, group VIII contains three elements in each of the three periods.
6. All the elements present in a particular group are chemically similar in nature. They also show a regular gradation in their physical and chemical properties from top to bottom.

2. What are the merits and demerits of the Mendeleev’s Periodic classification?

Ans: Merits of Mendeleev’s Periodic Classification

1. Classification of all elements: Mendeleev’s classification included all the 63 elements known at that time based on their atomic mass and facilitated the systematic study of elements.
2. Correction of atomic masses: Atomic masses of some elements, like Be (beryllium), Au (gold), and In (indium), were corrected based on their positions in the table.
3. Prediction of new elements: Mendeleev arranged the elements in the periodic table in increasing order of atomic mass, but whenever he could not find an element with expected properties, he left a blank space.
4. Valency of elements: Mendeleev’s classification helped in understanding the valency of elements. The
valency of elements is given by the group number. For example, all the elements in group 1, i.e. lithium, hydrogen, sodium, potassium, rubidium, caesium, have valency 1.

Defects of Mendeleev’s Periodic Table

1. Position of Hydrogen: The position of hydrogen, which is placed in group IA along with alkali metals, is ambiguous as it resembles alkali metals as well as halogens (group VII A).
2. Position of Isotopes: All the isotopes of an element have different atomic masses; therefore, each one of them should have been assigned a separate position. On the other hand, they are all chemically similar; hence, they should all be placed at the same position.
3. Anomalous* Pairs of Elements: At some places, an element with a greater atomic mass had been placed before an element with a lower atomic mass due to their properties.
4. Grouping of chemically dissimilar elements: Elements such as copper and silver have no resemblance to alkali metals (lithium, sodium, etc.), but have been grouped in the first group.
5. Separation of chemically similar elements: Elements that are chemically similar, such as gold and platinum, have been placed in separate groups.

3. Describe the modern periodic table in terms of groups and periods.

Ans:

Groups: There are 18 vertical columns in the periodic table. Each vertical column is called a group. The groups have been numbered from 1 to 18 (in Arabic numerals). All elements present in a group have similar electronic configurations and have the same number of valence electrons.
Periods: There are seven horizontal rows in the periodic table. Each row is called a period. The elements in a period have consecutive atomic numbers. The periods have been numbered from 1 to 7 (in Arabic numerals). In each period, a new shell starts filling up. The period number is also the number of the shell which starts filling up as we move from left to right across that particular period.

4. What are the following types of elements and where are they located in the periodic table?

(a) Main group elements

(b) Noble gases

(c) Transition elements

(d) Inner transition elements.

Ans:

(a) Main group elements: The elements present in groups 1 and 2 on the left side and groups 13 to 17 on the right side of the periodic table are called representative or main group elements. Their outermost shells are incomplete, which means their outermost shell has fewer than eight electrons.
(b) Noble gases: Group 18 on the extreme right side of the periodic table contains noble gases. Their outermost shells contain 8 electrons, except He, which contains only 2 electrons.
(c) Transition elements: The middle block of the periodic table (groups 3 to 12) contains transition elements. Their two outermost shells are incomplete. Since these elements represent a transition (change) from the most electropositive element to the most electronegative element, they are named as transition elements.
(d) Inner transition elements: These elements, also called rare-earth elements, are shown separately below the main periodic table. These are two series of 14 elements each.

5. Discuss the merits of the modern periodic table.

Ans:

1. Position of isotopes: All isotopes of an element have the same atomic number and therefore, occupy the same position in the modern periodic table.
2. Anomalous pairs: The anomaly regarding all these pairs disappears when the atomic number is taken as the basis for classification. For example, cobalt (at. no. 27) would naturally come before nickel (at. no. 28) even though its atomic mass is a little more than that of nickel.
3. Electronic configuration: This classification is according to the electronic configuration of elements, i.e., the elements having a certain pattern of electronic configuration are placed in the same group of the periodic table. It relates the properties of elements to their electronic configurations. This point will be further elaborated in the next section.
4. Separation of metals and non-metals: The position of metals, non-metals and metalloids is clearly established in the modern periodic table.
5. Position of transition metals: It makes the position of the transition metals quite clear.
6. Properties of elements: It reflects the differences, the trends and the variations in the properties of the elements in the periodic table.
7. This table is a simple, systematic and easy way of remembering the properties of different metals.

6. What is the relationship between the electronic configuration and the modern periodic table?

Ans: The elements having a certain pattern of electronic configuration are placed in the same group of the periodic table. It relates the properties of elements to their electronic configurations. The recurrence of similar electronic configurations is the cause of periodicity in the properties of elements.

8. Explain the variation of atomic size in a group and in a period.

Ans: The size of atoms decreases from left to right in a period but increases from top to bottom in a group.

In a group, as one goes down, a new shell is added to the atom, which is farther away from the nucleus. Hence, electrons move away from the nucleus. This increases the atomic size in a group from top to bottom.
In a period, the atomic number and therefore the positive charge on the nucleus increase gradually. As a result, the electrons are attracted more strongly and they come closer to the nucleus. This decreases the atomic size in a period from left to right.

9. How is metallic character related to ionization energy? Explain the variation of metallic character in the periodic table.Ans: Metallic character decreases in a period from left to right. It is because the ionisation energy increases in a period. This decreases the electropositive character and metallic nature. Eg., in the 3rd period, the first three elements, Sodium, Magnesium and Aluminium are metals. The fourth one, Silicon, is a metalloid. The last three elements, Phosphorus, Sulphur and Chlorine and non-metals.