NCERT Solutions For Class 10 Science Chapter 5 Periodic Classification Of Elements

NCERT Solutions For Class 10 Science Chapter 5 Periodic Classification Of Elements

Question 1. Why did Mendeleev have gaps in his periodic table?

1. State any three limitations of Mendeleev’s classification.
2. How does the electronic configuration of atoms change in a period with an increasein atomic number?

Answer:

1. Gaps were left for undiscovered elements in the periodic table.

2.

1. The position of hydrogen was not justified.
2. Increasing order of atomic mass could not be maintained.
3. Isotopes cannot be given in separate places.

Several shells remain the same number of valence electrons, increasing from left to right in a period till the octet is complete.

Question 2. An element X which is a yellow solid at room temperature shows catenation and allotropy. ‘X’ forms two oxides which are also formed during the thermal decomposition of ferrous sulphate crystal and are the major air pollutants.

1. Identify ‘X.
2. Write its electronic configuration.
3. Write the balanced chemical equation for the thermal decomposition of ferrous sulphate crystals.
4. What would be the nature (acidic/basic) of oxides formed?
5. Locate the position of the element in the periodic table.

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Answer:

1. X’ is sulphur.

2. Electronic configuration of ‘X’ is 2, 8, 6.

SO₂ and SO₃ are acidic oxides.

It belongs to Group 16 and 3rd period.

Question 3. An element placed in the 2nd group and 3rd period of the periodic table burns in presence of oxygen to form a basic oxide.

1. Identify the element.
2. Write its electronic configuration.
3. Write the balanced equation when it burns in the presence ofair.
4. Write a balanced equation when this oxide is dissolved in water.
5. Draw the electron dot structure for the formation ofthis oxide.

Answer:

1. Magnesium

2. Electronic configuration- 2, 8, 2

3. 2 Mg+O₂ → 2MgO

4. MgO+H₂O→ Mg (OH)₂

5.

⇒ \( \underset{2,8,2}{\mathrm{Mg}} \longrightarrow \underset{2,8}{\mathrm{Mg}^{2+}}+2 e^{-} \)

⇒ \( \underset{2,6}{\mathrm{O}}+2 \mathrm{c}^{-} \longrightarrow \underset{2,8}{\mathrm{O}^{2-}}\)

Question 4. An element X (Atomic number 17) reacts with an element Y (atomic number 20) to form a divalent halide.

1. Where in the periodic table are elements X and Y placed?
2. Classify X and Y as metal (s) and non-metals or metalloids.
3. What will be the nature of the oxide of element Y? Identify the nature of bonding in the compound formed.
4. Draw the electron dot structure ofthe divalent halide.

Answer:

1. ‘X’ is in group 17 and 3rd period. ‘Y’ is in group 2 and 4th period.
2. X is non-metal, V is metal.
3. X forms basic oxide. It has ionic bonding in the compound formed.

Question 5. Two elements ‘P’ and ‘Q’ belong to the same period ofthe modernperiodic table and are in Group-1 and Group-2 respectively. Compare their following characteristics in tabular form:

1. The number ofelectrons in their atoms.
2. The sizes oftheir atoms
3. Their metallic characters
4. Their tendency to lose electrons
5. The formula oftheir oxides
6. The formula oftheir chlorides

Answer:

Question 6.

1. Why do we classify elements?
2. What were the two criteria used by Mendeleev in creating his Periodic Table?
3. Why did Mendeleev leave some gaps in his Periodic Table?
4. In Mendeleev’s Periodic Table, why was there no mention ofNoble gases like Helium, Neon and Argon?
5. Wouldyou place the two isotopes of chlorine, Cl-35 and Cl-37in different slots because of their different atomic masses or in the same slot because their chemical properties are the same? Justify your answer.

Answer:

1. Classification of elements helps in:

1. Systematic study of the properties of elements.
2. I am understanding and remembering the properties of elements.

2. The two criteria used by Mendeleev were:

1. Increasing order of atomic mass.
2. Similarity in the properties ofthe elements.

3. Mendeleev left a gap in his periodic table for the undiscovered elements like germanium, and gallium.

4. There was no mention of noble gases like helium, neon and argon as they were notknown at that time, and were discovered much later.

5. According to Mendeleev’s periodic table Cl-35 and Cl-37 should be placed at different positions as the basis of classification is increasing order of atomic masses but since their chemical properties are the same they should be placed in the same position in the periodic table.

Question 7. The atomic radii ofthe element of second period are given below:

1. Arrange these elements in decreasing order oftheir atomic radii.
2. Are the elements now arranged in the pattern ofa period in the periodic table?
3. Name the element which has the (a) largest and (b) smallest atomic number.
4. From the above data, infer how the atomic size or atomic radius ofthe elements changes as we go from left to right in a period.
5. Name one metal, one non-metal and a metalloid out of these elements.
6. Why does atomic radius decrease as we move from left to right in a period?

Answer:

1.

2. Yes, the elements are now arranged in the pattern ofa period of the periodic table.

3.

1. Li has the smallest atomic number.
2. F has the largest atomic number.

4. Atomic radius decreases on moving from left to right in a period ofthe periodic table

5. Metal: Li, non-metal: C, Metalloid: B

6. Force of attraction for the outermost electrons increases because of increased nuclear charge.

Question 8. 

Consider the table given above and answer the following questions:

1. Name the most reactive metal.
2. How many shells does ‘d’ have?
3. Name the element(s) having valency 2.
4. How many valence electrons does j’ have?
5. Which is more non-metallic, ‘Id or ‘V?
6. The atom of which element is bigger, ‘e’ or ‘h’?

Answer:

1. d
2. 4
3. e/g
4. 2
5. h
6. e

Question 9. Given below are some elements ofthe modern periodic table: ₄Be, ₉F, ₁₄Si, ₁₉K, ₂₀Ca

1. Select the element that has one electron in the outermost shell and write its electronic configuration.
2. Select two elements that belong to the same group. Give a reason for your answer.
3. Select two elements that belong to the same period. Which one of the two has a bigger atomic size?

Answer:

₁₉K, its electronic configuration is: 2, 8, 8, 1

₄Be, ₂₀Ca (Both have two valence electrons and in a group valence electrons remain the same).

₄Be, ₉F belongs to the same period. The atomic size ofBe is bigger because in a period, atomic size decreases due to increased nuclear charge.

Question 10. Study the following table in which the positions of six elements A, B, C, D, E and F are shown as they are in the modern periodic table:

Based on the above table, answer the following questions:

1. Name the element which forms only covalent compounds.
2. Name the element which is a metal with valency three.
3. Name the element which is a non-metal with valency three.
4. Out ofD and E, which is bigger and why?
5. Write the common name for the family to which the elements C and F belong.

Answer:

1. E
2. D
3. B
4. D is bigger; atomic radius/size decreases from left to right along a period.
5. Noble gases

Question 11. The atomic number ofan element is 17. Predict (a) its valency, (b) whether it is a metalor non-metal, and (c) its relative size concerning other members of its group. Justify your answer in each case. (CBSE 2011, 2013)
Answer:

1. 1(-1).It has electronic configuration = 2, 8, 7. Therefore, it requires one more electron to become stable.

2. Non-metal. Because it will show the tendency to gain electrons.

3. It belongs to the third period and it has a smaller size as compared to other members of this period because the size of the atom decreases in a period due to an increase in the nuclear charge.

Note: It belongs to group 17, which is of non-metals and in this group, it is bigger than fluorine but smaller than another member.

Question 12. Atomic radii of the elements of the second period are given below:

1. Arrange them in decreasing order oftheir atomic radii.
2. Are the elements now arranged in the pattern of a period in the Periodic Table?
3. Which elements have the largest and the smallest atoms?
4. How does the atomic radius change as you go from left to right in a period?

Answer:

The elements of the second period in the decreasing order of their atomic radii are:

1.

2. Yes, the above elements are arranged in the pattern of a period in the periodic table.

3. Li has the largest (152 pm) and O has the smallest (66 pm) atoms.

4. The atomic radii decrease as we move from left to right in a period due to a corresponding increase in their nuclear charge.

Question 13. Give reasons for the following:

1. Lithium atoms are smaller than sodiuatomsom.
2. Chlorine (Atomic Number 17) is more electronegative than Sulphur (Atomic Number

Answer:

1 The electronicic configuration of Li is 2,1 and that of Na is 2, 8, 1. Since sodium has more shellit hashus a larger size.

2. E.C. of Cl = 2, 8,7

E.C. of S = 2, 8,6.

As sulphur is placed before chlorine in the periodic table, soit has a bigger size andits tendency to gain electrons is lesser than Cl.

Question 14. Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, Al, Si, and Ba.
Answer:

1. The element K (potassium) is in groups of Mendeldev’s Periodic Table in which the general formula of the oxides of elements is R₂ O.

• So, the formula of an oxide of K will be K₂O.

2. The element C (carbon) is in group IV of Mendeleev’s periodic table in which the general formula of the oxides of elements is RO₂.

• So, the formula of an oxide of C will be CO₂

3. The element Al (aluminium) is in group 3 of Mendeleev’s periodic table in which the general formula of the oxides of elements is R₂O₃.

• So, the formula of the oxide of Al will be Al₂O₃.

4 The element Si (Silicon) is in group IV of Mendeleev’s periodic table in which the general formula for the oxides of elements is RO₂.

• So, the formula of the oxide of Si will be SiO_2.

5. The element Ba (Barium) is in group II of Mendeleev’s periodic table in which the general formula for the oxides of elements is RO.

• So, the formula of the oxide of Ba will be BaO.

Question 15. An atom has electronic configurations 2, 8, and 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.) N(7) F(9) P(15) Ar(18)

Answer:

The atomic number (equal to number of electrons in a neutral atom) of the element would be 2 + 8 + 7 = 17.

Examine the electronic configuration of the given elements:

N(7) – 2,5

F(9) – 2,7

P(15) – 2,8,5

Ar(18) – 2,8,8

The electronic configuration of F(9) is similar to that of the given element (2, 8,

Therefore, F is chemically similar to the given element.

Question 16. The position of three elements A, B, and C in the Periodic Table are shown below

1. State whether A is a metal or non-metal.
2. State whether C is more reactive or less reactive than A.
3. Will C be larger or smaller in size than B?
4. Which type of ion, cation or anion, will be formed by element A?

Answer:

1. A is a non-metal.
2. C is less reactive than A.
3. C will be smaller in size than B.
4. A will form an anion.

Question 17. 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:

The electronic configuration of elements:

1. Atomic number 12: 2, 8, 2

2. Atomic number 19: 2, 8, 8, 1

3. Atomic number 20: 2, 8, 8, 2

4. Atomic number 21: 2, 8, 8, 3

5. Atomic number 38: 2, 8, 18, 8, 2

6. Element with atomic number 12 will have similar physical and chemical properties as element 38 as both have two electrons in the outermost shell.

Question 18. Compare and contrast the arrangement of elements in Mendeleev’s Periodic Table and the Modern Periodic Table.
Answer:

Question 19. Three elements ‘X’, and ‘Z’having atomic numbers 11,7 and 6 respectively react with oxygen to form their oxides.

1. Arrange these oxides in increasing order oftheir basic nature.
2. Give a reason for your answer.

Answer: ₁₁X = 2, 8, 1

₇Y = 2, 5

₆Z = 2, 4

160 Scionco-X

1. Y < Z < X

2. X is metallic hence, its oxide is basic. While Y and Z are non-metals and their oxides are acidic.

(In a period, acidic character and basic character decrease).

Question 20. An element belongs to the third period and second group ofthe periodic table:

1.  State the number of valence electrons in it.
2. Is it a metal or a non-metal?
3. Name the element.
4. Write the formula of its oxide.

Answer:

1. 2 valence electrons
2.  Metal
3. The element is Magnesium.
4. Magnesium oxide, MgO.

Question 21. The atomic number of K and Ca is 19 and 20 respectively and they belong to the same period.

1. Which of them would have a smaller atomic size?
2. Which one would be more electropositive?
3. To which group would each of them belong?

Answer:

1. Ca, as size decreases in a period due to an increase in the effective nuclear charge.
2. K, as electropositive character decreases in a period due to an increase in the effective nuclear charge.
3. K belongs to group 1 (Alkali Metals). Ca belongs to group 2 (Alkaline Earth Metals).

Question 22. Justify the following statements:

1. Cations are smaller in size than the corresponding atom.
2. The size ofthe atom increases as we move down a group.
3. Atomic size decreases as we move across a period.

Answer:

1. Because cations are formed bythe loss of valence electrons. This decreases the nuclear
2. charge and thus size of cations decreases.
3. Because new shells are being added as we go down the group.
4. It is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.

Question 23. The formula of magnesium oxide is MgO. State the formula of barium nitrate and barium sulphate, if barium belongs to the same group as magnesium.
Answer:

Answer:

Question 24. Apart TPeriodicdic Table has been shown below:

Answer the following questions based on the position of elements in the above table.

1. Which element is a noble gas? Give reason.
2. Which element is most electronegative? Give

Write the electronic configuration of(a) B and (b) E.

Answer:

1. G – It is a noble gas and has zero valency.

2. E- Smallest atomic size and more electron affinity.

3.

1. 2, 8, 1
2. 2,7

Question 25.

1. State Modern Periodic Law.

2. How many

1. Groups and
2. Periods are present in the modern periodic table?

Question 26. State how the problem of placing:

1. Hydrogen
2. Isotopes ofan element have been solved in this periodic table.

Answer:

1. The properties of elements are the periodic function of their atomic numbers.

2.

1. 18 groups
2. 7 periods

3.

1. Hydrogen in 1st group.
2. Isotopes ofan element at one place.

Question 27. Given below are four elements with their atomic numbers

1. Identify the elements which belong to the same group ofthe Modern Periodic Table.
2. Arrange the given elements in decreasing order of atomic size.
3. Write the formula ofthe oxides of B.
4. Which ofthe above elements is a metalloid?

Answer:

1. B and C
2. B > D > A > C
3. B₂O
4. D

Question 28. The following table shows elements represented by the letters A, B, C, D, E, F, G and H.

1. Which ofthe elements has the atomic size:

1. Biggest
2. Smallest

2. Which element has valency:

1. 3
2. Zero

Answer:

1.

1. A
2. G

2.

1. C
2. H

Question 29. Three elements A, B and C have atominumberser 7, 8 and 9 respectively.  What would be their positions in the modern periodic table? (Mention group and period both).

1. Arrange A, B and C in the decreasing order oftheir size.
2. Which one ofthe three elements is most reactive and why?

Answer:

1.

Position of A = 15th group, 2nd period

Position of R = 16th group, 2nd period

Position of C = 17th group, 2nd period

2. A > B > C

3. C is the most reactive as it has the smallest size, so it can easily gain electrons.

Question 30.

1. The elements ofthe second period along with their atomic number in parentheses are given below: B (5), Be (4), O (8), N (7), i (3), C (6), F (9)

1. Arrange them in the same order as they are in the periodic table.
2. Which element has the largest and smallest atom?

2. Why does the atomic radius change as we move from left to right in a period?

Answer:

1.

1. Li, Be, B, C, N, O, F
2. The largest atom is Lithium and the smallest is Fluorine.

2. The atomic radius decreases in moving from left to right along a period.

This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.

Question 31. An element X(atomic number 7) reacts with an element Y(atomic number 20) to form a divalent halide.

1. Where in the periodic table are elements X and Y placed?
2. Classify X and Y as metal(s), non-metal(s) or metalloid(s).
3. What will be the nature of the oxide of element Y? Identify the nature of bonding in the compound formed.
4. Draw the electron dot structure ofthe divalent halide.

Answer:

1. X belongs to group 17 and 3rd period

Y belongs to group 2 and 4th period

2. X-non metal

Y-metal

3. Basic oxide; Ionic bonding

Question 32. Atomic number ofa few elements is given below 10,20,7,14

1. Identify the elements.
2. Identify the Group number of these elements in the PeriodicIable.
3. Identify the Periods of these elements in the PeriodicIable.
4. What would be the electronic configuration for each of these elements
5. Determine the valency of these elements.

Answer:

1. Elements—Neon (Ne), Calcium (Cu), Nitrogen (N), Silicon (Si)

2. Group— 18, 2, 15, 14

3. Period— 2,4,2,3

4. Electron configuration

₁₀Ne = 2, 8

₂₀Ca = 2, 8, 8, 2

₇N = 2, 5

₁₄Si = 2, 8, 4

Valency— Ne(O), Ca(2), N(3), Si(4)

Question 33. Complete the following crossword puzzle

Across:

1. An element with atomic number 12.

3. Metal used in making cans and member ofGroup 14.

4. A lustrous non-metal which has 7 electrons in its outermost shell.

Down:

2. Highly reactive and soft metal which imparts a yellow colour when subjected to flame and is kept in kerosene.

5. The first element of the second Period.

6. An element which is used in making fluorescent bulbs and is the second member of Group18 in the Modern Periodic Table.

7. A is a radioactive element which is the last member of the halogen family.

8. Metal which is an important constituent of steel forms rust when exposed to moist air.

9. The first metalloid in the Modern Periodic Table whose fibres are used in making bulletproof vests.

Question 34. 

1. In this ladder symbols of elements jumbled up. Rearrange these symbols of elements in the increasing order oftheir atomic number in the Periodic Table.
2. Arrange them in the order of their group also.

Answer:

1. H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca

2.

Group 1 — H, Li, Na, K

Group 2 — Be, Mg, Ca

Group 13 — B, Al

Group 14—C, Si

Group 15—N, P

Group 16—O, S

Group 17—F, Cl

Group 18 —He, Ne, Ar

Question 35. Mendeleev predicted the existence of certain elements not known at the time and named two of them as Eka-silicon and Eka-aluminium.

1. Name the elements which have taken the place of these elements.
2. Mention the group and the period of these elements in the Modern Periodic Table.
3. Classify these elements as metals, non-metals or metalloids.
4. How many valence electrons are present in each one of them?

Answer:

Germanium (Ge) and Gallium (ga)

Group 14; Period 4 and Group 13; Period 4

Ge-Metalloid; Ga-Metal

Ga-3, Ge-4

Question 36.

1. The electropositive nature ofthe element(s) increases down the group and decreases across the period.
2. The electronegativity of the element decreases down the group and increases across the period.
3. Atomic size increases down the group and decreases across a period (left to right).
4. Metallic character increases down the group and decreases across a period. On the basis ofthe above trends ofthe Periodic Table, answer the following about the elements with atomic numbers 3 to 9.
   1. Name the most electropositive element among them.
   2. Name the most electronegative element.
   3. Name the element with the smallest atomic size.
   4. Name the element which is a metalloid.
   5. Name the element which shows maximum valency.

Answer:

1. Lithium
2. Fluorine
3. Fluorine
4. Boron
5. Carbon

Question 37. An elementX which is a yellow solid at room temperature shows catenation and allotropy. X forms two oxides which are also formed during the thermal decomposition offer sulphate crystals and are the major airpollutants.

1. Identify the element X.
2. Write the electronic configuration of X.
3. Write the balanced chemical equation for the thermal decomposition of ferrous sulphate.
4. What would be the nature (acidic/ basic) of oxides formed?
5. Locate the position ofthe element in the Modern Periodic Table.

Answer:

1. Element X is sulphur (atomic Number 16)

2. K-2, L- 8-6 M

3.

4. Acidic

5. 3rd period, group 16

Question 38. An element X of group 15 exists as a diatomic molecule and combines with hydrogen at 773K in the presence ofthe catalyst to form a compound ammonia which has a characteristic pungent smell.

1. Identify the element X. How many valence electrons does it have?
2. Draw the electron dot structure of the diatomic molecule of X. What type of bond is formed in it?
3. Draw the electron dot structure for ammonia and what type of bond is formed in it.

Answer:

1. Nitrogen (atomic number 7)

2,5; it has 5 valence electrons

2.

3.

Question 39. Which group of elements could replaced in Mendeleev’sPeriodic Table without disturbing the original order? Give reason.
Answer:

Noble gases

According to Mendeleev’s classification, the properties of elements are the periodic function of their atomic masses and there is a periodic recurrence of elements with similar physical and chemical properties. Noble gas being inert, could be placed in a separate group without disturbing the original order.

Question 40. Give an account of the process adopted by Mendeleev for the classification of elements. How did he arrive at “Periodic Law”?
Answer:

• 63 elements were known.
• Compounds of these elements with oxygen and hydrogen were studied (formation of oxides and hydrides)
• Elements with similar properties were arranged in a group
• Mendeleev observed that elements were automatically arranged in the order of increasing atomic masses.

Question 41. The properties of the elements are given below. Where would you locate the following elements in the periodic table?

1. A soft metal stored under kerosene.
2. An element with variable (more than one) valency stored under water.
3. An element which is tetravalent and forms the basis of organic chemistry.
4. An element which is an inert gas with atomic number 2.
5. An element whose thin oxide layer is used to make other elements corrosion-resistant by the process of”anodising”.

Answer:

1. Sodium (Na) Group 1 and Period 3 or Potassium (K) Group 1 and Period 4
2. Phosphorus (P) Group 15 and Period 3
3. Carbon (C) Group 14 and Period 2
4. Helium (He) Group 18 and Period 1
5. Aluminium (Al) Group 13 and Period 3

Question 42. Given below are the meltingpoints and atomic radii of three elements X, Y, and Z of the periodic table having V electrons in the outermost shells of their atoms:
Answer:

Question 43. What similarity was observed by Newlands in the properties of elements arranged in the increasing order of their atomic masses and the octaves of music? How many elements were known at the time of Newlands?
Answer:

Newlands observed that as in music eighth note is repeated in an octave of music, when the elements are arranged in the order of increasing atomic masses, the properties of the eighth element (starting from a given element) are a repetition of the properties of the first element.

From the above table, it is observed that sodium at serial number 8 has similar properties to the first element, lithium. There were only 56 elements known at the time of Newlands.

Question 44. Three elements A, B, and C have atomic numbers 7, 8, and 9 respectively.

1. What would be their positions in the Modern Periodic Table (Mention group and period both)?
2. Arrange A, B, and C in the decreasing order oftheir size.
3. Which one ofthe three elements is most reactive and why?

Answer:

1.

Element A- group (5A or 15), period 2

Element B- group (6A or 16), period 2

Element C- group (7A or 17), period 2

2. A > B > C

3. C is the most reactive. It can easily attract one electron to acquire a stable electronic configuration of Ne.

Question 45. What is a group in a periodic table? In what part ofgroup would you separately expect the elements to have

1. The greatest metallic character
2. The largest atomic size

Answer:

The group is a vertical column in the periodic table. The elements placed in a vertical column in the periodic table are said to belong to the same group.

As the metallic character increases on going down a group, the greatest metallic character is expected to be shown by the elements in the lowest part of a group.

Atomic size increases on moving down a group. So element at the bottom of a group would be of the largest size in that group.

Question 46. Barium is an element of atomic number 56. Look up its position in the periodic table and answer the following?

1. Is it a metal or a non-metal?
2. Is it more reactive than calcium or less?
3. What will be its valency?
4. What will be the formula of barium chloride?
5. Will it be larger than its neighbour (caesium) or smaller?

Answer:

1. It is a metal as it is placed under calcium and strontium.
2. It is more reactive than calcium because in this case outermost shell is farther away from the nucleus and it can more easily lose electrons.
3. Valency is 2 as it belongs to group 2.
4. BaCl₂

It is smaller in size than its neighbour Caesium as it is next to Caesium iththe e n 6th period of the periodic table. Atomic size decreases as we move from left to right in a period.

Question 47. Properties of some elements are given below. Identify in each case the element and group in the periodic table to which it belongs.

1. A soft metal stored under kerosene.
2. An element with variable valency stored under water.
3. An element which is tetravalent and forms the basis of organic chemistry.
4. An element which is an inert gas with atomic number 2.
5. A metal which burns with brilliant light when ignited.
6. An element which is yellow solid at room temperature that shows catenation and allotropy.

Answer:

Question 48. Dobereiner arranged the elements in the order of increasing atomic masses in a triad. The arrangement was modified by Newlands and later by Mendeleev. Even Mendeleev’s arrangement was later modified by Mosley resulting in the modern periodic table.

There are many other similar examples in the history ofscientific advancement where theories and, novels were modified by later scientists and original contributions accepted these modifications happily.

Answer the following based on the above data:

1. How does Modern periodic law differ from periodic law given by Mendeleev?
2. What scientific values can be associated with the above anecdotes?
3. Give an example from your life experiences so far that reflects the display of such values by you.

Answer:

1. Mendeleev’s periodic law states that the properties of elements are the periodic function of their atomic masses. Mosley’s modern periodic law states that the properties of elements are a periodic function of their atomic number.
2. Open-mindedness, ability to accept failure, intellectual honesty, respect for other’s views, spirit of enquiry.
3. Acceptance of wrong traditional beliefs initially and change in these beliefs at a later stage.

Question 49. Ria and Rama are students of Class-10 Ria is very much organised and maintained. The teachers love her. She earns great respect in the class whereas Rama is unorganised and always faces a lot of problems in handling life situations. Answer the following questions based on the above information:

1. In your opinion how does organisation help in daily life?
2. How can you relate the above fact with the Chapter classification of elements?
3. How does the classification of elements help us in studying them properly?

Answer:

The organisation makes our life simple, easy and systematic

As classification and organisation help in our daily life in the same way classification of elements has made the study of elements easier and simple.

We can know about the properties of elements because of this ‘classification.

Question 50. Given below are three groups ofthree elements each with their respective atomic masses. Can you find which of these groups form the Dobereiner triad?

Answer:

Only group B forms Dobereiner triad

⇒ \(\left(\mathrm{Sr}=87.6 \frac{40.1+137.3}{2}=\frac{1774}{2}=88.7\right)\)

Question 51. Elements A, B, C, D, E, F, G, H, I, and J have atomic weights as 3, 7, 9, 13, 35, 19, 25, 30, 50, and 97 respectively. Which are the two elements having similar properties according to Newlands’ law of octaves? What are their atomic weights? Explain.
Answer:

According to Newlands’ law of octaves, if the elements are arranged in the increasing order of their atomic weights, then the first and eighth elements have similar properties.

Thus, we have:

The first and eighth elements are A and E—which have similar properties. Their atomic weights are 3 and 35 respectively. Similarly, elements B, I and C, J are other pairs.