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Class 10 Science Chapter 5 Periodic Classification of Elements
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Periodic Classification of Elements
Chapter – 5
GENERAL SCIENCE
Textual Questions and Answers:
Page – 81
Q.1. Did Dobereiner’s triads also exist in the columns of Newlands Octaves? Compare and find out.
Ans: Yes, Dobereiner’s triads also exist in the columns of Newland’s octaves.
For example: Li, Na, k.
Q.2. What the limitations of Dobereiner’s classification?
Ans: Limitations of Dobereiner’s classification of elements:
It could identify only three triads from the elements which were known at that time. It failed to arrange all the elements which were known then in the form of triads of elements with the same chemical properties. So, his classification of elements was not much successful.
Q.3. What were the limitations of Newland’s law of octaves?
Ans: The major limitations of Newland’s law of octaves are:
(i) Newlands ‘law of octaves was applicable to the classification of elements up to calcium only. After calcium every eight elements did not posses the properties similar to that of the first element. Thus Newlands’ law of octaves worked well with lighter elements only.
(ii) Newands’ assumed that only 56 elements existed in nature bet and no more elements would be discovered in the future. But later on, several new elements were discovered whose proportics did not fit into Newlands, law of octaves.
(ii) In order to fit elements into his table, Newlands put even two elements together in one slot and that too in the column of unlike elements having very different properties.
(iv) Iron element which resembles cobalt and nickel elements in properties was placed far away from these elements.
Page – 85
Q.1. Use Mendeleev’s periodic table to predict the formula for the oxides of the following elements:
K, C, AI, Si, Ba
Ans: Oxygen is a member of group VIA in Mendeleev.s periodic table. Its valency is 2. Similarly, the valencies of all the elements listed can be predicted from their respective groups. This can help in writing the formulae of their oxides.
(i) The element K is in group 1 of Mendeleev’s periodic table is which the general formula of the oxides of elements is R₂ O. So, the formula of oxide of K will be K₂ O.
(ii) The element C is in group IV of Mendeleev’s periodic table in which the general formula of the oxides of elements is RO₂So , the formula of oxide of C will be CO₂.
(iii) The element Al is in group III of Mendeleev’s periodic table in which the general formula of the oxides of elements is R₂O₃. So the formula of oxide of Al will be Al₂O₃.
(iv) The element Si is in group IV of Mendelev’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 SiO₂.
(v) The element Ba 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 oxide of Ba will be BaO .
Q.2. Besides gallium, which other elements have since been discovered that were left by Mendeleev in this periodic table? (any two)
Ans: Scandium and Germanium.
Q.3. What were the criteria used by Mendeleev in creating his periodic table?
Ans: Mendeleev’s periodic table:
- Mendeleev arranged the elements in vertical columns and horizontal rows.
- All the elements discovered at that time were included in his table.
Criteria of Mendeleev’s periodic table:
- Mendeleev’s periodic table was based on the atomic weights of the elements.
- He arranged the elements in the increasing atomic weights as he considered that the physical and chemical properties of the elements are dependent on their atomic weights.
Q.4. Why do you think the noble gases are placed in a separate group?
Ans: Noble gases like helium, neon and argon are chemically inert and are present in atmosphere in extremely low concentrations. Thus owing to their similar inert behaviour and similar electronic configuration. They are justified to be placed in a separate group.
Page – 90
Q.1. How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?
Ans: Mendeleev was unable to give fixed position to hydrogen and isotopes in the periodic table. In Mendeleev’s periodic table, the increasing manner of atomic mass of the elements is not always regular from one to its next. It was believed that a more fundamental property than atomic mass could explain periodic properties in a better manner.
It was Henry Moseley who demonstrated that atomic number of an element could explain periodic properties in a better way than atomic mass of an element and arranged the elements in increasing order of their atomic numbers. Then it was found that the various anomalies of Mendeleev’s periodic table were removed by the modern periodic table.
Q.2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis of your choice?
Ans: Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg). This is because the number of valence electrons (2) is same in all these three elements. And since chemical properties are due to valence electrons, they show same chemical reactions.
Q.3. Name
( a ) Three elements that have a single electron in their outermost shells.
( b ) Two elements that have two electrons in their outermost shells.
( c ) Three elements with filled outermost shells.
Ans (a) Lithium, sodium, potassium.
(b) Magnesium, calcium.
(c) Helium, Neon, Argon.
Q.4. (a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
(b) Helium is an unreactive gas and neon in a gas of extremely low reactivity. What if anything, do their atoms have in common?
Ans: (a) The atoms of lithium, sodium, potassium all have only one electron in their outer most shells.
(b) The atoms of helium and neon have their outer most shell completely filled.
Q.5. In the Modern Periodic Table, which are the metals among the first ten elements?
Ans: As, the metallic character decreases across the period so lithium and beryllium are the metals among the first ten elements in the Modern Periodic Table.
Q.6. By considering their position in he periodic table , which one of the following elements would you expect to have maximum metallic characteristic?
Ga, Ge, As, Se, Be
Ans: Be.
EXERCISES
Q.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.
(a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides become more acidic.
Ans: (c) The atoms lose their electrons easily when going left to right is not correct.
Q.2. 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 the Periodic Table as
(a) Na
(b) Mg
(c) AI
(d) Si
Ans: (b) Mg.
Q.3. Which element has
(a) Two shells, both of which are completely filled with electrons?
(b) The electronic configuration 2, 8, 2?
(c) A total of three shells, with four electrons in its valence shell?
(d) A total of two shells, with three electrons in its valence shell?
(e) Twice as many electrons in its second shell as in its first shell?
Ans: (a) Ne (2, 8)
(b) Mg
(c) Si (2, 8, 4)
(d) B (2, 3)
(e) C (2, 4)
Q.4. (a) What property do all elements in the same column of the periodic table as born have in common?
( b ) What property do all elements in the same column of the periodic Table as fluorine have in common?
Ans: (a) They have the same valence and are metalloids.
(b) They form acidic oxides and have seven electrons in their outermost shells.
Q.5. An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of these elements?
(b) 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)
Ans: (a) The atomic number of the element = 2 + 8 + 7
= 17
(b) F (9) Because the noof electrons of the outermost shell of F is 7
Q.6. 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.
(b) State whether C is more reactive or less reactive that A.
(c) Will C be larger or smaller in size than B?
(d) Which type of ion , cation or anion will be formed by element A?
Ans: (a) Element A is in group 17. Now , group 17 is on the right side or the periodic table where non – metals are placed. So, element A is a non – metal.
(b) In group 17 of halogens, the chemical reactivity decreases on going down in a group. Thus element C will be less reactive than element A.
(c) On going from left to right in a period , the size of atoms, decreases. So, the atom of C will be smaller in size than an atom of B.
(d) Element A of group 17 has 7 valence electrons. So, it will accept 1 electrons to form a negatively charged ion, A. The negatively charged ion is called anion. Thus, element A will form an anion.
Q.7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration A these two elements. Which of these will be more electronegative? Why?
Ans: Electronic configuration of Nitrogen = 2,5
Electronic configuration of phosphorous = 2,8,5
Nitrogen will be more electronegative because outermost shell is nearer to nucleus.
Q.8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Ans: In the Modern Periodic Table, the electronic configuration of an element determines the number of valence electrons whereas the position of the atom is determined by the valence electrons.
Q.9. 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?
Ans: The electronic configuration of calcium = 2, 8, 8, 2
The electronic configuration of atomic number 12 = 2, 8,2
The electronic configuration of atomic number 19 = 2, 8, 8, 1
The electronic configuration of atomic number 21 = 2, 8, 8, 3
The electronic configuration of atomic number 38 = 2, 8, 18, 8, 2
Elements with atomic number 12 38 will have similar physical and chemical properties as calcium.
Q.10. Compare and contrast the arrangement of elements in Mendeleev’s Periodic Table and Modern Periodic Table.
Ans: From your chemistry lessons you have learned about the Mendeleev’s Periodic Table and its modification which is known as the Modern Periodic Table. Let us compare both of them in the tabular form.
| Mendeleev’s Periodic Table | Modern Periodic Table |
| 1. Mendeleev’s periodic table is based on atomic masses of based on atomic number of elements . | 1. Modern periodic table is based on atomic number of elements . |
| 2. Mendeleev’s periodic table does not explain the reason for the periodicity in the properties of elements. | 2. Modern periodic table says that since the electronic configurations of elements are repeated at regular intervals therefore , the properties, of elements are also repeated at regular intervals. |
| 3. There are nine vertical columns called groups . | 3. There are eighteen vertical columns called groups. |
| 4. Mendeliev’s periodic table had a number of defects . | 4. There are no defeats in the modern periodic table . |
Multiple Choice Questions:
Q.1. Which of the following statements is not a correct statements about the trends when going from left to right across the periods of the periodic table?
(a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms loss their electrons more easily.
(d) The oxides become more acidic.
Ans: (c) The atoms loss their electrons more easily.
Q.2. The electronic configuration of the atom of an element X is 2, 8, 4. In modern periodic table, the element X is placed in:
(a) 2nd group.
(b) 4th group.
(c) 14th group.
(d) 8th group.
Ans: (c) 14th group.
Q.3. The atomic number of an element is 20. In modern periodic table, this element is placed in:
(a) 2nd period.
(b) 4th period.
(c) 3rd period.
(d) 1st period.
Ans: (b) 4th period.
Q.4. Five elements A, B, C, D and E have atomic numbers of 2, 3, 7, 10 and 18 respectively. The elements which belong to the same period of the periodic table are:
(a) A, B, C
(b) B, C, D
(c) A, D, E
(d) B, D, E
Ans: (b) B, C, D
Q.5. The elements A , B , C , D and E have atomic number 9, 11, 17, 12 and 13 respectively. The pair of elements which belongs to the same group of the periodic table is:
(a) A and B
(b) B and D
(c) A and C
(d) D and E
Ans: (c) A and C
Q.6. Which of the following element would lose an electron easily?
(a) Mg
(b) Na
(c) K
(d) Ca
Ans: (c) K
Q.7. Which of the following element does not lose an electron easily?
(a) Na
(b) F
(c) Mg
(d) A1
Ans: (b) F
Q.8. Where would you locate the element with electronic configuration 2, 8 in the modern periodic table?
(a) Group 8
(b) Group 2
(c) Group 18
(d) Group 10
Ans: (c) Group 18
Q.9. An element which is an essential constituent of all organic compounds belongs to following group of modern periodic table:
(a) Group 4
(b) Group 14
(c) Group 15
(d) Group 16
Ans: (b) Group 14
Q.10. Which of the following is the valence shell for the elements of second period of the modern periodic table?
(a) M shell.
(b) k shell.
(c) L shell.
(d) N shell.
Ans: (c) L shell.
Q.11. The element which has the maximum number of valence electrons is:
(a) Na
(b) P
(c) Si
(d) AI
Ans: (b) P
Q.12. The correct increasing order of the atomic radii of the elements oxygen, fluorine and nitrogen is:
(a) O, F, N
(b) N, F, O
(c) O, N, F
(d) F, O, N
Ans: (d) F, O, N.
Q.13. The atomic numbers of the elements Na , Mg, K and Ca are 11, 12, 19 and 20 respectively. The element having the largest atomic radius is:
(a) Mg
(b) Na
(c) K
(d) Ca
Ans: (c) K
Q.14. In Mendeleev’s periodic table, gap was not left for one of the following elements. This element is:
(a) Gallium.
(b) Beryllium.
(c) Germanium.
(d) Scandium.
Ans: (b) Beryllium.
Q.15. The Newlands law of octaves for the classification of elements was found to be applicable only up to the element:
(a) Potassium.
(b) Calcium.
(c) Cobalt.
(d) Phosphorus.
Ans: (b) Calcium.
Q.16. According to Mendeleev’s periodic law , the elements were arranged in the periodic table in the order of
(a) Decreasing atomic numbers.
(b) Increasing atomic numbers.
(c) Decreasing atomic masses.
(d) Increasing atomic masses.
Ans: (d) Increasing atomic masses.
Q.17. The three elements having chemical symbols of Si, B and Ge are:
(a) Al metals.
(b) All non – metals.
(c) All metalloids.
(d) Si is metalloids, B is metal and Ge is non – metal.
Ans: (c) All metalloids .
Q.18. In Mendeleev’s periodic table, gaps were left for the elements to be discovered later on. An element which found a vacant place in the periodic table later on is:
(a) Be
(b) Si
(c) Ge
(d) Se
Ans: (c) Ge.
Q.19. The three imaginary elements X, Y and Z represent a Dobernener’s triad. If the atomic mass of element X is 14 and that of element Y is 46, then the atomic mass of element Z will be
(a) 28
(b) 60
(c) 78
(d) 72
Ans: (C) 78
Q.20. The atomic numbers of four elements? P, Q, R and S are 6, 8, 14 and 16 respectively. Our of these, the element known as metalloid is:
(a) P
(b) Q
(c) R
(d) S
Ans: (c) R
Q.21. Which of the following statement is correct in regard to Sign the classification of elements?
(a) Elements in modern periodic table are arranged on the basis of increasing atomic masses.
(b) Elements in Mendeleev’s periodic table are arranged on the basis of increasing atomic numbers.
(c) In modern periodic table, the element nickel of lower atomic mass is kept before the element cobalt of higher atomic mass.
(d) In modern periodic table, the isotopes of chlorine having different atomic masses are kept in the seme group.
Ans: (d) In modern periodic table, the isotopes of chlorine having different atomic masses are kept in the seme group.
Q.22. Which of the following statement about the modern periodic table is correct?
(a) It has 18 horizontal rows known as periods.
(b) It has 7 vertical columns known as periods.
(c) It has 18 vertical columns known as groups .
(d) It has 7 horizontal rows known as groups .
Ans: (c) It has 18 vertical columns known as groups .
Q.23. An element X forms an oxide X₂O₃. In which Mendeleev’s periodic table is this element placed?
(a) Group II
(b) Group III
(c) Group V
(d) Group VIII
Ans: (b) Group III
Q.24. The modern periodic table was prepared by:
(a) Dobereiner.
(b) Newlands.
(c) Bohr
(d) Mendeleev.
Ans: (c) Bohr.
Q.25. The atomic particle whose number in the atoms of an element always remains the same and which forms the real basis for the modern classification of elements is:
(a) Electron.
(b) Proton.
(c) Neuton.
(d) Meson.
Ans: (b) Proton.
Q.26. Which of the following are the correct characteristics of isotopes of an element?
(i) same atomic mass.
(ii) same atomic number.
(iii) same physical properties.
(iv) same chemical properties.
(a) (i), (ii) and (iv)
(b) (ii), (iii) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)
Ans: (d) (ii) and (iv)
Q.27. The correct formula of the oxide of Eka – aluminium element predicted by Mendeleev was:
(a) EaO₃
(b) Ea₃O₂
(c) Ea₂O₃
(d) EaO
Ans: (c) Ea₂O₃
Q.28. The element which can form an acidic oxide should be on the one whose atomic number is:
(a) 6
(b) 16
(c) 12
(d) 19
Ans: (b) 16
Q.29. The element which forms a basic oxide has the atomic number of:
(a) 18
(b) 17
(c) 14
(d) 19
Ans: (d) 19
Q.30. Which one of the following does not increase while oily lo moving doesn the group of the periodic table?
(a) Atomic radius.
(b) Metallic character.
(c) Valence electrons.
(d) Shells in the atoms.
Ans: (c) Valency electrons.
Q.31. On moving from left to right in a period of the periodic A table, the atomic number of elements increases. What happens to the size of atoms of elements on moving from left to right in a period?
(a) Increases.
(b) Decreases.
(c) Remains he same.
(d) First increases then decreases.
Ans: (b) Decreases.
Q.32. Which of the following set of elements is written correctly in the order of their increasing metallic character?
(a) Mg, Al, Si
(b) C, O, N
(c) Na, Li, K
(d) Be, Mg, Ca
Ans: (d) Be, Mg, Ca.
Short Questions :
Q.1. Write two merits of the modern periodic table.
Ans: The merits of the modern periodic table are:
1. The classification of elements is based on the atomic number, which is a more fundamental property.
2. The reason for placing isotopes in one place is justified as the classification is on the basis of atomic number.
Q.2. Write the three similar chemical properties of lithium, sodium and potassium.
Ans: (i) All these elements are metals.
(ii) All of them react with water to form alkalis and hydrogen gas.
(iii) All of them have a valency of 1.
Q.3. Write newlands law of octaves.
Ans: Newland’s Law of Octaves states that when Elements are arranged in increasing order of Atomic Mass, the properties of every eighth Element starting from any Element are a repetition of the properties of the starting Element. Law of Octaves was true only for Elements up to Calcium.
Q.4. Write three merits of Mendeleev’s Classification of elements.
Ans: 1. Classification helps in the identification of the position of an element in the periodic table through which the properties of their compounds can be predicted.
2. Mendeleev’s periodic law predicted the existence of some elements that had not been discovered at that time.
3. It helps in learning the physical and chemical properties of elements and finding their uses. It explains why elements have periodicity and explains why all elements in a group have similar features that differ between the groups and periods.
Q.5. Write the two limitations of mendeleev’s classification of elements.
Ans: The limitations of Mendeleev’s periodic table are:
(i) Elements that show large variations in their physical and chemical properties were added to the same group of the periodic table.
(ii) Mendeleev could not fit hydrogen at a particular place in the periodic table.
Q.6. Why all the elements of a group show similar chemical properties?
Ans: All the elements of a group in the periodic table have similar electronic configurations that is having the same number of valence electrons , so all the elements of a group show similar chemical properties.
Q.7. Write two advantages of the periodic table.
Ans: Advantages of modern periodic table:
- The table correlates the position of the elements with their electron configuration much more clearly.
- The periodic table is now based on the fundamental property or we should say the atomic numbers of the elements.
Q.8. Which of the following belong to:
(i) The same period.
(ii) the same group?
| Element | Atomic number |
| A | 2 |
| B | 10 |
| C | 5 |
Ans: Electronic configuration of A =
Electronic configuration of B = 2, 8
Electronic configuration of C = 2, 3
(i) The elements B and C have the same valence shell(L) therefore B and C belong to the same period.
(ii) Elements A and B belong to the same group because their valence shells are completed.
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