Periodic Classification of Elements: Class 10 Science Chapter 5 notes

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In this chapter, Class 10 Science Chapter 5: Periodic Classification of Elements (NCERT, SEBA, NBSE, TBSE etc.), you will learn how the different elements found in nature can be grouped and arranged according to their properties. The concept of the Periodic Table has a long history, with several scientists contributing to its development over time. These include Johann Wolfgang Dobereiner, John Newlands, Dmitri Ivanovich Mendeleev, and Henry Moseley. Earlier versions of the Periodic Table had limitations and became outdated as new elements were discovered. Get here the questions, answers, textbook solutions, PDF of the chapter.

Periodic Classification of Elements

Intext / Page 85

1. Did Dobereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.

Answer: Yes, Dobereiner’s triads exist in the columns of Newlands’ octaves. The triad formed by the elements Li, Na, and K of Dobereiner’s triads also occurred in the columns of Newlands’ octaves.

2. What were the limitations of Dobereiner’s classification?

Answer: One of the limitations of Dobereiner’s classification was that not all known elements could be classified into groups of triads on the basis of their properties. For elements with very low mass or very high mass, Dobereiner’s triads are not applicable.

3. What were the limitations of Newlands’ Law of Octaves?

Answer: Limitations of Newlands’ law of octaves were: 

a) it was not applicable throughout the arrangements, only up to calcium;
b) elements discovered after Newlands’ octaves did not follow the law of octaves;
c) the position of cobalt and nickel in the group of elements (F, Cl) with different properties could not be explained;
d) the placement of iron far away from cobalt and nickel, which have similar properties as iron, also could not be explained.

Intext / Page 89

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

Answer: The oxide of K will be K2O. The oxide of C will be CO2. The oxide of Al will be Al2O3. The oxide of Si will be SiO2. The oxide of Ba will be BaO.

2. Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (any two)

Answer: Two elements that have been discovered since Mendeleev’s time and were not included in his periodic table are scandium and germanium.

3. What were the criteria used by Mendeleev in creating his Periodic Table?

Answer: Mendeleev based his periodic table on the observation that the properties of elements are periodic functions of their atomic masses. This means that if elements are arranged in increasing order of their atomic masses, their properties repeat at regular intervals.

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

Answer: The noble gases are placed in a separate group because they are inert elements with properties that are different from all other elements.

Intext / Page 94

1. How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?

Answer: The modern periodic table, based on the atomic number of elements rather than their atomic mass, was able to remove the anomalies present in Mendeleev’s periodic table. Mendeleev was unable to give hydrogen and isotopes fixed positions in his periodic table, and the arrangement of elements by atomic mass was not always regular. Henry Moseley demonstrated that the atomic number of an element better explained periodic properties, and the modern periodic table arranges elements in increasing order of their atomic numbers.

2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?

Answer: Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg) because all three elements have the same number of valence electrons (2). Chemical properties are due to valence electrons, so these elements are expected to have similar chemical reactions.

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, and (c) three elements with filled outermost shells.

Answer: (a) Lithium (Li), sodium (Na), and potassium (K) have a single electron in their outermost shells. (b) Magnesium (Mg) and calcium (Ca) have two electrons in their outermost shells. (c) Neon (Ne), argon (Ar), and xenon (Xe) have filled outermost shells.

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 is a gas of extremely low reactivity. What, if anything, do their atoms have in common?

Answer: (a) Yes, the atoms of all three elements lithium, sodium, and potassium have a single electron in their outermost shells. 

(b) Both helium (He) and neon (Ne) have filled outermost shells. Helium has a duplet in its K shell, while neon has an octet in its L shell.

5. In the Modern Periodic Table, which are the metals among the first ten elements?

Answer: Among the first ten elements in the Modern Periodic Table, lithium (Li) and beryllium (Be) are metals.

6. By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic: Ga, Ge, As, Se, Be?

Answer: Beryllium (Be) is expected to have the most metallic characteristics among the given elements because it is located on the far left side of the periodic table.

Exercise

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?

Answer: (c) The atoms lose their electrons more easily is not a correct statement. (On moving from left to right across the periods of the periodic table, the non-metallic character increases. Hence, the tendency to lose electrons decreases.)

2. Element X forms a chloride with the formula XCl2, which is a 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) Al, (d) Si.

Answer: (b) X would most likely be in the same group of the Periodic Table as magnesium (Mg).

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?

Answer: (a) Neon has two shells, both of which are completely filled with electrons (2 electrons in K shell and 8 electrons in L shell).
(b) Magnesium has the electronic configuration 2, 8, 2.
(c) Silicon has a total of three shells, with four electrons in its valence shell (2 electrons in K shell, 8 electrons in L shell, and 4 electrons in M shell).
(d) Boron has a total of two shells, with three electrons in its valence shell (2 electrons in K shell and 3 electrons in L shell).
(e) Carbon has twice as many electrons in its second shell as in its first shell (2 electrons in K shell and 4 electrons in L shell).

4. (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?

Answer: (a) All the elements in the same column as boron have the same number of valence electrons (3). Therefore, they all have valency equal to 3.
(b) All the elements in the same column as fluorine have the same number of valence electrons (7). Therefore, they all have valency equal to 1.

5. An atom has electronic configuration 2, 8, 7.

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

Answer: (a) The atomic number of this element is 17.
(b) It would be chemically similar to F (9) with configuration as 2, 7.

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

Group 16Group 17
A
BC

(a) State whether A is a metal or non-metal.
(b) State whether C is more reactive or less reactive than 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?

Answer: (a) A is a non-metal.
(b) C is less reactive than A, as reactivity decreases down the group in halogens.
(c) C will be smaller in size than B as moving across a period, the nuclear charge increases and therefore, electrons come closer to the nucleus.
(d) A will form an anion as it accepts an electron to complete its octet.

7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?

Answer: Nitrogen has the following electronic configuration: 2 5. Phosphorus has the following electronic configuration: 2 8 5. Nitrogen is more electronegative than phosphorus because, as you move down a group in the periodic table, the number of shells increases and the valence electrons move further away from the nucleus. This decreases the effective nuclear charge and the tendency to attract electrons, leading to a decrease in electronegativity.

8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?

Answer: The electronic configuration of an atom is related to its position in the periodic table because elements with similar electronic configurations are placed in the same column. In a group, the number of valence electrons remains the same, while across a period, the number of valence electrons increases.

9. Explain why:

(a) The elements of the same group have similar chemical properties.
(b) The elements of the same period have different properties.

Answer: a) The elements of the same group have similar chemical properties because they have the same number of valence electrons in their outermost shells.

b) The elements of the same period have different properties because they have different numbers of valence electrons in their outermost shells.

10. The electronic configuration of an element ‘X’ is 2, 8, 8, 2. To which period and group of the modern periodic Table does ‘X’ belong? State its valency. Justify your answer in each case.

Answer: The electronic configuration of element X is 2, 8, 8, 2, so it belongs to period 4 and group 2 of the periodic table. Its valency is 2 because it can acquire a stable gas configuration by losing 2 electrons.

11. (a) State two main characteristics of elements on which the Modern Periodic Table is based.
(b) No fixed position can be assigned to hydrogen in the periodic table. Why?

Answer: (a) The two main characteristics of elements on which the Modern Periodic Table is based are atomic mass and chemical properties.

(b) Hydrogen cannot be assigned a fixed position in the periodic table because it exhibits properties of both alkali and halogen elements.

12. The elements Be, Mg, and Ca each have two electrons in their outermost shells and are in periods 2, 3, and 4, respectively, of the Modern Periodic Table. Now, answer the following questions, giving justification in each case:

(i) Write the group to which these elements belong.
(ii) Name the least reactive element.
(iii) Name the element with the largest atomic radius.

Answer: (i) The elements Be, Mg, and Ca belong to group 2 of the periodic table because they all have 2 valence electrons.

(ii) Be is the least reactive element because it has a full valence shell and is therefore more stable.

(iii) Ca has the largest atomic radius because it has the highest atomic number and therefore has more energy shells.

13. 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 element with atomic number 12 (Magnesium) has physical and chemical properties resembling calcium because they both have the same number of valence electrons (2).

14. Compare and contrast the arrangement of elements in Mendeleev’s Periodic Table and the Modern Periodic Table.

Answer: The arrangement of elements in Mendeleev’s periodic table and the modern periodic table differs in several ways:

  • In Mendeleev’s periodic table, elements were arranged in increasing order of atomic mass, while in the modern periodic table, they are arranged in increasing order of atomic number.
  • Mendeleev’s periodic table had only 8 vertical columns (called groups), while the modern periodic table has 18 vertical columns (also called groups).
  • In Mendeleev’s periodic table, each group was subdivided into subgroups ‘a’ and ‘b’, while in the modern periodic table, groups are not subdivided into subgroups.
  • The noble gases, which were not known at the time of Mendeleev, are placed at the end of the periodic table in a separate group (group 18) in the modern periodic table.
  • Mendeleev’s periodic table did not have a place for isotopes, while in the modern periodic table, slots are determined according to atomic number, which allows for the inclusion of isotopes.

Tick (✓) the correct option

1. In Mendeleev’s Periodic Table, gaps were left for the elements to be discovered later. Which of the following elements found a place in the periodic table later?

Answer: Germanium

2. The element with atomic number 14 is hard and forms acidic oxide with a covalent halide. To which of the following categories does the element belong?

Answer: (b) Metalloid

3. Doberiener’s system of classification into triad was not found to be useful as he could identify only:

Answer: (b) Three triads

4. The atomic number of an element is a more fundamental property than its atomic mass was showed by:

Answer: (a) Henry Moseley

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