NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements

Chapter 5 Periodic Classification of Elements NCERT Solutions for Class 10 Science that will be helpful in knowing about the new topics and things. Whenever you have any doubts in any question, you can take help from the given NCERT Solutions. You can track your progress anytime and provide complete assistance through the help of Class 10 Science NCERT Solutions.

Chapter 5 Periodic Classification of Elements Class 10 Science NCERT Solutions

In Text Questions

Page No: 81

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

Solution

Yes, Dobereiner’s triads also exist in the columns of  Newlands’ classification of elements based on the law of octaves.

Consider the elements lithium (Li), sodium (Na) and potassium (K) which are present in the second column of Newlands' classification of elements. Now, if we start with lithium as the 1st  element, then the 8th element from it is sodium, and according to Newlands' law of octaves, the properties of 8th element, sodium should be similar to those of the 1st element, lithium. Again, if we take sodium as the 1st element, then the 8th element from it is potassium, and according to Newlands' law of octaves, the properties of 8th element, potassium should be similar to those of the 1st element, sodium. This means that according to Newlands' law of octaves, the elements lithium, sodium and potassium should have similar chemical properties. We also know that lithium, sodium and potassium form a Dobereiner's triad having similar chemical properties. From this, we conclude that Dobereiners triads also exist in the columns of Newlands Octaves.

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

Solution

The main limitation of Dobereiner's classification of elements was:

(i) It failed to arrange all the known elements in the form of triads of elements having similar chemical properties.

(ii) Dobereiner could identify only three triads from the elements known at that time.

(iii) Dobereiner failed to explain the relation between atomic masses of elements and their chemical properties.

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

Solution

The limitations of Newlands' Law of Octaves was:

(i) Newland's law of octaves was applicable to only the first few elements' means only up to calcium out of total 56 elements known at that time.

(ii) Newland adjusted two elements in the same slot having different properties in order to fit elements into his table. For example: iron, whose properties resembles with cobalt and nickel, was placed far away from these elements.

(iii) Newlands' octaves did not have provision to accommodate the newly discovered elements.

Page No: 85

1. Use Mendeleev's Periodic Table to predict the formulae for the oxides of the following elements:

K, C, Al, Si, Ba.

Solution

K is in group 1. Therefore, the oxide will be K₂O.
C is in group 4. Therefore, the oxide will be CO₂.
Al is in group 3. Therefore, the oxide will be Al₂O₃.
Si is in group 4. Therefore, the oxide will be SiO₂.
Ba is in group 2. Therefore, the oxide will be BaO.

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

Solution

Germanium and Scandium.

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

Solution

Mendeleev’s periodic table was based on the observation that the properties of elements are a periodic function of their atomic masses. This means that if elements are arranged in the increasing order of their atomic masses, then their properties get repeated after regular intervals.

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

Solution

Noble gases are inert elements. Their properties are different from all other elements. Therefore, the noble gases are placed in a separate group.

Page No: 90

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

Solution

→ Elements are arranged in the increasing order of their atomic number in Modern Periodic Table, thus there was no need for keeping more than one element in one slot.

→ In Modern Periodic Table, there was no problem of the place of isotopes, as isotopes have same atomic mass with different atomic numbers.

→ Elements having same valence electron are kept in same group.

→ Elements having same number of shells were put under the same period.

→ Position of hydrogen became clarified in as it is kept in the group with the elements of same valence electrons.

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

Solution

Since the chemical properties are due to valence electrons present in there outermost shell. Magnesium belongs to group 2 in a periodic table having valency 2. So, the two elements which show same kind of chemical reactivity as magnesium are beryllium (Be) and calcium (Ca), which belongs to the same group as that of magnesium and have valency 2.

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.

Solution

(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?

Solution

(a) Yes. The atoms of all the three elements lithium, sodium, and potassium have one 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?

Solution

The first ten elements in the modern periodic table are hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine and neon. Out of which only two are metals i.e. lithium (Li) which is an alkali metal and beryllium (Be) which is an alkaline earth metal.

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

Solution

Since Be lies to the extreme left-hand side of the periodic table, Be is the most metallic among the given elements.

Page No: 91

Exercises

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.

Solution

(c) The atoms lose their electrons more easily.

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

Solution

(b) 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?

Solution

(a) Neon
(b) Magnesium
(c) Silicon
(d) Boron
(e) Carbon

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?

Solution

(a) Valency equal to 3.
(b) 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)

Solution

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

Page No: 92

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 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?

Solution

(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?

Solution

Nitrogen (N) and phosphorous (P), both the elements belongs to 15th group in a periodic table. The electronic configuration of these elements are:

N (7)- 2,5

P(15)-2,8,5

The number of shells increases on moving down the group, which decreases the tendency to attract the electrons. Therefore, electronegativity decreases. As the order of the elements present in the periodic table from up to down is nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi), and moscovium (Mc). Nitrogen comes first on moving down the group so nitrogen will be more electronegative than phosphorous.

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

Solution

In the modern periodic table, elements are arranged in the increasing order of their atomic number. Those elements which have similar electronic configuration are placed in same group. In a group, all the elements have same number of electrons in their valence shell, hence possess same valency. So the position of an atom is related to the number of electrons present in the outermost shell.

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?

Solution

The element with atomic number 12 has same chemical properties as that of calcium. This is because both of them have same number of valence electrons (2).

10. Compare and contrast the arrangement of elements in Mendeleev's Periodic Table and the Modern Periodic Table.

Solution

	Mendeleev's Periodic Table	Modern Periodic Table
Basis	Elements are arranged in the increasing order of their atomic masses.	Elements are arranged in the increasing order of their atomic numbers.
Number of elements	It contains presently 63 elements.	It contains presently 118 elements.
Number of groups and periods	There are total 7 groups (columns) and 6 periods (rows).	There are total 18 groups (columns) and 7 periods (rows).
Position of elements	Those elements which were having similar properties were placed directly into a single group.	Those elements which were having same valence shell were placed in the same period and elements those were having same number of valence electrons were placed in a same group.
Position of Hydrogen	It was not fixed.	Hydrogen is placed above alkali metals.
Position of Transition elements	Transition elements were included with the other elements.	Transition elements are placed in a separate block.
Position of Isotope	Isotopes with different atomic weights, should be placed in different positions, but are not given any position and no justified reason was explained.	Isotopes were assigned same position as they have same atomic number.

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