5. Periodic Classification of Elements

Class 10 Science Chapter 5. Periodic Classification of Elements

Chapter 5. Periodic Classification of Elements

Class 10 Science Chapter 5. Periodic Classification of Elements Internal Questions and Answers :

Internal Questions :

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

Answer : Yes, Döbereiner's triads also existed in the columns of Newlands' Octaves. Both Döbereiner and Newlands were early attempts at classifying elements based on their properties.

Döbereiner's triads grouped three elements with similar chemical properties, and their atomic weights showed a pattern where the middle element's atomic weight was approximately the average of the other two. For example, lithium (Li), sodium (Na), and potassium (K) form a triad with atomic weights: Li (6.9), Na (23), and K (39). The atomic weight of Na (23) is close to the average of Li (6.9) and K (39).

Newlands' Octaves, on the other hand, attempted to classify elements in groups of eight. He noticed that every eighth element had similar properties, which he referred to as "octaves." However, this pattern worked only up to a certain point and did not cover the entire periodic table.

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

Answer : Döbereiner's classification had some limitations are :

(i) Not all elements could be grouped into triads, as some elements didn't fit well with others in terms of properties and atomic weights.

(ii) Döbereiner's classification didn't cover all known elements at that time, leaving gaps in the periodic table.

(iii) There was no clear pattern or explanation for why certain elements formed triads while others didn't.

(iv) Döbereiner only focused on chemical properties and atomic weights, ignoring other important characteristics of elements.

(v) The triads concept worked well only for a few elements and couldn't be extended to all elements known at that time.

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

Answer : The limitations of Newlands' Law of Octaves were:

(i) Newlands' law worked well for the elements known at that time but could not accommodate all the elements discovered later.

(ii) Newlands observed that every eighth element showed similar properties, but he couldn't explain why this pattern occurred or what determined the periodicity.

(iii) Newlands' law did not include the transition elements, which are located in the middle of the periodic table and have unique properties.

(iv) In some cases, elements were placed in the wrong groups because of their close resemblance to elements in the eighth position, even though they didn't exhibit true periodicity.

(v) Newlands' law assumed a fixed group size of eight elements, which didn't hold true for all elements. Some groups had more than eight elements, making the pattern inaccurate.

Internal Questions :

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

Answer : On Mendeléev’s Periodic Table, we can predict the formulae for the oxides of the given elements are :

K (Potassium) -

C (Carbon) -

AI (Aluminum) -

Si (Silicon) -

Ba (Barium) -

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

Answer : Two elements that were left by Mendeléev in his Periodic Table but have since been discovered are:

Scandium (Sc): Scandium was discovered after Mendeléev's Periodic Table . It was found to fit into the table in Group 3.

Germanium (Ge): Mendeléev predicted its existence and named it "eka-silicon" in his periodic table. Germanium is now placed below silicon in Group 14.

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

Answer : Mendeléev used two main criteria in creating his Periodic Table:

(i) Mendeléev arranged the elements in order of increasing atomic mass. He noticed that elements with similar properties occurred at regular intervals when arranged in this way.

(ii) Mendeléev observed that when elements were arranged in order of increasing atomic mass, there was a periodic repetition of their properties. He called this the "Periodic Law," which formed the basis for organizing elements into groups and periods in the Periodic Table.

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 have very stable and unreactive electronic configurations. They do not easily form compounds with other elements due to their full outer electron shells, making them chemically inert. Placing them in a separate group emphasizes their unique and distinct characteristics compared to other elements in the periodic table.

Internal Questions :

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

Answer : The Modern Periodic Table removed anomalies by arranging elements based on atomic number, not atomic mass. This made it more accurate and consistent, showing elements' properties better and filling gaps, unlike Mendeléev's table.

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

Answer : Calcium and Strontium are two elements that show chemical reactions similar to magnesium. They are in the same group (Group 2) of the periodic table, meaning they have the same number of valence electrons, leading to similar chemical properties.

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.

Answer : (a) Hydrogen (H) , Lithium (Li) and Sodium (Na) .

(b) Beryllium (Be) and Magnesium (Mg) .

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, there is a similarity in the atoms of these elements. Lithium, sodium, and potassium all belong to Group 1 of the periodic table, known as the alkali metals. They have similar electronic configurations, with one valence electron in their outermost energy level. This similarity in electronic structure is what makes them react similarly with water to liberate hydrogen gas.

(b) Both helium and neon are noble gases located in Group 18 of the periodic table. Their atoms have full outer electron shells, making them highly stable and unreactive. This shared electronic configuration is the reason for their low reactivity.

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

Answer : In the Modern Periodic Table, the metals among the first ten elements are:

Lithium (Li)

Beryllium (Be)

Sodium (Na)

Magnesium (Mg)

Aluminum (Al)

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 : Among the given elements, beryllium (Be) would be expected to have the maximum metallic characteristic. Beryllium is located on the far left of the periodic table, in Group 2, which is the alkaline earth metals group. Elements in Group 2 are known for having strong metallic characteristics, and beryllium is no exception. It has a high tendency to lose electrons and form cations, making it a typical metallic element.

Class 10 Science Chapter 5. Periodic Classification of Elements Exercise Questions and Answers :

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.

2. Element X forms a chloride with the formula , 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) AI (d) Si

Answer : (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?

Answer : (a) Neon (Ne) [The electronic configuration of element is 2.8 ].

(b) Magnesium (Mg) . [The electronic configuration of Mg is 2.8.2]

(d) Boron (B) [The electronic configuration of element is 2.3]

(e) Carbon (C) [The electronic configuration of element is 2.4]

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 elements in the same column as boron in the Periodic Table (Group 13) have in common that they have three valence electrons.

(b) All elements in the same column as fluorine in the Periodic Table (Group 17) have in common that they have seven valence electrons.

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 the given element is 17 (2+8+7=17) .

(b) The electronic configuration of N is 2.5 .

The electronic configuration of F is 2.7 .

The electronic configuration of P is 2.8.5 .

The electronic configuration of Ar is 2.8.8 .

N and P are chemically similar .

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?

Answer : (a) A is a non-metal .

(b) C is less reactive than A .

(d) Anion .

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 :

Atomic numbers	Name of elements	Electronic configuration
7	Nitrogen	2.5
15	Phosphorus	2.8.5

Nitrogen is more electronegative than phosphorus . Because the outermost electrons are nearest from the nucleus .

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 determines its position in the Modern Periodic Table. Elements with similar electronic configurations are placed in the same column (group), showing similar chemical properties. The number of valence electrons determines the group number.

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?

Answer :

Atomic numbers	Name of elements	Electronic configuration
20	Ca	2.8.8.2
12	Mg	2.8.2
19	K	2.8.8.1
21	Sc	2.8.8.3
38	Rb	2.8.8.18.2

The elements of atomic number 12 and 38 have the same physical and chemical properties . Because both the element have same number of valency .

10. Compare and contrast the arrangement of elements in Mendeléev’s Periodic Table and the Modern Periodic Table.

Answer : In Mendeléev's Periodic Table:

(i) Elements were arranged based on increasing atomic mass.

(ii) Elements with similar chemical properties were grouped together, but there were some anomalies.

(iii) There were gaps for yet-to-be-discovered elements.

(iv) The position of hydrogen was uncertain.

In the Modern Periodic Table:

(i) Elements are arranged based on increasing atomic number.

(ii) Elements with similar chemical properties are grouped together in the same column (group).

(iii) Gaps for undiscovered elements were filled, and the table is more complete.

(iv) Hydrogen is placed in its own separate group due to unique properties.