5. Periodic Classification of Elements – Class 10 Science SEBA Assam Board
Chapter 5: Periodic Classification of Elements -SEBA/ASSEB Class 10 Science Textbook, Assam Board
Chapter 5: Periodic Classification of Elements
Internal Question and Answer:
1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Answer: Yes, Döbereiner’s triads also appeared in Newlands’ Octaves. For example, Li, Na, K formed a triad and also occurred in the same column of Newlands’ arrangement, showing similar chemical properties.
2. What were the limitations of Dobereiner’s classification ?
Answer: The limitations of Dobereiner’s classification are -
(i) Dobereiner showed that when the three elements in a triad were written in the order of increasing atomic masses ; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements .
(ii) Dobereiner could identify only three triads from the elements known at time .
Hence, this system of classification into traids was not found to be useful .
3. What were the limitations of Newland’s Law of Octaves ?
Answer: The limitations of Newland’s law of Octaves are -
(i) It was found that the law of Octaves was applicable only upto calcium, as after calcium every eighth element did not possess properties similar to that of the first .
(ii) It was assumed by Newland’s that only 56 elements existed in nature and no more elements would be discovered in the future .
(iii) In order to fit elements into his table , Newland’s adjusted two elements in the same slot, but also put some unlike elements under the same note .
Internal Question and Answer:
1. Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, AI, Si, Ba.
Answer: Using Mendeléev’s Periodic Table, the formulae of oxides are:
|
Name of Elements |
Name of oxide |
Valency |
|
K |
K₂O |
1 |
|
C |
CO₂ |
4 |
|
Al |
Al₂O₃ |
3 |
|
Si |
SiO₂ |
4 |
|
Ba |
BaO |
2 |
2. Besides gallium, which other elements have since been discovered that were left by Mendeléev in his Periodic Table? (any two)
Answer: Besides gallium, two elements that were discovered later and had been left as gaps in Mendeléev’s Periodic Table are Germanium (Ge) and Scandium (Sc) .
3. What were the criteria used by Mendeléev in creating his Periodic Table?
Answer: Mendeléev used two main criteria:
(i) Atomic mass: He arranged the elements in the increasing order of their atomic masses.
(ii) Chemical properties: He placed elements with similar chemical properties in the same group and left gaps for elements that had not yet been discovered.
4. Why do you think the noble gases are placed in a separate group?
Answer: Noble gases are placed in a separate group because their valency is zero. They have a completely filled outermost shell, so they are very stable and do not easily react with other elements. Therefore, they are placed separately in Group 18 of the Modern Periodic Table.
Internal Question and Answer:
1. How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?
Answer: The Modern Periodic Table removed the anomalies of Mendeléev’s Periodic Table by arranging elements in the increasing order of their atomic numbers. This gave a proper position to isotopes and corrected the order of elements such as cobalt and nickel. Elements with similar electronic configurations and properties are placed in the same group.
2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Answer: The two elements expected to show chemical reactions similar to magnesium (Mg) are calcium (Ca) and beryllium (Be).
Magnesium, calcium and beryllium belong to the same group of the Modern Periodic Table. They have two valence electrons and therefore show 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:
|
Electronic configuration |
Atomic numbers |
Name of elements |
|
|
(a)
|
2.1 2.8.1 2.8.8.1 |
3 11 19 |
Lithium Sodium Potassium |
|
(b) |
2.2 2.8.2 |
4 12 |
Beryllium Magnesium |
|
(c) |
2 2.8 2.8.8 |
2 10 18 |
Helium Neon Argon |
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 lithium, sodium and potassium. All three have one electron in their outermost shell. Hence, they have similar chemical properties and react with water to liberate hydrogen gas.
(b) The atoms of helium (He) and neon (Ne) have a completely filled outermost shell of electrons. Therefore, they are chemically stable and show very low reactivity.
5. In the Modern Periodic Table, which are the metals among the first ten elements?
Answer: Among the first ten elements of the Modern Periodic Table, the metals are lithium (Li) and beryllium (Be).
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: By considering their position in the Modern Periodic Table, the element with maximum metallic character is Be (Beryllium) .
Exercises of Chapter 5 : Periodic Classification of Elements
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. [SEBA 2018]
(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.
[The atoms do not lose their electrons 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
[ The formula XCl₂ shows that X has a valency of 2. Magnesium (Mg) also has a valency of 2 and belongs to Group 2 of the Modern Periodic Table. Therefore, X would most likely be in the same group as 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
The electronic configuration of neon is 2.8 .
(b) Magnesium (Mg) .
(c) Silicon (Si)
The electronic configuration of silicon is 2.8.4 .
(d) Boron (B) .
The electronic configuration of neon is 2.3 .
(e) Carbon (C)
The electronic configuration of neon 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 the elements in the same column as boron belong to Group 13 and have three electrons in their outermost shell.
(b) All the elements in the same column as fluorine belong to Group 17 and have seven electrons in their outermost shell.
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 element is 17 .
(b)
|
Name of elements |
Atomic numbers |
Electronic configuration |
|
N |
7 |
2.5 |
|
F |
9 |
2.7 |
|
P |
15 |
2.8.5 |
|
Ar |
18 |
2.8.8 |
Nitrogen and phosphorus have the same chemical properties . Because both the element have same number of valency .
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-metyal .
(b) C is less reactive than A .
(c) C is smaller than B , Because we move from left to right , then the atomic size decreases .
(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 is directly related to its position in the Modern Periodic Table.
(a) The number of electron shells in an atom determines its period.
(b) The number of valence electrons determines its group for the main-group elements.
(c) Elements having the same number of valence electrons are placed in the same group and show similar chemical properties.
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:
|
Mendeléev’s Periodic Table |
Modern Periodic Table |
|
Elements were arranged in the increasing order of atomic masses. |
Elements are arranged in the increasing order of atomic numbers. |
|
It had 8 groups, with some groups divided into sub-groups. |
It has 18 groups and 7 periods. |
|
There was no proper place for isotopes. |
Isotopes are placed at the same position because they have the same atomic number. |
|
Some elements had to be placed out of atomic mass order to maintain similar properties. |
Elements are arranged according to atomic number, which removes this problem. |
|
The position of hydrogen was not clearly fixed. |
Hydrogen is placed separately at the top of Group 1, though its position is still considered unique. |
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