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NCERT Solution for Class 10 Science Chapter 5 Periodic Classification of Elements
Q1. Did Dobereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Ans. Only one triad of Dobereiner ’s triads exists 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.
Q2. What were the limitations of Dobereiner’s classification?
Ans. All known elements could not be classified into groups of triads on the basis of their properties.
Did Doberiener’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Yes, some of the Doberiener’s triads did exist in the columns of .Newlands’ Octaves. For example,
Li, Na, K
BE, Mg, Ca
What were the limitations of Doberiener’s Triads?
Limitations of Dobereiner’s Triad
All known elements could not be classified into groups of triads on the basis of their properties.
Not all groups obeyed the Law of Triads.
What were the limitations of Newlands’ Law of Octaves?
Limitations of Newlands’ Law of Octaves
1. The law of octaves was found to be applicable only up to calcium. It was not applicable to elements of higher atomic masses.
2. He thought that only 56 elements existed in nature and no more elements were discovered in future. But the discovered elements were not accommodated in the octaves.
3. In order to fit elements into octaves, Newland adjusted two elements in the same slot which were not similar.
Use Mendeleev’s periodic table to predict the formulae for the oxides of the elements: K, C, Al, Si, Ba.
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.
1. Potassium (K) is a member of group IA. Its valency is 1. Therefore, the formula of its oxide is K2O.
2. Carbon (C) is a member of group IVA. Its valency is 4. Therefore, the formula of its oxide is C2O4 or CO2.
3. Aluminium (Al) belongs to groups IIIA and its valency is 3. The formula of the oxide of the element is Al2O3.
4. Silicon (Si) is present in group IVA after carbon. Its valency is also 4. The formula of its oxide is Si2O4 or SiO2.
5. Barium (Ba) belongs to group IIA and the valency of the element is 2. The formula of the oxide of the element is Ba2O2 or BaO.
Besides gallium, which two other elements have since been discovered that fill the gaps left by Mendeleev in his periodic table?
Two other elements are scandium (Sc) and germanium (Ge). In their gaps, the elements with names Eka-boron and Eka-silicon were placed.
What were the criteria used by Mendeleev in creating his periodic table?
Mendeleev used atomic masses of the elements as the criteria for creating his periodic table. In this table, the elements were arranged in order of increasing atomic masses.
Why do you think the noble gases are placed in a separate group?
In Mendeleev’s Periodic Table, the elements have been arranged in different groups on the basis of valency. For example, the elements placed in group I (IA and IB) have valency equal to one. Same is the case with the elements placed in other groups. Since the noble gas elements He, Ne, Ar, Kr, Xe and Rn have zero valency, they could not find a place in Mendeleev’s Periodic Table. They have been placed in a separate group called zero group in the periodic table. Please note that the noble gas elements were not a part of the Mendeleev’s Periodic Table. They were added later on.
How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?
1. Modern Periodic Table or Long Form of Periodic Table: In the Modern Periodic Table, the elements are arranged in increasing order of their atomic numbers.
The Modern Periodic Table was formulated by Henry Moseley in 1913.
Modern Periodic Law: It states that “the properties of elements are a periodic function of their atomic numbers.”
2. Main Features of Modern Periodic Table
# The elements are arranged according to their electronic configurations.
# There are seven periods, numbering 1 to 7. The period number signifies the number of electron shells of an element. Thus, the period one elements have only one shell. The period two elements have only two shells, and so on. Period 1 is the shortest period with only two elements. Periods 2 and 3 are short periods with eight elements each. Periods 4 and 5 are long periods
Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Magnesium (Mg) belongs to group 2 known as Alkaline Earth Family. The two other elements belonging to the same group are calcium (Ca) and strontium (Sr). The basis of choice is the electronic distribution in the valence shell of these elements. All of them have two electrons each.
For example,
K L M N O
Mg 2 8 2
Ca 2 8 8 2
Sr 2 8 18 8 2
Name:
1. three elements that have a single electron in their outermost shells.
2. three elements that have two electrons in their outermost shells.
3. three elements with filled outermost shells.
1. Lithium, sodium, potassium (alkali metals)
2. Beryllium, magnesium, calcium (alkaline earth metals)
3. Helium, neon, argon (noble gases).
In the Modern Periodic Table, which are the metals among the first ten elements?
Metals among the first ten elements are lithium (Li) and beryllium (Be). These are placed towards the left of the table.
By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristics?
Ga, Ge, As, Se, Be
Before identifying the metallic character from the list of the elements, we must remember two points:
The metallic character of an element is related to the electron releasing tendency of its atoms. Greater the tendency, more will be the metallic character.
In general, metallic character of the elements increases down the group and decreases along a period. With the help of the Modern
