Chapter 5 Periodic Classification of Elements Solutions
Question - 1 : - Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Answer - 1 : -
Döbereiner’s triads did exist in the columns of Newlands’ Octaves; For example, the elements
Lithium (Li), Potassium (K) and Sodium (Na) constitute a Dobereiner’s Triad but are also found in the second column of Newland’s Octaves
Question - 2 : - What were the limitations of Döbereiner’s classification?
Answer - 2 : -
(i) They were not applicable for very low mass or very high mass elements.
(ii) All the elements couldn’t fit into Dobereiner’s triads.
(iii) As the methods to calculate atomic mass improved, Dobereiner’s triads validity began to decrease. For example, in the triad of F, Cl and Br, the arithmetic mean of atomic masses of F and Br are not equal to the atomic mass of CI.
Question - 3 : - What were the limitations of Newlands’ Law of Octaves?
Answer - 3 : -
• Limitations of Newlands’ Law of Octaves are as follows
• Newlands’ Law of Octaves applicable to elements up to Calcium
• Newland assumed there are 56 elements in the nature and no more elements would be discovered in the future.
• To fit elements into table Newland put two elements into one slot. Newland introduced unlike elements with different properties into one column.
• Iron (Fe) was placed away from elements that resembles in properties. Ex: Nickel and cobalt
Question - 4 : - Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, AI, Si, Ba.
Answer - 4 : -
K- K2O
C-C2O4 or CO2
Al- Al2O3
Si-Si2O4 or SiO2
Ba2O2 or BaO
Oxygen is a member of group VI A 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 will help in writing the formulae of their oxides.
(i) Potassium (K) is a member of group IA. Its valency is 1. Therefore, the formula of it is K2O.
(ii) Carbon (C) is a member of group IV A. Its valency is 4. Therefore, the formula of it is C2O4 or CO2.
(iii) Aluminium (Al) belongs to groups III A and its valency is 3. The formula of its oxide is Al2O3.
(iv) Silicon (Si) is present in group IV A after carbon. Its valency is also 4. The formula oxide is Si2O4 or SiO2.
(v) Barium (Ba) belongs to group II A and the valency of the element is 2. The formula of oxide of the element is Ba2O2 or BaO.
Question - 5 : - Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (Any two)
Answer - 5 : - Germanium and Scandium are the element that are left by Mendeleev in his Periodic Table since its discovery.
Question - 6 : - What were the criteria used by Mendeleev in creating his Periodic Table?
Answer - 6 : -
Mendeleev concentrated on various compounds formed by the elements with Hydrogen and
Oxygen. Among physical properties, he observed the relationship between the atomic masses
of various elements while creating his periodic table.
Question - 7 : - Why do you think the noble gases are placed in a separate group?
Answer - 7 : - Noble gases are placed in a separate group because of their inert nature and low concentration in our atmosphere. They are kept in a separate group called Zero group so that they don’t disturb the existing order.
Question - 8 : - How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?
Answer - 8 : -
(a) In the Modern Periodic table elements are arranged in the increasing order of their atomic number. This removes the anomaly regarding certain pairs of elements in Mendeleev’s periodic table.
(b) Atomic number of cobalt is 27 and nickel is 28. Hence, cobalt will come before nickel even though its atomic mass is greater.
c) All isotopes of the same elements have different atomic masses, but same atomic number; therefore they are placed in the same position in the modern periodic table.
Question - 9 : - Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Answer - 9 : - Calcium and Beryllium are similar to Magnesium because all the three elements belong to the same group and have 2 valence electrons in their outer shell.
Question - 10 : - 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 - 10 : -
1. Lithium (Li), Sodium (Na) and potassium (k) have a single electron in their outermost shells.
2. Magnesium (Mg) and Calcium (Ca) have two electrons in their outermost shells
3. Neon (Ne), Argon (Ar and Xenon (Xe) filled outermost shells