Extra Questions for Class 10 Science Chapter 5 Periodic Classification of Elements
Q.1. On what basis Mendeleev classify the elements?
Answer: Mendeleev classified the elements on the basis of their fundamental property, i.e. atomic mass and chemical properties. He reacted the elements with hydrogen and oxygen and formulaeof the hydrides and oxides of the elements were considered as a basic property for their classification.
Q.2. Why was the position of hydrogen not fixed in Mendeleev’s periodic table?
Answer: Electronic configuration of hydrogen is similar to that of alkali metals. Just like alkali metals, it reacts with halogens, oxygen and sulphur to form compounds of similar formulae. For e.g.: H2S and Na2S. On the other hand, hydrogen reacts with metals and non-metals just like halogens. Due to this anomalous behaviour of hydrogen, Mendeleev was unable to fix a position for it.
Q.3. What is Newlands’ law of octaves? Why is it called so?
Answer: Newland arranged the known elements in increasing order of atomic mass. He found that the every eighth element had properties similar to that of first one. He compared it to the octaves found in music and thus it is called Newland’s law of octaves.
Q.4. Elements of the same group have similar properties. Why?
Answer: Elements present in the same group have a same number of electrons in their valence shells. On moving down a group, the number of shells increases but the number of electrons in the valence shell remains the same. That’s why the elements of the same group have similar properties.
Q.5. State Dobereiner’s law of triads.
Answer: Dobereiner created some groups having three elements in each group. He called these groups triads. He stated that when the elements in a group are arranged in increasing order of their atomic masses then the atomic mass of the middle element is approximately the average of the atomic masses of two other elements.
Q.6. Why did Mendeleev left some gaps in his periodic table?
Answer: Mendeleev left some gaps in his periodic table because he predicted the existence of some elements which had not been discovered at that time.
Q.7. In which period and group would you place the elements with the following electronic configurations.
(i) 2,8 (ii) 2,5
(i) The electronic configuration is 2,8. It means that the given element contains 2 electrons in K shell and 8 electrons in L shell. Thus the element contains 2 shells. Thus it belongs to the second period. Secondly, there are 8 electrons in the valence shell (completely filled) which is true for the 18th group.
(ii) The electronic configuration is 2,5. It means that the given element contains two shells, thus the period is 2nd. Secondly, there are 5 electrons in the valence shell which is true for the 15th group.
Q.8. How many groups and periods are present in the modem periodic table?
Answer: Vertical columns in the periodic table are called “groups”. There are 18 such groups in modern periodic table. Horizontal rows are called “periods”. There are 7 such periods in modern periodic table.
Q.9. A metal M forms an oxide having the formula M2O3. It belongs to the third period in the modern periodic table. Write the atomic number and valency of the metal.
Answer: Since the Molecular formula of oxide of a metal (M) is M2O3 where valency of oxygen is 2 and the valency of the metal is 3. Since the metal has a valency of 3 and belongs to the third period, so the metal is Aluminium.
Q.10. What is the main difference between Mendeleev’s and modern periodic law?
Answer: The main difference between the Mendeleev’s and modern periodic law is that Mendeleev considered the properties of elements as periodic function of their atomic masses while modern periodic law considers the properties of elements as periodic function of their atomic numbers.
Q.11. How does the atomic size of the elements very along a period?
Answer: The atomic size increases on moving down the group. This is because of the increase in a number of shells due to which the distance between the outermost electron and the nucleus increases.
The atomic size decreases on moving along a period. This is because of the increase in the effective nuclear charge on moving from left to right which tends to pull the electrons towards the nucleus.