Question1. What are the merits and demerits of newland’s classification?
Solution:
Merits
(i) This classification worked well for lighter elements only up to Ca.
(ii) This classification gave us a relation between the properties of the elements and their atomic masses.
(iii) It was shown by this classification for the first time that there exists a periodicity in the properties of the elements.
Demerits
(i) This classification failed when the heavier elements beyond Ca were arranged according to Newland’s law of octaves.
(ii) At the time of this law, noble gases were unknown. When noble gases were discovered, neon (Ne) between F and Na, and argon (Ar) between Cl and K, it becomes the ninth element and not the eighth which has the similar properties.
Question2. What is achievement of Dobereiner’s law of traids?
Solution:
The law recognised the relation between atomic weight of an element and its chemical properties for the first time.
Question3. State two reasons for rejecting law of octaves.
Solution:
(i) The law did not extend properly beyond the element calcium.
(ii) The law did not provide any specific place for hydrogen.
Question4. What are Metalloids?
Answer:
Those elements which show some properties of both metals and non-metals are called metalloids. Examples: Boron, Silicon, Germanium, Arsenic, Antimony, Tellurium and Polonium.
Question5. Explain clearly, why atomic number of an element is most important to the chemist than its relative atomic mass.
Answer:
Atomic number corresponds to the number of electron in an atom or it reflects the electronic configuration of the element. The elements having similar electronic configurations can then be placed together in the same group, it helps in the systematic classification of elements.
Question6. Write two reasons responsible for the late discovery of noble gases. [CBSE 2013]
Answer:
Noble gas elements were not present in earth crust as minerals like other elements and were present in air to a very small extent.
Their atoms have stable electronic configuration of their outermost shells also called valence shells. (2 in case of He and 8 for other elements). They do not combine with atoms of other elements.
Question7. Identify the elements with the following property and arrange them in increasing order of their reactivity.
(a) An element which is a soft and reactive metal.
(b) The metal which is an important constituent of limestone.
(c) The metal which exists in liquid state at room temperature. [NCERT Exemplar]
Answer:
(a) Alkali metals are soft and reactive. For example, Na, K, etc.
(b) Limestone is calcium carbonate, therefore, the important constituent of limestone is calcium.
(c) Metals which exists in the liquid state at room temperature are Mercury (Hg).
Question8. List three main anomalies of Mendeleev’s classification of elements.
Answer:
Anomalies or defects of Mendeleev’s periodic classification of elements are as follows:
Position of hydrogen.
Position of isotopes: Isotopes of an element were not assigned separate places.
Elements which are chemically similar such as gold and platinum have been placed in separate groups.
Question9. (a) State modern periodic law.
(b) What are the advantages of the long form of the periodic table over Mendeleev’s periodic table?
Answer:
(a) According to modern periodic law “physical and chemical properties of elements are periodic function of their atomic numbers”.
(b) (i) Classification of elements on the basis of atomic number is more closer to chemical properties.
(ii) No separate places for isotopes of an element are required.
(iii) In case of Ar and K, Ar has less atomic number so it should be placed before K according to the increasing order of atomic number.
Question10.Two elements X and Y have atomic number 12 and 17 respectively.
(i) Write the electronic configuration of both.
(ii) Which type of bond will they form?
(iii) Write the formula of the compound formed by their combination (in terms of X and Y).
Answer:
(i) The electronic configuration of element X (Z = 12) = 2, 8, 2
The electronic configuration of element Y (Z = 17) = 2, 8, 7
(ii) The bond formed will be of ionic nature. One atom of X will transfer 1 electron each to two atoms of Y.
(iii) The formula of the compound is:
Periodic Classification of Elements Class 10 Extra Questions with Answers Science Chapter 5, 4
Question11. 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) Lithium (Li), Sodium (Na), Potassium (K)
(b) Magnesium (Mg), Calcium (Ca)
(c) Helium (He), Neon (Ne), Argon (Ar)
Question12.Two elements X and Y belong to group 1 and 2 respectively in the same period of periodic table. Compare them with respect to: [CBSE 2011]
(i) the number of valence electrons in their atoms
(ii) their valencies
(iii) metallic character
(iv) the sizes of their atoms
(v) the formulae of their oxides
(vi) the formulae of their chlorides.
Answer:
X and Y belong to same period.
X belongs to group ‘1’.
Y belongs to group ‘2’.
(i) Valence electron in X is 1 whereas valence electrons in Y are 2.
(ii) The valency of X is 1 whereas valency of Y is 2.
(iii) X is more metallic than Y because metallic character decreases on moving from left to right in a period.
(iv) The size of X is more than Y because size of the atom decreases on moving from left to right in a period.
(v) Oxide of X = X2O, Oxide of Y = YO
(vi) Chloride of X = XCl, Chloride of Y = YCl2
Question13.The element Li, Na and K, each having one valence electron, are in period 2, 3 and 4 respectively of the Modem Periodic Table.
(a) In which group of the periodic table should they be?
(b) Which one of them is least reactive?
(c) Which one of them has the largest atomic radius? Give reason to justify your answer in each case. [CBSE 2013]
Answer:
(a) Since the elements have one valence electron, they are placed in group 1 (Alkali metals) in the order Li (period = 2); Na (period = 3); K (period = 4).
(b) Since the reactivity of the elements increases down a group, the element Li is the least reactive chemically.
(c) Since the atomic size increases down a group, the element K has the large atomic size or atomic radius out of these elements.
Question14.Explain why are the following statements not correct:
(a) All groups contain metals and non-metals.
(b) Atoms of elements in the same group have the same number of electron(s).
(c) Non-metallic character decreases across a period with increase in atomic number.
(d) Reactivity increases with atomic number in a group as well as in a period. [2010]
Answer:
(a) Because all groups do not contain metals and non-metals. For example, Alkali metal group contains metals only.
(b) Atom of the elements in the same group have same number of electrons in the valence shell.
(c) Non-metallic character increases across a period with increase in atomic number.
(d) Reactivity increases with atomic number in a group but not across a period.
Question15. Why did Mendeleev leave some gaps in his Periodic Table?
Answer:
Mendeleev left some gaps in is Periodic Table for the elements yet to be discovered. He even predicted the properties of these elements by studying the properties of the neighbouring elements.
