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NCERT Solutions for Class 10 Science Chapter 5
Important Questions with answers for CBSE Exams – 2018
Periodic Classification of elements – Class 10 Chemistry
What are the discrepancies in Mendeleev’s periodic table?
List two observations which posed a challenge to Mendeleev’s Periodic law.
(i) Position of isotopes: Isotopes of various elements cannot be filled into Mendeleev’s table. Isotopes are the elements having similar chemical properties but different atomic masses.
(ii) Anomalies of atomic weights and properties of elements: If arranged according to gradation in chemical properties, elements of higher atomic weights are to be placed before those of lower atomic weights at some place. This is contrary to Mendeleev’s law. For example, argon with atomic mass of 40 comes before potassium with atomic mass 39. Iodine (at. mass = 127) comes after tellurium (at. mass = 127.8).
(iii) Falling of dissimilar elements in same group: Some chemically similar elements fall in different group chemically dissimilar elements are placed together. For example, copper, silver and gold are placed in the same group with alkali metals. Mercury, lead and barium which possess similar properties fall into different groups.
(iv) Hydrogen resembles both alkali metals (group I) and halogens (group VII) and thus no fixed position could be given to hydrogen in the periodic table.
Write two reasons responsible for the late discovery of noble gases.
(a) Noble gasses are inert.
(b) These are present in nature in minute amounts.
State Modern Periodic Law. What is the Modern Periodic Table called?
Modern Periodic Law : the physical and chemical properties of elements are the function of their atomic numbers. The Modern Periodic Table is called ‘long form’ of the periodic table.
Mention the position of (i) hydrogen and (ii) isotopes of same element in the modern periodic table.
(i) Hydrogen is placed at the top of group IA.
(ii) Isotopes of the same elements are placed in the same slot.
Why is atomic number of an element more important to a chemist than its atomic mass?
Atomic number is considered to be a more appropriate parameter than atomic mass for classification of elements in a periodic table. Why?
The properties of element depend upon the number of electrons present in the valence shell which are related to atomic number. Thus the properties of different elements can be compared if we know their atomic numbers. On the other hand, atomic mass can in no way determine the chemical properties of elements, because is does not vary regularly with gradation in the chemical properties of elements.
The atomic number of an element ‘X’ is 20. Write
(a) its valency,
(b) whether it is a metal or non-metal,
(c) the formula of compound formed when the element ‘X’ reacts with an element ‘Y’ of atomic number 8.
Justify your answer in each case.
(a) Electronic configuration of X is 2, 8, 8, 2. Its valency is 2.
(b) It is a metal. It belongs to group 2 and can lose 2 electrons easily.
(c) Formula of compound of X and Y is XY. Since valency of Y is -2 (2, 6).
The element Li, Na and K, each having one valence electron, are in period 2, 3 and 4 respectively of Modern 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.
(a) Group 1. These all have one valency.
(b) Li. Its outermost orbit is very near to the nucleus and so removal of electron is a little difficult.
(c) K has largest atomic radius. It occupies three shells.
Give reasons for the following:
(a) Metals have tendency to form cations.
(b) Fluorine atom is smaller than chlorine atoms.
(c) Non-metals have tendency to gain electrons.
(a) As the effective nuclear charge on the valence electrons in the metals is comparatively less, they have tendency to form cations.
(b) Fluorine atom is smaller than chlorine atom as it has less number (2) of shells compared to that in chlorine (shells 3).
(c) Non-metals have high nuclear attraction on the outermost shells and thus these have tendency to attract/gain electrons.