Solutions of Periodic Classification Of Elements (Page No - 305) - Chemistry Lakhmir Singh, Class 10 | Extra Documents, Videos & Tests for Class 10 PDF Download

Question 35:
(a) Explain why, the first period of the modern periodic table has only two elements whereas second period has eight elements
 (b) Why do elements in the same group show similar properties but the elements in different groups show different properties ?
 (c) For each of the following triads, name the element with the characteristics specified below :

(d) State one reason for keeping fluorine and chlorine in the same group of the periodic table.
 (e) What are the merits of the modern periodic table of elements ?
 Solution :
(a) The 1st period has two elements because the 1st electron shell of an atom c a n take a maximum of two electrons only. The 2nd period of the periodic table has 8 electrons because the maximum no. of electrons which can be put in the 2nd shell of an atom is 8.
(b) The elements in the same group show similar properties because they have similar electronic configuration (having the same number of valence electrons) whereas the elements of different groups have different electronic configurations (different number of valence electrons) due to which they show different properties.
(c) (i) F; Br
(ii) Li; Li
(d) Fluorine and chlorine have been placed in the same group because both of them have 7 valence electrons.
(e) Merits of modern periodic table:
(i) The modern periodic table is based on the atomic numbers of elements which is the most fundamental property of elements.
(ii) It helps us to understand why elements in a group show similar properties but elements in different groups show different properties.
(iii) It explains the reasons for the periodicity in properties of elements.
(iv) It tells us why the properties of elements are repeated after 2, 8, 18 and 32 elements.

Question 36:
(a) What is a group in the periodic table ? In which part of a group would you separately expect the elements to have (i) the greatest metallic character (ii) the largest atomic size ?
 (b) In what respects do the properties of group 1 elements differ from those of group 17 elements ? Explain with examples by taking one element from each group.
 (c) From the standpoint of atomic structure, what determines which element will be the first and which the last in a period of the periodic table ?
 (d) Explain why, the properties of elements are repeated after 2, 8,18 and 32 elements in the periodic table.
 (e) What are the advantages of the periodic table ?
 Solution :
(a) The vertical columns in a periodic table are called groups.
(i) The greatest metallic character is found in the elements in the lowest part of the group.
(ii) The largest atomic size is found in the lowest part of the group.
(b) Group 1 elements have 1 valence electron and are ionic in chemical reactions. Their chemical reactivity increases down the group. They are electropositive in nature and it increases down the group.
Whereas, the elements of group 17 have 7 valence electrons. They all are non-metals. Their chemical reactivity decreases down the group. They are electronegative in nature and it decreases down the group.
(c) The no. of valence electrons in the atoms of elements decides which element will be the 1st element in a period and which will be the last in a period.
(d) The properties of elements are repeated after 2, 8, 18 and 32 elements in the periodic table because the electronic configurations of the elements are repeated in this manner.
(e) Advantages of the periodic table:
(i) It has made the study of chemistry systematic and easy.
(ii) It is easier to remember the properties of an element if its position in the periodic table is known.
(iii) The type of compounds formed by an element can be predicted by knowing its position in the periodic table.
(iv) It is used as a teaching aid in chemistry in schools and colleges.