CBSE Class 10 Science 2017 · Paper 531/1 · Question 20
(a) Write the number of groups and periods in the Modern Periodic Table. How does the atomic size vary down a group in the periodic table ? State its reason.
(b) The electronic configurations of four elements A, B, C and D are as follows :
A – (2, 8, 7); B – (2, 8, 1); C – (2, 8, 2); D – (2, 8, 8, 2)
- (i)Which amongst these elements will form acidic oxide and why ?
- (ii)Which amongst these elements has the smallest atomic radius and why ?
- (iii)Out of these select those two elements which have same valency and form compounds by ionic bonds.
Answer
18 groups, 7 periods; atomic size increases down a group; A forms the acidic oxide and is smallest; C and D form ionic compounds.
(a) There are 18 groups and 7 periods. Atomic size increases down a group, because a new shell is added at each step down, which increases the distance between the outermost electrons and the nucleus; this outweighs the increase in nuclear charge.
(b) (i) A (2, 8, 7): it has seven valence electrons and gains one electron to complete its octet, so it is a non-metal, and oxides of non-metals are generally acidic. B, C and D are metals, whose oxides are basic. (ii) A: A, B and C all have three shells, and A has the greatest nuclear charge (17 protons), which pulls the electrons closest; D has four shells and is the largest. (iii) C and D: each has two valence electrons and so a valency of 2, and both lose these electrons to form positive ions, that is, they form compounds by ionic bonds.
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