Q.3 (ii) Answer the following sub-questions regarding the modern periodic table :
(a) Write modern periodic law.
(b) Write the number of periods and groups.
(c) What is the valency of halogens ?
Best answer
Answer: (a) Properties of elements are a periodic function of their atomic number; (b) 7 periods and 18 groups; (c) 1.
(a) Modern periodic law: the physical and chemical properties of the elements are a periodic function of their atomic numbers. Elements are arranged in increasing order of atomic number.
(b) The modern periodic table has 7 horizontal rows called periods and 18 vertical columns called groups.
(c) Halogens (F, Cl, Br, I) are in group 17 and have 7 valence electrons. They need one more electron to complete their octet, so their valency is 1.
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