Elements have been arranged in the following sequence on the basis of their increasing atomic masses. F, Na, Mg, Al, Si, P, S, Cl, Ar, K
(a) Pick two sets of elements which have similar properties.(b) The given sequence represents which law of classification of elements?

(a) The two sets of elements that have similar properties are

(i) F and Cl - Fluorine, Chlorine and Bromine have the same valence electrons and have a non-metallic character.  All three atoms exist as diatomic molecules. The valence electrons are 7 , and these all atoms try to gain one electron to attain the nearest noble or inert gas configuration. So, the valency will be +1.

(ii) Na and K - The elements of potassium and sodium have similar chemical properties because they have the same number of valence electrons and form +1 ions and belong to the same group in the periodic table.

(b) The given sequence represents Newland's Law of Octave of classification of elements- Newland's Law of Octaves states that when Elements are arranged in increasing order of Atomic mass, the properties of every eighth element starting from any element are a repetition of the properties of the starting element. The Law of Octaves was true only for Elements up to Calcium.