Question

1) Element C,N,O have been placed in 2nd period.

2) Hologen are monovalent

Answer 1

1) because all these have 2 valence shell in its outermost shell.and hs 2 no. of shell

2)their valency is -1

Answer 2

(a) Element C, N, O and F are all placed in the second period in the periodic table because all these elements contain same number of electron shells i.e. two shells.

(b) Group 17 elements i.e. F, Cl, Br and I have seven electrons in their outermost shells. By gaining one electron, they achieve stable noble gas configuration. So, they have valency = 1 and are hence monovalent.