16. What is the number of elements in the:
(a) first period, and
(b) third period of the Modern Periodic Table?
17. (a) What are groups in the periodic table?
(b) State the property which remains same on moving
down in a group in the periodic table.
18. What is the common feature of electronic configurations
of the elements at the end of periods 2 and 3?
19. Give two examples each of:
(a) group 1 elements
(b) group 17 elements
(c) group 18 elements
Answers
16.a) 2 elements :- hydrogen and helium
b) 8 elements
17.a) In chemistry, a group is a column of elements in the periodic table of the chemical elements. There are 18 numbered groups in the periodic table; the f-block columns are not numbered
b) As we move down in a group the number of shells increase and valence electrons in a group remains the same. Valency: Valency of all elements is the same.
18. the last elements of period 2 and period 3 are neon and argon. there are 8 valence electron in this element and hence has fully filled electronic configuration and is a stable element.
19.a) Group 1A (or IA) of the periodic table are the alkali metals: hydrogen (H), lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). These are (except for hydrogen) soft, shiny, low-melting, highly reactive metals, which tarnish when exposed to air.