Elements of group 1 and 17 have same valency.Explain
Answers
Let us take one element from group 1 and Group 17 to compare their properties
From group 1: Hydrogen
Number of electrons=1
Nearest noble gas-Helium(Atomic number 2)
From group 2:
Number of electrons=9
Nearest noble gas-Neon(Atomic number 10)
Valency is the number of electrons needed to be gained or loosed in order to reach the nearest noble gas configuration
thus, Hydrogen tries to gain 1 electron to attain helium electron configuration
and becomes hydrogen -1 by gaining an electron.
In case of Florine it also tries to gain 1 electron to attain neon's atomic configuration and becomes fluorine -1.
Thus, Elements in group 1 and group 17 have same valency number but not the same sign always.
Try doing this method for other elements and you will find that group 1 will have +1 and group 17 will have -1....