Question

An element $X$ belongs to the ${3^{rd}}$ period and group 17 of the periodic table. State its (i) electronic configuration, (ii) valency. Justify your answer with reasoning.

Answer 1

Counting the periods and matching it with the periods of that of halogens, we find that period three is for Chlorine. So the element is Chlorine. Chlorine has an atomic number equal to 17.

Complete step by step answer: Since the element is in group 17
So, it is a halogen.
Halogens start from the second period.
Counting the periods and matching it with the periods of that of halogens, we find that period 3 has Chlorine.
So, the element is Chlorine.
Element X has 17 electrons i.e. 7 electrons in its valence shell, so it will accept 1 electron, hence, its valency is 1.
The electronic configuration of chlorine is $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}$
Element X is chlorine which requires only one electron to complete its octet. Hence, its valency is 1.
The first two electrons will go in the 1s orbital.
Since 1s orbital can only hold two electrons, the next two electrons for Chlorine will move into the 2s orbital.
The next six electrons will move in the 2p orbital.
The p orbital can hold up to six electrons.
The further two electrons will occupy the 3s.
Since the 3s is full, the remaining 5 electrons will move into a 3p orbital.

Note: Do not get confused between group and period, make sure to remember that groups are present vertically and periods are present horizontally. Since the element is in group 17. So, it is a halogen. Halogens start from the second period. There is chlorine in the third period.