Question

Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be ________ and ________ respectively.
A) 1,1
B) 1,0
C) 0,1
D) 0,0

Answer 1

The valency of an element is its combining capacity, in form of gain, loss or share of electrons to complete its octet.
The valency can be known by writing electronic configuration. The last shell electrons will tell us how many electrons have to be lost, gained or shared.
Group 17 has valency of 1, as it can accept 1 electron and complete octet. While group 18 are called noble gases or inert gases, and it has a valency of 0, as they don’t need any electrons to complete octet, as it is already full octet.

Complete answer:
Valency is the capacity of an element or atom to hold or make bonds with other atoms.
This bond formation mainly involves gain of electron, loss of electron or sharing of electron.
This transaction of electrons is mainly due to achieving stability by completing octet or nearest noble gas configuration.
Group 1 elements have 1 electron in valence shell, so loses 1 electron and valency is 1.
Group 2 elements have 2 electrons in valence shell, so loses 2 electrons and valency is 2.
Group 13 elements have 3 electrons in valence shell, so loses 3 electrons and valency is 3.
Group 14 elements have 4 electrons in valence shell, so it can neither lose 4 electrons or gain 4 electrons, so it shares 4 electrons with other atoms, hence valency is 4.
Group 15 elements have 5 electrons in valence shell, so they can gain 3 electrons to achieve octet and hence valency is 3.
Group 16 elements have 6 electrons in valence shell, so they can gain 2 electrons to achieve octet and hence valency is 2.
Group 17 elements have 7 electrons in valence shell, so they can gain 1 electron to achieve octet and hence valency is 1.
Group 18 elements have 8 electrons in the valence shell, so they can neither gain electrons or lose an electron. They already have octet and hence valency is 0.
Let us write the electronic configuration of Neon, and Chlorine using atomic number(Z).
Neon (Z=10) = 2,8
Chlorine (Z=17) = 2,8,7
Here, we are asked valency of Neon and Chlorine, which is group 18 and group 17 element, hence the valency becomes 0 and 1 respectively, which we can see from configuration also, that Neon has already 8 electrons in valence shell, and Chlorine has 7 electrons, so 1 electron is needed to complete octet.

So the correct option is (C) 0, 1.

Note: In periodic table, the periodic trend for valency is that it increases from group 1 to group 14 with value of 1 to 4, and then decreases from group 15 to group 18 with value of 3 to 0 respectively.