Question

What is the valency of potassium with atomic number 19?
A. 0
B. 1
C. 2
D. 3

Answer 1

Hint: Valency of an element is determined by octet rule. To solve this question we should first write the electronic configuration of the element with respect to its shell and then compare it with nearest noble gas configuration.

Complete step by step answer:
Valency: The combining capacity of an atom is known as its valency. The number of bonds that an atom can form as part of a compound is expressed by the valency of the element. Valency is a measure of the ability of an atom to bond with other atoms.
To find valency of potassium we follow these steps mentioned below:
So, potassium has atomic number 19. So by applying octet rule we get:-
In K shell: - 2 electrons (fully filled)
In L shell: - 8 electrons (fully filled)
In M shell: - 8 electrons (fully filled)
In N shell: - 1 electron (not filled)
Electron configuration will be: - 2,8,8,1
So, the valence shell contains one valence electron. So, it will donate this electron to attain noble gas configuration of argon. It will become ${{K}^{+}}$.

Additional information:
Noble gases have a completely filled outermost shell and that’s why they are least reactive. Other element’s reactivity depends upon their ability to attain the noble gas configuration. If the outermost shell has 8 electrons then the element is said to have a complete octet. By gaining, sharing and losing the electrons the atoms complete their outermost orbital and make an octet. The reactivity of an atom is described by the total number of electrons lost, gained or shared to complete its octet and it also determines the valency of the atom.

Note: Difference between Valency and Oxidation Number: Valency is different from the oxidation number, and it has no sign.
For example, the valency of nitrogen is 3, whereas it can have oxidation numbers from -3 to +5. The oxidation number is the hypothetical charge of an atom in a molecule or ion, and it is a measure of its apparent capacity to gain or lose electrons within that species.