Question

Potassium (at no. 19) and chlorine (at no. 17) react to form a compound. Explain on the basis of electronic concepts.

Answer 1

Hint Potassium loses an electron as it undergoes the process of oxidation and on losing an electron it forms a cation. On the other hand chlorine gains an electron and becomes an anion by undergoing reduction. The loss of electrons by atoms or molecules during a reaction is known as Oxidation and the gain of electrons by atoms or molecules during a reaction is known as Reduction.

Complete Step By Step Solution: Electronic configuration of chlorine is: $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{5}}$
p subshell contains 3 orbitals i.e. at maximum or a fully filled p-subshell can have six electrons according to Pauli Exclusion Principle. So, on gaining an electron the configuration of chlorine becomes $3{{p} ^ {6}}$and it becomes an anion by undergoing reduction. Here chlorine behaves as an Oxidizing Agent because it removes the electrons from other reactants in a reaction and gains them for itself. Thus it reduces itself by gaining electrons.
Electronic configuration of potassium is: $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}4{{s}^{1}}$
Here potassium loses an electron of 4s subshell and forms a cation. On losing an electron the atom acquires positive charge that’s why it became a cation. The potassium atom undergoes oxidation which is the process of losing electrons. The potassium atom behaves as Reducing Agent i.e. it itself loses an electron to provide it to the recipient reactant which gains electrons from it.
So, potassium gets oxidized but behaves as a Reducing Agent and the chlorine gets reduced but behaves like an Oxidizing Agent.

Additional Information:
To check whether an atom got oxidized in a chemical reaction or not simply look at its oxidation number before and after the reaction, if the oxidation no. in the product is more than the oxidation no. in the reactant then the atom got oxidized i.e. it lost electron, example of oxidizing agents include halogens, oxygen, etc. Good oxidizing agents are those which attract electrons easily.

Note: Pauli Exclusion Principle states that not more than two electrons can occupy the same orbital and the two electrons in an orbital must have opposite spins. According to this principle no two electrons can have the same set of all four quantum numbers. It is called the exclusion principle because other electrons are excluded from having the same set of quantum numbers as assigned to any other electron.