Question

Fluorine has the highest electronegativity among the $n{{s}^{2}}n{{p}^{5}}$ group on the Pauling scale, but the electron affinity of fluorine is less than that of chlorine because:

(A) the atomic number of fluorine is less than that of chlorine.

(B) fluorine being the first member of the family behaves in an unusual manner.

(C) chlorine can accommodate an electron better than fluorine by utilising its vacant 3d- orbital.

(D) small size, high electron density and an increased electron repulsion makes addition of electrons to fluorine less favourable than that in the case of chlorine.

Answer 1

Hint: For an element to have high electronegativity it must have high electron density to pull the shared pair of electrons towards itself. However, for an element to have high electron affinity, the atom must readily accept an electron with relatively less energy.

Complete step by step answer:

Electronegativity is a measure of the tendency of an atom to attract the shared pair of electrons. The Pauling scale is commonly used to measure electronegativity.

Electron affinity is defined as the change in energy (kJ/mole) of a neutral atom present in a gaseous state when an electron is added to the atom to form a negative ion. It basically tells the tendency of the atom to gain an electron.

Fluorine is above chlorine in the 17th group of the modern periodic table. Hence it has a smaller size than chlorine atom although it has 7 electrons in its valence shell. Due to this the electron density increases. Due to high electron density fluorine can attract the shared pair of electrons hence having higher electronegativity.

Due to small size, the inter electronic repulsions are very high in fluorine atoms and hence cannot easily gain an electron. On the other hand, chlorine has the same number of electrons and larger size. This is the reason why chlorine can easily accept an electron in its valence shell i.e. has higher electron affinity than fluorine.

So, the correct answer is “Option C”.

Note: Fluorine has an electronegativity of 4 and chlorine has an electronegativity of 3.16 as measured on the Pauling scale. In the options given above option (A) is a valid statement. However, it does not explain why electronegativity of fluorine is more than chlorine and electron affinity of chlorine is more than fluorine.