Question

Reactivity of transition elements decreases almost regularly from Sc to Cu because of:
(A)- lanthanides contraction
(B)- regular increase in ionisation enthalpy
(C)- regular decrease in ionisation enthalpy
(D)- increase in number of oxidation states

Answer 1

Hint: As along the period, the size decreases along with the increase in the electronegativity. The energy to remove an electron required increases, which in turn determines the reactivity of the element.

Complete step by step solution:
The elements from Scandium to Copper belong to the First transition series of the d-block, where with the increase in the atomic number, the nuclear charge increases, and the atomic radius decreases along the period.
Then, the ionisation energy is the amount of energy required to remove the outermost electron from the orbit in the isolated gaseous state of the atom to form the ion. It is related to the force of attraction between the nucleus and the electrons. As more the nuclear charge, more the attraction of the electrons, and higher is the ionisation energy needed to form the ion. Thus, along the period the ionisation enthalpy increases.
This increase in the ionisation enthalpy thus, leads to the decrease in the reactivity of the element. As along the period, with the decrease in the atomic size, the electronegativity increases, so the enthalpy of hydration of the cation decreases, making the element unreactive.

Therefore, the reactivity of the transition elements decreases almost regularly from Sc to Cu because of option (B)- regular increase in ionisation enthalpy.

Note: The lanthanide contraction is seen in the 5d-series of the d-block elements, so the ionisation enthalpy as well as the reactivity of the elements of 3d is not affected by it.
Also, the oxidation state of the 3d-series elements corresponds to the loss of electrons till Mn, but after that again the number of oxidation states decreases, with the decrease in the atomic size. But along the period, the reactivity of the elements is not dependent on the oxidation state.