Problem 43 - A Complete Resource Book in Chemistry for JEE Main

Question

Why Sc \((Z=21)\) is not considered as a transition element? (a) properties of \(\mathrm{Sc}\) are similar to alkali metals (b) \(3 \mathrm{~d}\) orbitals are empty in its stable compound (c) stable oxidation number of \(\mathrm{Sc}\) is \(+2\) (d) atomic volume of \(\mathrm{Sc}\) is very large

Step-by-Step Solution

Answer

Sc is not considered a transition element because its 3d orbitals are empty in its stable compound, ruling out partially filled d-orbitals.

Step 1: Definition of transition elements

Transition elements are defined as those elements which have partially filled d-orbitals in their atoms or at least one of their oxidation states.

Step 2: Electronic configuration of Scandium

The electronic configuration of scandium (Sc, Z=21) is \([\text{Ar}] 3d^1 4s^2\).

Step 3: Oxidation states of Scandium

In its most stable oxidation state, scandium loses all of its outer electrons resulting in \([\text{Ar}] 3d^0\), thus the 3d-orbitals become empty.

Step 4: Identify the correct reason

Option (b) states that the 3d orbitals are empty in its stable compound. Since a transition element must have partially filled d-orbitals in one of its oxidation states, option (b) correctly explains why Sc is not considered a transition element.

Key Concepts

Scandium

Scandium (Sc), with an atomic number of 21, is a metal found in the d-block of the periodic table, right at the beginning of the transition elements series. Despite its position, scandium is unique because it does not behave like the typical transition elements.
Transition elements are characterized by the presence of partially filled d-orbitals in at least one of their oxidation states. However, scandium stands out due to its peculiarity when forming compounds.
In its most stable oxidation state, scandium forms compounds where its d-orbitals are empty, deviating from the common trait among transition elements. This is why scandium is often not regarded as a true transition element, despite its d-block positioning in the periodic table.

d-orbitals

The d-orbitals are a set of five orbitals that can hold a total of 10 electrons. They have unique shapes and orientations, which contribute to the complex chemistry of transition metals. In the context of scandium, the d-orbitals play a critical role in classifying it as a non-transition element.
For an element to be a transition metal, it must have a partially filled d-orbital in at least one oxidation state. This means there must be some electrons in the d-orbitals, enabling interesting chemical characteristics such as variable oxidation states and colored compounds.
In scandium, the 3d-orbitals are occupied when it is in its neutral atomic state, specifically having 1 electron, as its electron configuration is represented as \([ ext{Ar}] \,3d^1 4s^2\). However, when scandium forms its most stable ion, it loses these electrons, resulting in empty 3d-orbitals, confirmed by its configuration \([ ext{Ar}] \,3d^0\). This absence of electrons in the d-orbitals in its stable state solidifies why scandium is an exception among transition elements.

Electronic Configuration

Electronic configuration refers to the distribution of electrons across the various atomic orbitals. Understanding the electronic configuration of an element like scandium gives insight into its chemical behavior.
Scandium has the electronic configuration \([ ext{Ar}] \,3d^1 4s^2\) in its atomic form. This means that after the argon core, there is one electron in its 3d-orbitals, and two electrons in its 4s-orbitals.
When scandium forms compounds, it commonly does so in the +3 oxidation state, as is typical for Group 3 elements. This involves losing its 3d electron and both 4s electrons. Thus, its configuration in the common Sc³⁺ ion becomes \([ ext{Ar}]\), with completely emptied d-orbitals.
This complete electron loss from the d-orbitals when forming the ion further underscores why scandium doesn't fit the transition metal criteria.

Oxidation States

An oxidation state for an element in a compound represents the degree of oxidation it has, i.e., how many electrons it has gained or lost. Understanding oxidation states helps clarify why scandium is not a transition element.
Scandium typically exhibits a +3 oxidation state, which means it loses three electrons — one from the 3d orbital and two from the 4s orbital. In this +3 state, its electron configuration is \([ ext{Ar}]\), with empty 3d-orbitals.
The lack of partially filled d-orbitals in its most stable oxidation state means that scandium does not display some of the unique features of transition metals, such as displaying various oxidation states or forming colored, complex ions.
This distinct singular oxidation state of +3 without partially filled d-orbitals is why scandium does not meet the criteria for being a transition element, unlike many of its d-block counterparts which exhibit multiple oxidation states.