Question

Why \(\mathrm{Sc}(\mathrm{Z}=21)\) 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

Short answer

Scandium's 3d orbitals are empty in stable compounds.

Step-by-step solution

Define a Transition Element

Transition elements, often referred to as transition metals, are elements that have partially filled d-subshells either in their elemental form or in one of their common oxidation states.

Evaluate the Electron Configuration of Scandium

The electron configuration of Scandium ( \(\mathrm{Sc}\)) is \([\mathrm{Ar}] \, 3d^1 \), meaning it has one electron in the 3d subshell. When forming stable compounds, Scandium typically assumes a +3 oxidation state, leaving the 3d orbitals empty.

Analyze the Oxidation States

Scandium's most stable oxidation state is +3, which results in the complete removal of its d electron: \( \mathrm{Sc}^{3+} \) has the configuration \([\mathrm{Ar}] \, 3d^0 \), creating an empty 3d orbital, which is atypical for transition elements, as they usually have partially filled d orbitals.

Explanation of Why Scandium is Not a Transition Element

Since Scandium's stable compound forms have empty 3d orbitals, it doesn't meet the criteria for transition elements, which require having partially filled d-orbitals in at least one stable oxidation state.

Key concepts

Scandium Electron Configuration

Scandium, with the atomic number 21, is a curious case when it comes to the study of transition elements. To understand why Scandium isn't typically classified as a transition element, it's important to delve into its electron configuration.
Scandium's electron configuration is denoted by \[ ext{Sc: } [ ext{Ar}] \, 3d^1 4s^2 \], meaning it has one electron residing in the 3d orbital. This initial configuration seems to suggest it could qualify as a transition element, because it exhibits a partially filled d orbital.
However, as Scandium forms bonds and transitions to a stable +3 oxidation state, this electron is removed from the 3d orbital, resulting in a \[ ext{Sc}^{3+}: [ ext{Ar}] \, 3d^0 \] configuration.
This change results in Scandium having empty d orbitals, which deviates from the typical behavior of transition elements. For an element to be considered a transition metal, it must have partially filled d orbitals in at least one of its stable oxidation states, and it's this requirement that Scandium doesn't fulfill. Thus, despite its initial electron configuration, Scandium's properties under stable conditions don't align with those of transition metals.

Oxidation States

Oxidation states refer to the degree of oxidation (or loss of electrons) of an element in a compound. Transition elements often showcase a variety of oxidation states due to the accessibility of their d electrons.
Scandium, however, predominantly stabilizes in a +3 oxidation state. In this state, all valence electrons are removed, achieving the configuration \[ ext{Sc}^{3+}: [ ext{Ar}] \, 3d^0 \], meaning the 3d orbital is completely empty. This lack of electrons in the 3d subshell under stable conditions is what differentiates scandium from other transition elements, which characteristically have partially filled d orbitals even after oxidation.
Three key points about Scandium's oxidation state:

• Primarily exhibits a +3 oxidation state.
• The only electron in the 3d orbital is lost when oxidized.
• Does not present partially filled d orbitals in its stable state, unlike true transition metals.

In essence, the absence of partially filled d orbitals in its stable oxidation states precludes Scandium from being classified as a true transition element, aligning it with the criteria defined by chemists for transition metals.

Partially Filled d Orbitals

The defining trait of a transition element is the presence of partially filled d orbitals. These orbitals are pivotal in dictating the unique properties and chemistry of transition metals.
When discussing Scandium, although its ground state ( ext{Sc: } [ ext{Ar}] \, 3d^1 4s^2) hints at a partially filled d orbital, this is not maintained in its stable states. Upon forming the \[ ext{Sc}^{3+} \] ion, all electrons are removed from the d orbital, resulting in a completely empty 3d orbital.
Partially filled d orbitals are crucial for several reasons:

• They allow for variable oxidation states, giving transition metals their versatile chemistry.
• They contribute to the formation of colored compounds due to d-d electron transitions.
• They enable strong metallic bonding, allowing for unique magnetic and catalytic properties.

As Scandium lacks these characteristics in its stable compound forms (specifically, no partial occupancy of the d orbitals), it does not completely fit into the transition element category, which relies heavily on the chemistry imparted by those partially filled d orbitals.