Q.On what ground can you say that scandium (Z = 21) is a transition element but zinc (Z = 30) is not?

Concept: Transition Element Definition

A transition element is defined as an element that has an incomplete d-subshell in its ground state or in any of its common oxidation states.

Reasoning

1. Scandium (Z = 21) has the ground-state configuration $[\text{Ar}]\,3d^1 4s^2$. In its common +3 oxidation state, it loses the $4s^2$ and $3d^1$ electrons, giving $[\text{Ar}]$ — a completely empty d-subshell. However, scandium is still considered a transition element because its ground state has an incomplete d-subshell ($3d^1$). …

Definition used (IUPAC). An element is a transition element if it has a partially filled $d$-subshell in the neutral atom or in any of its stable oxidation states.

Scandium $(Z=21)$:

• Atom: $[\text{Ar}]\,3d^1\,4s^2$ — the $3d$ subshell holds one electron, i.e. it is incomplete.
• Therefore scandium satisfies the definition and is a transition element (even though its common ion $\text{Sc}^{3+}$ is $[\text{Ar}]\,3d^0$, the neutral atom already has a partially filled $3d$).

Zinc $(Z=30)$:

• Atom: $[\text{Ar}]\,3d^{10}\,4s^2$ — the $3d$ subshell is completely filled.
• Common ion: $\text{Zn}^{2+} = [\text{Ar}]\,3d^{10}$ — still completely filled. …

Method: Electronic Configuration & Definition-Based Analysis

This method uses the IUPAC definition of transition elements and compares the ground-state electronic configurations of the two elements.

Definition (The "Why")

A transition element is defined as an element that has an incomplete d-subshell either in its neutral atom or in any of its common oxidation states.

Steps

Step 1: Write the ground-state electronic configuration of scandium (Z = 21)

• Sc: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^1$
• In shorthand: $[\text{Ar}] \, 4s^2 3d^1$

Step 2: Check the d-subshell in the neutral atom

• The 3d subshell has only 1 electron — it is incomplete (maximum capacity is 10).
• Therefore, scandium satisfies the definition in its neutral state.

Step 3: Write the ground-state electronic configuration of zinc (Z = 30)

• Zn: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10}$
• In shorthand: $[\text{Ar}] \, 4s^2 3d^{10}$

Step 4: Check the d-subshell in the neutral atom

• The 3d subshell has 10 electrons — it is completely filled.
• Zinc does not satisfy the definition in its neutral state.

Step 5: Check the common oxidation state of zinc …

The Correct Definition (CBSE / NCERT Standard)

A transition element is defined as an element that has a partially filled d-subshell either in its ground state or in any of its common oxidation states.

This is the key: partially filled d-orbital — not just "belongs to d-block."

Common Mistake #1: Confusing d-block with transition elements

The error:

Students say: "Scandium is in the d-block, so it is a transition element. Zinc is also in the d-block, so it should also be a transition element."

Why it's wrong:

Being in the d-block is necessary but not sufficient. The definition requires a partially filled d-subshell in the atom or a common ion.

• Scandium (Z = 21):

  Ground state: $[Ar]\,3d^1\,4s^2$ → d-subshell is partially filled (1 electron).

  Common oxidation state: $\ce{Sc^{3+}}$ has $[Ar]$ → d-subshell is empty.

  But since the ground state has a partially filled d-orbital, it qualifies.

• Zinc (Z = 30):

  Ground state: $[Ar]\,3d^{10}\,4s^2$ → d-subshell is completely filled.

  Common oxidation state: $\ce{Zn^{2+}}$ has $[Ar]\,3d^{10}$ → still completely filled.

  So it never has a partially filled d-subshell → not a transition element.

How to avoid:

Always check both the atom and its common ions for a partially filled d-orbital. Don't just look at the periodic table block.

Common Mistake #2: Forgetting to check common oxidation states

The error:

Students only check the ground state configuration and ignore ions.

Why it's wrong:

Some elements (like copper, silver, gold) have a filled d-subshell in the ground state but a partially filled d-subshell in a common oxidation state. They are transition elements.

• Example: Copper ($[Ar]\,3d^{10}\,4s^1$) has a filled d-subshell in ground state, but $\ce{Cu^{2+}}$ ($[Ar]\,3d^9$) is partially filled → it is a transition element.

How to avoid:

Always write the electronic configuration of the most common ion(s) and check for partial filling.

Common Mistake #3: Miswriting electronic configurations

The error:

Students write $\ce{Sc}$ as $[Ar]\,4s^2\,3d^1$ and then say "d-orbital has 1 electron, so it's partially filled" — correct.

But for $\ce{Zn}$, they write $[Ar]\,4s^2\,3d^{10}$ and say "d-orbital has 10 electrons, so it's filled" — correct.

However, they sometimes forget the 4s orbital and write $\ce{Sc}$ as $[Ar]\,3d^3$ (wrong) or $\ce{Zn}$ as $[Ar]\,3d^{12}$ (impossible).

How to avoid:

Memorise the Aufbau order: 4s fills before 3d, but 4s is written after 3d in notation.

Use the noble gas core method:

• Sc: $[Ar]\,3d^1\,4s^2$
• Zn: $[Ar]\,3d^{10}\,4s^2$

Common Mistake #4: Confusing "partially filled" with "unfilled"

The error: …