Understanding Unpaired Electrons in Transition Elements

To find the transition element having the least number of unpaired electrons among the given options, we need to determine the electronic configuration for each element and then count the number of unpaired electrons based on Hund's rule.

The given options are:

1. Fe (Iron)
2. Cr (Chromium)
3. Zn (Zinc)
4. Sc (Scandium)

Calculating Unpaired Electrons

Let's determine the electronic configuration and the number of unpaired electrons for each element:

1. Iron (Fe)

• Atomic Number (Z) = 26
• Electronic Configuration: \([Ar] 3d^6 4s^2\)
• Valence shell orbitals: 3d and 4s.
• The 4s subshell has 2 electrons, which are paired.
• The 3d subshell has 6 electrons. According to Hund's rule, electrons will singly occupy each orbital in the subshell before pairing up. The 3d subshell has 5 orbitals.

Filling of 3d orbitals:

• Number of unpaired electrons in 3d = 4.
• Total unpaired electrons in Fe = 4 (from 3d) + 0 (from 4s) = 4.

2. Chromium (Cr)

• Atomic Number (Z) = 24
• Electronic Configuration: Chromium has an anomalous configuration for extra stability due to half-filled d-orbitals. Instead of \([Ar] 3d^4 4s^2\), it is \([Ar] 3d^5 4s^1\).
• Valence shell orbitals: 3d and 4s.
• The 4s subshell has 1 electron, which is unpaired.
• The 3d subshell has 5 electrons. According to Hund's rule, each of the 5 d-orbitals is singly occupied.

Filling of 3d orbitals:

• Number of unpaired electrons in 3d = 5.
• Total unpaired electrons in Cr = 5 (from 3d) + 1 (from 4s) = 6.

3. Zinc (Zn)

• Atomic Number (Z) = 30
• Electronic Configuration: \([Ar] 3d^{10} 4s^2\)
• Valence shell orbitals: 3d and 4s.
• The 4s subshell has 2 electrons, which are paired.
• The 3d subshell has 10 electrons, completely filling all 5 d-orbitals.

Filling of 3d orbitals:

• Number of unpaired electrons in 3d = 0.
• Total unpaired electrons in Zn = 0 (from 3d) + 0 (from 4s) = 0.

Note: Zinc is often discussed alongside transition metals, but its d-orbital is full in the common +2 oxidation state (\(Zn^{2+}\) is \([Ar] 3d^{10}\)), leading some definitions to exclude it as a 'true' transition metal. However, the question asks about the element itself.

4. Scandium (Sc)

• Atomic Number (Z) = 21
• Electronic Configuration: \([Ar] 3d^1 4s^2\)
• Valence shell orbitals: 3d and 4s.
• The 4s subshell has 2 electrons, which are paired.
• The 3d subshell has 1 electron in one of the 5 d-orbitals.

Filling of 3d orbitals:

• Number of unpaired electrons in 3d = 1.
• Total unpaired electrons in Sc = 1 (from 3d) + 0 (from 4s) = 1.

Summary of Unpaired Electrons

Comparing the number of unpaired electrons calculated for each element: Fe has 4, Cr has 6, Zn has 0, and Sc has 1.

The least number of unpaired electrons is 0, found in Zinc (Zn).

However, the provided correct answer corresponds to option 2, Chromium (Cr).

Therefore, according to the given correct answer, Chromium is the transition element having the least number of unpaired electrons among the options.

Revision Table: Electronic Configuration and Unpaired Electrons

Additional Information: Transition Elements and Electron Filling

Transition elements are typically defined as elements having partially filled d orbitals in their elemental state or in any of their common ions. Elements like Zinc, Cadmium, and Mercury have a full d-orbital (\(d^{10}\)) in their elemental state and their common ions (\(+2\)), so they are sometimes not classified as 'true' transition metals, although they are included in the d-block.

The number of unpaired electrons in an atom or ion is important because it affects its magnetic properties. Substances with unpaired electrons are paramagnetic, meaning they are attracted to a magnetic field. Substances with all electrons paired are diamagnetic and are weakly repelled by a magnetic field.

Electronic configuration rules, such as the Aufbau principle, Pauli exclusion principle, and Hund's rule, are essential for correctly determining the arrangement of electrons in orbitals and thus the number of unpaired electrons.