Question

The group \(4 \mathrm{~B}\) metal nitrides (TiN, ZrN, and HfN) as well as the group \(5 \mathrm{~B}\) metal nitrides \((\mathrm{V} \mathrm{N}, \mathrm{Nb} \mathrm{N}\), and \(\mathrm{TaN})\) are all superconductors at low temperature. Niobium(III) nitride, which has the highest \(T_{c}\), superconducts below \(16.1 \mathrm{~K}\). All of these compounds have crystal structures that are analogous to the sodium chloride structure. Scandium nitride also adopts the sodium chloride structure, but it is not a superconductor. (a) At room temperature will NbN be a metallic conductor or an insulator? (b) At room temperature will ScN be a metallic conductor or an insulator? (c) Why do you think the properties of \(\mathrm{ScN}\) are so different than the group \(4 \mathrm{~B}\) and \(5 \mathrm{~B}\) metal nitrides? (Hint: Consider the electron configuration of the metal cation.)

Short answer

At room temperature, NbN will be a metallic conductor, while ScN will be an insulator. The properties of ScN are different from those of group 4B and 5B metal nitrides due to the electron configuration of the metal cations. Scandium forms a compound with mainly ionic character, resulting in fewer free electrons available for electrical conduction, making it an insulator and not a superconductor like the group 4B and 5B metal nitrides.

Step-by-step solution

Determine the electron configuration

First, we need to determine the electron configurations for the metal cations in the given compounds: - Nb (Z=41) has an electron configuration of: \([Kr] 4d^4 5s^1\) - Sc (Z=21) has an electron configuration of: \([Ar] 3d^1 4s^2\) Now, let's analyze their electron configurations to find out whether they are metallic conductors or insulators at room temperature. #a# NbN

Identify the type of bonding

The electron configuration of Nb is \([Kr] 4d^4 5s^1\). This means it can lose 1 electron from the 5s orbital and 4 electrons from the 4d orbital, making it have a +5 oxidation state. Since nitrogen has a -3 oxidation state, the compound NbN will have a partial ionic character, meaning that it's a metallic conductor, as there are free electrons available for electrical conduction.

Conclusion for NbN

At room temperature, NbN will be a metallic conductor. #b# ScN

Identify the type of bonding

The electron configuration of Sc is \([Ar] 3d^1 4s^2\). This means it can lose 2 electrons from the 4s orbital and 1 electron from the 3d orbital, making it have a +3 oxidation state. Since nitrogen has a -3 oxidation state, the compound formed will have a predominantly ionic character, meaning that there are no free electrons available for electrical conduction.

Conclusion for ScN

At room temperature, ScN will be an insulator. #c# Different Properties of ScN

Compare electron configurations

The key difference between group 4B and 5B metal nitrides and ScN is the electron configuration of the metal cations. Group 4B and 5B metals have partially filled d-orbitals, which allow them to lose more electrons and form compounds with partial ionic character promoting the movement of free electrons and superconducting behavior at low temperatures.

Explain the properties of ScN

Scandium, on the other hand, has only one electron in its 3d orbital. When it forms ScN, it loses all its valence electrons, forming a compound with mainly ionic character. This results in fewer free electrons available for electrical conduction, making it an insulator at room temperature and not a superconductor at low temperatures like the group 4B and 5B metal nitrides.

Key concepts

Superconductivity

Superconductivity is a fascinating phenomenon encountered in certain materials at extremely low temperatures. It is a state where the material exhibits zero electrical resistance, allowing electrons to flow without any energy loss. This unique property makes superconductors highly efficient for carrying electricity, typically transforming them into powerful electromagnets.

• In metal nitrides like Niobium(III) nitride (NbN), superconductivity is observed below a critical temperature, denoted as \(T_c\), which in the case of NbN is 16.1 K.
• At this temperature, the thermal vibrations within the metal lattice are minimal, allowing electron pairs, known as Cooper pairs, to move freely through the crystal lattice.

This transition into the superconducting state happens because electron pairs overcome repulsive forces due to lattice vibrations, creating a collective electron flow that eliminates electrical resistance completely.
Even though many metal nitrides can demonstrate superconductivity, the exact \(T_c\) varies across different compounds.
Interestingly, not all materials with similar structures exhibit superconductivity, as demonstrated by Scandium nitride (ScN), which remains a non-superconductor.

Crystal Structure

Metal nitrides, like niobium nitride (NbN), zirconium nitride (ZrN), and tantalum nitride (TaN), typically crystallize into structures resembling that of sodium chloride (NaCl). This particular crystal structure is characterized by a highly ordered arrangement of atoms.

• The NaCl structure is a cubic crystal where each metal atom is surrounded by six nitrogen atoms, forming an octahedral geometry.
• This arrangement creates a strong ionic lattice due to the electrostatic interactions between the metal cations and nitrogen anions.

A kernel aspect of crystal structure in determining electric properties is how the atoms are spatially organized. If this layout supports good overlap between neighboring atoms' electron orbitals, it facilitates easier movement of electrons, contributing to metallic conduction or even superconductivity.
Despite adopting the NaCl crystal structure, ScN does not display superconducting properties. This is a clear indication that crystal structure alone does not determine superconductivity, as electron configuration and other factors also play crucial roles.

Electron Configuration

Electron configuration is key in understanding the chemical properties and conductive behavior of elements. It refers to the distribution of electrons within an atom across different energy levels or orbitals.

• For Nb (Z=41), the electron configuration is \([Kr] 4d^4 5s^1\). When forming NbN, niobium exhibits a tendency to lose electrons easily, achieving a +5 oxidation state.
• Sc (Z=21) has the configuration \([Ar] 3d^1 4s^2\), meaning it attains a +3 oxidation state when forming ScN.

The ability of an element to lose, gain, or share electrons determines the degree of ionic or covalent character in its compounds. In NbN, there are free electrons available due to its partially filled d-orbitals, leading to its metallic character and superconducting capabilities at low temperatures.
Conversely, in ScN, Scandium loses all its valence electrons resulting in no free electrons to facilitate conduction. This results in an insulator at room temperature. The electron configuration thus provides insights into electronic properties and helps explain why certain compounds might or might not exhibit superconductive behavior.