Question

The heats of atomization, \(\Delta H_{\text {atom }}\), in \(\mathrm{kJ} / \mathrm{mol}\), of the first transition series of elements are as follows: \(\begin{array}{lllllllllll} \text { Element } & \text { Ca } & \text { Sc } & \text { Ti } & \text { V } & \text { Cr } & \text { Mn } & \text { Fe } & \text { Co } & \text { Ni } & \text { Cu } \\ \hline \Delta H_{\text {atom }} & 178 & 378 & 471 & 515 & 397 & 281 & 415 & 426 & 431 & 338 \end{array}\) (a) Write an equation for the process involved in atomization, and describe the electronic and structural changes that occur. (b) \(\Delta H_{\text {atom }}\) varies irregularly in the series following \(\mathrm{V}\). How can this be accounted for, at least in part, using the electronic configurations of the gaseous atoms? (Hint: Recall the discussions of Sections \(6.8\) and \(6.9 .)\)

Short answer

(a) The general equation for atomization is: \(\text{Element}_{(s)} \rightarrow \text{Element}_{(g)}\). The electronic and structural changes that occur during atomization involve the electron distribution changing from a "sea of electrons" in the solid state to being confined within individual electron clouds in the gaseous state, and the structural alteration from a close-packed crystal lattice to dispersed isolated atoms in the gaseous state. (b) The irregular variation in \(\Delta H_{\text{atom}}\) values can be partially explained by the electron configurations of the gaseous atoms, particularly the stabilization of half-filled or completely filled 3d and 4s orbitals. By analyzing these unique electron configurations for each element in the first transition series, we can account for the observed variations in heats of atomization.

Step-by-step solution

Part (a): Writing the equation for atomization

Atomization is the process of turning a substance into isolated atoms. In this case, we are considering the atomization of the metallic element in its solid form. The general equation for atomization is: $$ \text{Element}_{(s)} \rightarrow \text{Element}_{(g)} $$ where the element symbol represents one mole of the element atoms, 's' denotes the solid state, and 'g' denotes the gaseous state.

Part (a): Describing electronic and structural changes

The electronic changes that occur during the atomization process involve a change in the atoms' electron distribution. In the solid state, the electrons are distributed among the metallic atoms, forming a "sea of electrons." When the atom is isolated in the gaseous state, its electrons are confined within its own electron cloud around the nucleus. Structurally, in the solid state, the metallic atoms are arranged in a close-packed crystal lattice structure. During atomization, the solid metal is converted into individual atoms, which have no regular arrangement and are dispersed in the gaseous state.

Part (b): Explaining the irregular variation in \(\Delta H_{\text{atom}}\)

The irregular variation in \(\Delta H_{\text{atom}}\) values for the elements can be explained using the electronic configurations of the gaseous atoms. The first transition series of elements has the general electron configuration: $$ [\text{Ar}] 3d^{n} 4s^{2} $$ where n = 0 to 10, representing the number of electrons in the 3d orbitals. From Sc to V, the 3d orbitals are half-filled or completely filled, while the 4s orbitals are completely filled. This arrangement contributes to the stability of these atoms. Furthermore, Cr and Cu have electron configurations with more stable half-filled and completely filled 3d orbitals, resulting in lower \(\Delta H_{\text{atom}}\) values. The irregularity in \(\Delta H_{\text{atom}}\) can be at least partly attributed to the electron configuration of the gaseous atoms. In particular, the stabilization of half-filled or completely filled orbitals can cause variations in the energy required to separate an atom from its neighbors in the crystal lattice. By understanding and analyzing the unique electron configurations of each element in the first transition series, we can explain the observed variations in heats of atomization, as suggested by Sections 6.8 and 6.9 from the textbook.

Key concepts

Transition Metals

Transition metals comprise one of the most fascinating groups in the periodic table, characterized by their ability to form a variety of different compounds with unique properties. These elements, which include familiar metals such as iron, copper, and nickel, are defined by their partially filled d-orbitals. These partially filled d-orbitals enable them to exhibit various oxidation states, which is fundamental to their role in redox reactions and catalysis.

When looking at the heats of atomization, transition metals tend to have high values because the process involves breaking the metallic bonds that hold the atoms together in the solid state. The strength of these metallic bonds can be attributed to the delocalization of d-electrons, contributing to a strong metallic lattice. Additionally, in their pure form, transition metals are often found as lustrous, hard, and have high melting and boiling points, again a testament to the strong bonds between their atoms. These characteristics ensure that a significant amount of energy, known as the heat of atomization, is required to convert them from a solid to a gaseous state.

Electron Configuration

Understanding electron configuration, particularly for transition metals, is pivotal in explaining their chemical behavior, including their heats of atomization. Electron configurations describe the distribution of electrons among the orbitals of an atom. In the case of transition metals, electrons are added to the 3d orbitals after the 4s orbitals.

Upon the atomization of transition metals, the electron configuration becomes a crucial factor since the energy required to disperse the atoms into the gaseous state is affected by the distribution of these electrons. The relatively filled d-subshells, which may be completely filled, half-filled, or have a single electron, can greatly influence the stability of the atom. The presence of completely filled or half-filled d-orbitals makes atoms such as chromium (Cr) and copper (Cu) exceptionally stable, reflected in their irregularly lower heat of atomization values. The electron configuration thus provides a valuable insight into the energy relationships in atomization and the overall reactivity of transition metals.

Chemical Bonding

Chemical bonding is at the heart of the atomization process. In the context of transition metals, the types of bonding and the strength of these bonds can explain why certain metals require more energy to atomize. Transition metals exhibit metallic bonding, where valence electrons are shared and dispersed over a lattice of positively charged ions. This 'sea of electrons' is mobile, allowing for the conductivity and malleability that metals are known for.

The variation of heats of atomization among the transition metals is directly linked to the nature of metallic bonding in these elements. Factors such as the number of valence electrons and their ability to participate in the bonding, as well as the size of the metal ions and lattice structure, play a significant role. More specifically, d-electrons contribute to the strength of the metallic bonds, as their delocalization leads to stronger interatomic attractions within the metal's crystal lattice. Therefore, an irregular heat of atomization can often be explained by the electron configuration influencing the strength of the metallic bonds, making it a key topic when discussing the chemical properties of transition metals.