The discovery of hafnium, element number 72, provided a controversial episode in chemistry. G. Urbain, a French chemist, claimed in 1911 to have isolated an element number 72 from a sample of rare earth (elements 58–71) compounds. However, Niels Bohr believed that hafnium was more likely to be found along with zirconium than with the rare earths. D. Coster and G. von Hevesy, working in Bohr's laboratory in Copenhagen, showed in 1922 that element 72 was present in a sample of Norwegian zircon, an ore of zirconium. (The name hafnium comes from the Latin name for Copenhagen, Hafnia). (d) Using their electron configurations, account for the fact that Zr and Hf form chlorides MCl₄ and oxides MO₂.

Verified step by step guidance

1. Step 1: Understand that the electron configuration of an atom determines its chemical behavior. The electron configuration of an atom is the distribution of electrons in its atomic orbitals. The electron configuration of an atom in its ground state is the most stable arrangement of electrons around the nucleus.
2. Step 2: Recall that Zirconium (Zr) and Hafnium (Hf) are in the same group (Group 4) of the periodic table. Elements in the same group have similar electron configurations, especially in their outermost (valence) electron shell, which leads to similar chemical properties.
3. Step 3: Write down the electron configurations for Zr and Hf. Zr has the electron configuration [Kr] 5s2 4d2 and Hf has the electron configuration [Xe] 6s2 4f14 5d2. Both Zr and Hf have 4 electrons in their outermost shell (2 in the s orbital and 2 in the d orbital).
4. Step 4: Understand that in forming compounds, atoms tend to achieve a stable electron configuration, often that of the nearest noble gas. In the case of Zr and Hf, they can achieve a stable electron configuration by losing the 4 electrons in their outermost shell.
5. Step 5: Conclude that Zr and Hf form chlorides MCl4 and oxides MO2 because in these compounds, Zr and Hf have lost their 4 outermost electrons, achieving a stable electron configuration. In MCl4, each Cl atom gains one electron and in MO2, each O atom gains two electrons, satisfying the octet rule for all atoms involved.

Key Concepts

Electron Configuration

Electron configuration describes the distribution of electrons in an atom's orbitals. For transition metals like zirconium (Zr) and hafnium (Hf), understanding their electron configurations is crucial for predicting their chemical behavior, including their ability to form compounds. Zr has the electron configuration [Kr] 4d² 5s², while Hf is [Xe] 4f¹⁴ 5d² 6s², indicating their similar valence electron arrangements that influence their bonding characteristics.

Chloride and Oxide Formation

Zirconium and hafnium can form chlorides (MCl₄) and oxides (MO₂) due to their ability to achieve stable oxidation states. In these compounds, Zr and Hf typically exhibit a +4 oxidation state, allowing them to bond with four chloride ions or two oxide ions. This behavior is a result of their electron configurations, which enable them to lose electrons and form stable ionic compounds.

Periodic Trends and Similarities

Zirconium and hafnium are in the same group of the periodic table, which means they share similar chemical properties due to their comparable electron configurations. This periodic trend leads to similarities in their reactivity and the types of compounds they form, such as chlorides and oxides. Understanding these trends helps explain why these two elements behave similarly in chemical reactions despite being in different periods.