When neptunium (Np) and plutonium (Pu) were discovered, the periodic table did not include the actinides, so these elements were placed in Groups \(7 \mathrm{~B}(7)\) and \(8 \mathrm{~B}(8) .\) When americium (Am) and curium (Cm) were synthesized, they were placed in Groups \(8 \mathrm{~B}(9)\) and \(8 \mathrm{~B}(10) .\) However, during chemical isolation procedures, Glenn Seaborg and his colleagues, who had synthesized these elements, could not find their compounds among other compounds of members of the same groups, which led Seaborg to suggest they were part of a new inner transition series. (a) How do the electron configurations of these elements support Seaborg's suggestion? (b) The highest fluorides of \(\mathrm{Np}\) and \(\mathrm{Pu}\) are hexafluorides, and the highest fluoride of uranium is also the hexafluoride. How does this chemical evidence support the placement of \(\mathrm{Np}\) and \(\mathrm{Pu}\) as inner transition elements rather than transition elements?

Step by step solution

01

- Write the Electron Configurations

Determine the electron configurations for the elements in question: neptunium (Np), plutonium (Pu), americium (Am), curium (Cm), and uranium (U). Ensure to include their ground-state configurations.

02

- Compare Electron Configurations

Examine the electron configurations of these elements and any similarities they may have with known series, like the lanthanides which follow a pattern of filling the 4f orbitals.

03

- Identify Inner Transition Elements

Identify the characteristic electron configurations of inner transition elements, focusing on the filling of 5f orbitals, and compare them to the configurations obtained in Step 1.

04

- Evaluate Seaborg's Suggestion (Part a)

Assess how the electron configurations of Np, Pu, Am, and Cm support the idea of being part of the actinide series, specifically noting the 5f orbital filling, which indicates inner transition metals.

05

- Analyze Chemical Properties (Part b)

Compare the chemical properties of uranium hexafluoride with neptunium and plutonium hexafluorides. Note the similarity in compound formation, especially the ability to form hexafluorides, which is typical of actinides rather than d-block transition metals.

06

- Conclusion on Chemical Evidence

Using the information from Step 5, conclude how the chemical properties (formation of hexafluorides) substantiate the placement of Np and Pu as inner transition elements.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Electron configuration

The electron configuration of an element shows how its electrons are distributed among the various orbitals. For neptunium (Np), plutonium (Pu), americium (Am), curium (Cm), and uranium (U), their electron configurations play a crucial role in understanding their placement in the periodic table.
For instance, the configurations are:

• Neptunium (Np): [Rn] 5f4 6d1 7s2
• Plutonium (Pu): [Rn] 5f6 7s2
• Americium (Am): [Rn] 5f7 7s2
• Curium (Cm): [Rn] 5f7 6d1 7s2
• Uranium (U): [Rn] 5f3 6d1 7s2

When comparing these configurations to elements in the lanthanide series, which fill their 4f orbitals, we see that these elements predominantly fill their 5f orbitals. This pattern is a strong indicator of their classification as inner transition metals, and it aligns with the characteristics of lanthanides, thus supporting the placement of these actinides in a similar category.

Inner transition metals

Inner transition metals include the lanthanides and actinides. These elements are primarily characterized by the filling of their f-orbitals. For actinides, this means the filling of 5f orbitals.
The main feature distinguishing inner transition metals from other elements is their electron configuration. Their unique properties arise from the electrons in these f-orbitals, which are more shielded from the nucleus than s- or d-electrons. This influences their chemical behavior and bonding.
When Seaborg and colleagues synthesized elements like neptunium and plutonium, they noticed discrepancies in expected chemical behaviors. Their electron configurations revealed the filling of 5f orbitals, consistent with inner transition elements, rather than the typical d-block transition metals. This observation was pivotal in recognizing these elements as a part of a new series—the actinides.

Chemical properties of actinides

Actinides exhibit unique chemical properties that also support their classification as inner transition metals. One clear piece of evidence is their ability to form hexafluorides.
For example, neptunium, plutonium, and uranium all form hexafluorides (NpF6, PuF6, UF6). This is a characteristic behavior not commonly observed in d-block transition metals.
The formation of these compounds demonstrates a high oxidation state, typically +6. This consistency among neptunium, plutonium, and uranium in producing hexafluorides strengthens the argument for their classification as actinides. Their chemical properties align more closely with those of their neighbors in the actinide series rather than those of traditional transition metals.
Therefore, the chemical evidence of forming similar high oxidation state compounds such as hexafluorides further supports Seaborg's placement of Np and Pu as inner transition metals, specifically actinides.