ChemistryChapter 6: The Transition Metals
6.1 Electron Configurations of the d-block elements
- Transition Elements:
Explanation level
The transition elements are located in the central block of the periodic table. They are characterized by the filling of their inner d subshells. Understanding their electron configurations is fundamental to explaining their unique properties, such as variable oxidation states, catalytic activity, and the formation of colored complexes. This section explains the electron configurations of the first-row d-block elements and the exceptions to the general rules.
Definition
- Transition Elements: Elements that have an incomplete d subshell in their most stable ionic states.
- d-block Elements: Elements in which the d subshell is being filled. This includes Group 3 to Group 12.
Important Note: The textbook states that while zinc is a d-block element, it is not a transition element because it forms only one ion, Zn^{2+}, which has a completely filled 3d subshell.
Core Concept
The core concept is that the electron configuration of transition metals follows the Aufbau principle, which states that electrons fill the lowest energy orbitals first. However, there are exceptions to this rule, particularly for chromium and copper. The reason for these exceptions is that atoms or ions with a half-filled (e.g., 3d5) or filled (e.g., 3d10) d subshell are relatively more stable. This stability arises from the symmetrical distribution of electrons and the exchange energy associated with having many unpaired electrons with parallel spins.
Detailed Explanation
The Aufbau Principle and Exceptions
The textbook states that the electronic configuration of transition metals follows the Aufbau principle. However, there are some exceptions to this rule. For example, chromium and copper break the Aufbau principle.
- Chromium (Cr): Expected configuration: [Ar] 3d4 4s2. Actual configuration: [Ar] 3d5 4s1.
- Copper (Cu): Expected configuration: [Ar] 3d9 4s2. Actual configuration: [Ar] 3d10 4s1.
Reason for the Exceptions: The reason is that atoms or ions with a half-filled 3d sub-level (3d5) or a filled 3d sub-level (3d10) are relatively stable, avoiding the inter-electronic repulsion in the 4s orbital that occurs with the 4s2 configuration. Moreover, in both cases, the 3d electron cloud is symmetrical, which shields the nucleus relatively in a more effective way.
Electron Configurations of the First-Row d-Block Elements and Their Ions
The textbook provides a table showing the electron configurations of some of the first-row d-block elements and their ions. A key piece of information is:
Key Information: When forming cations, electrons are always removed from the higher energy sub-shell, i.e., 4s in the first row of d-block elements.
Table: Electron Configurations of First-Row d-Block Elements and Their Ions
| Element | Electron Configuration | Ion | Electron Configuration |
|---|---|---|---|
| Sc | [Ar] 3d1 4s2 | Sc^{1+} | [Ar] 3d1 4s1 |
| Sc^{2+} | [Ar] 3d1 | Ti | |
| [Ar] 3d2 4s2 | Ti^{2+} | [Ar] 3d2 | |
| V | [Ar] 3d3 4s2 | V^{4+} | [Ar] 3d1 |
| Cr | [Ar] 3d5 4s1 | Cr^{2+} | [Ar] 3d4 |
| Cr^{3+} | [Ar] 3d3 | Mn | |
| [Ar] 3d5 4s2 | Mn^{2+} | [Ar] 3d5 | |
| Fe | [Ar] 3d6 4s2 | Fe^{2+} | [Ar] 3d6 |
| Fe^{3+} | [Ar] 3d5 | Co | |
| [Ar] 3d7 4s2 | Co^{2+} | [Ar] 3d7 | |
| Ni | [Ar] 3d8 4s2 | Ni^{2+} | [Ar] 3d8 |
| Cu | [Ar] 3d10 4s1 | Cu^{1+} | [Ar] 3d10 |
| Cu^{2+} | [Ar] 3d9 | Zn |
Relative Energies of 4s and 3d Sub-shells
The textbook explains the energy difference between 3d and 4s orbitals changes across the period of transition metals. In calcium, the 4s orbital has lower energy than the 3d orbital because the 4s electron experiences full attraction of the unscreened nucleus. This makes the 4s electrons more penetrating than the 3d electrons.
Along the 3d period, after calcium, the 4s electrons are shielded by the 3d electrons, slightly decreasing their energy, causing a small rise in the first and second ionization energies. On the contrary, the 3d electrons face a higher effective nuclear charge due to less shielding, strongly attracted by the nucleus. This results in a potential increase of 3d electrons, reflected in a higher rise of third ionization energy.
Intermediate Level
The electron configuration of first-row transition metals involves the filling of the 3d subshell after the 4s subshell has been filled. The Aufbau principle predicts configurations like [Ar] 3d4 4s2 for chromium and [Ar] 3d9 4s2 for copper. However, the actual configurations are [Ar] 3d5 4s1 and [Ar] 3d10 4s1, respectively. This is because a half-filled or fully-filled d subshell provides extra stability due to exchange energy and symmetry. When forming cations, electrons are removed from the 4s orbital first, as it is higher in energy than the 3d orbital in these ions. For example, Fe^{2+} is [Ar] 3d6 and Fe^{3+} is [Ar] 3d5.
Important Points
- - Transition Element: Has an incomplete d subshell in its most stable ionic state.
- - Aufbau Principle: Electrons fill lower energy orbitals first.
- - Exceptions: Cr ([Ar] 3d5 4s1) and Cu ([Ar] 3d10 4s1).
- - Reason for Exceptions: Half-filled and fully-filled d subshells are more stable.
- - Ion Formation: Electrons are removed from the 4s orbital first.
- - Zinc: [Ar] 3d10 4s2, not a transition element because it has a full d subshell.
- - Scandium: Included as a transition element by IUPAC.
Remember
- - Cr: 3d5 4s1 (not 3d4 4s2).
- - Cu: 3d10 4s1 (not 3d9 4s2).
- - Reason: Half-filled (d5) and fully-filled (d10) are more stable.
- - Ion formation: Remove from 4s first.
- - Example: Fe^{2+} is 3d6, Fe^{3+} is 3d5.
- - Zn is not a transition element (full d subshell).
- - Sc is a transition element (by IUPAC).
Common Mistakes
- - Thinking that 4s is always filled before 3d. While generally true for the neutral atoms, the order of filling can be more complex, and for ions, electrons are removed from 4s first.
- - Forgetting the exceptions for Cr and Cu. These are the most common exceptions and are frequently tested.
- - Removing electrons from the 3d orbital instead of the 4s orbital when forming ions. Always remove from the outermost shell first (4s in this case).
- - Thinking that Zn is a transition element. Zn has a full 3d subshell in its common +2 oxidation state, so it is not a transition element according to the definition.
- - Assuming that all elements in the d-block are transition elements. The textbook clearly distinguishes between d-block elements and transition elements.
Quick Revision
The electron configurations of the first-row d-block elements generally follow the Aufbau principle, but chromium and copper are exceptions. Chromium has a configuration of [Ar] 3d5 4s1 and copper has [Ar] 3d10 4s1, because half-filled and fully-filled d subshells are more stable. When these elements form cations, electrons are removed from the 4s orbital first. For example, Fe^{2+} is [Ar] 3d6 and Fe^{3+} is [Ar] 3d5. Zinc is a d-block element but not a transition element because it has a full 3d subshell ([Ar] 3d10 4s2).
60-Second Revision
The first-row d-block elements mostly follow the Aufbau principle, but Cr (3d5 4s1) and Cu (3d10 4s1) are exceptions because half-filled and fully-filled d subshells are more stable. When forming ions, electrons are removed from the 4s orbital first (e.g., Fe^{2+} is 3d6). Zinc is not a transition element because it has a full 3d subshell.
Frequently Asked Questions (FAQs)
Q1. What is the electronic configuration of chromium and why is it an exception to the Aufbau principle? Answer: The electronic configuration of chromium is [Ar] 3d5 4s1, not [Ar] 3d4 4s2 as predicted by the Aufbau principle. This is because a half-filled 3d subshell (3d5) is more stable due to the symmetrical distribution of electrons and the exchange energy associated with having five unpaired electrons.
Q2. What is the electronic configuration of copper and why is it an exception? Answer: The electronic configuration of copper is [Ar] 3d10 4s1, not [Ar] 3d9 4s2. This is because a fully-filled 3d subshell (3d10) is more stable than a 3d9 configuration due to the symmetrical distribution of electrons and the exchange energy.
Q3. When transition metals form ions, which electrons are removed first and why? Answer: When transition metals form cations, electrons are always removed from the 4s orbital first. This is because, in the context of the ions, the 3d orbital is lower in energy than the 4s orbital. The 4s electrons are also further from the nucleus and are the outermost electrons.
Q4. Why is zinc not considered a transition element? Answer: Zinc is a d-block element but not a transition element because it forms only one common ion, Zn^{2+}, which has a completely filled 3d subshell ([Ar] 3d10). A transition element is defined as having an incomplete d subshell in its most stable ionic state.
- For Fe^{2+}, two electrons are removed from the 4s orbital: [Ar] 3d6.
- For Fe^{3+}, three electrons are removed (two from 4s and one from 3d): [Ar] 3d5.
Q6. Why is scandium considered a transition element? Answer: Scandium was initially controversial because its common +3 oxidation state (Sc^{3+}) has an empty d subshell. However, it was later included in the transition metals by the IUPAC system when compounds with +1 and +2 oxidation states, such as CsScCl_3, were synthesized. In these compounds, scandium has an incomplete d subshell.
Q7. What is the relative energy of the 4s and 3d orbitals in transition metals? Answer: The energy difference between 3d and 4s orbitals changes across the period. In calcium, the 4s orbital is lower in energy than the 3d orbital. As the 3d subshell begins to fill, the increased nuclear charge and shielding effects lower the energy of the 3d orbitals relative to the 4s. This is why the 4s electrons are removed first when forming cations.
Q8. What is the electron configuration of Cu^{2+}? Answer: The electron configuration of neutral copper is [Ar] 3d10 4s1. When it forms Cu^{2+}, it loses two electrons: one from the 4s orbital and one from the 3d orbital, giving [Ar] 3d9.
Q9. What is the key information to remember when writing electron configurations for transition metal ions? Answer: The key information is that when forming cations, electrons are always removed from the higher energy sub-shell, which is the 4s orbital in the first row of d-block elements, before removing electrons from the 3d subshell.
Multiple Choice Questions (MCQs)
- Q1. Which one of the following species has a d10 sub-shell?
- A. Cu^{2+}
- B. Zn^{2+}
- C. Mn
- D. Cr
- Correct Answer: B
- Explanation: Zn^{2+} has the electron configuration [Ar] 3d10, so its d subshell is completely filled.
Q2. Which one is a correct statement about the zinc element? 1. It has a complete 3d sub-shell. 2. It is a transition metal. 3. It forms colourless complexes. 4. It can exist in one stable oxidation state. A. 1 and 2 only B. 1, 3, and 4 only C. 1 and 4 only D. All are correct Correct Answer: B Explanation: Zinc has a complete 3d sub-shell (1), forms colourless complexes (3), and exists in only one stable oxidation state (+2) (4). It is not a transition metal (2 is incorrect).
Q3. The electronic configuration of the iron cation in the complex [Fe(CN)_6]3- is: A. [Ar] 3d6 B. [Ar] 3d5 C. [Ar] 3d6 4s1 D. [Ar] 3d4 4s2 Correct Answer: B Explanation: In [Fe(CN)_6]3-, the iron is in the +3 oxidation state. Neutral Fe is [Ar] 3d6 4s2. Removing three electrons gives [Ar] 3d5.
Q4. The cyanide ligand (CN^-) can form two complexes with iron with formulae, [Fe(CN)_6]3- and [Fe(CN)_6]4-. What is the oxidation state of iron in them? A. +2 and +3 respectively B. +4 and -2 respectively C. +2 and +4 respectively D. +3 and +2 respectively Correct Answer: D Explanation: In [Fe(CN)_6]3-, the overall charge is -3. Six CN⁻ ligands contribute -6, so Fe must be +3. In [Fe(CN)_6]4-, six CN⁻ ligands contribute -6, so Fe must be +2.
Q5. Transition metals are different from alkali metals and alkaline earth metals in many ways. Which one is an incorrect statement about transition metals when compared with group 1 and group 2 metals? A. They form coloured compounds. B. They show variable oxidation states. C. They have high melting points. D. They are more reactive. Correct Answer: D Explanation: Transition metals are generally less reactive than Group 1 and Group 2 metals. The other statements are true.
Q6. Which one of the following properties of transition metals is held responsible for their catalytic behaviour? A. They form coloured complexes. B. They show variable oxidation states. C. They have high melting points. D. They have high density. Correct Answer: B Explanation: The ability to easily change between different oxidation states allows transition metals to act as catalysts by providing an alternative reaction pathway with a lower activation energy.
- Q7. The highest oxidation state shown by first-row transition metals is shown by:
- A. Copper
- B. Manganese
- C. Nickel
- D. Cobalt
- Correct Answer: B
- Explanation: Manganese has the electron configuration [Ar] 3d5 4s2 and can show a maximum oxidation state of +7 (e.g., in KMnO_4).
Q8. The weakest field ligand among the following is: A. H_2O B. CN^- C. NH_3 D. OH^- Correct Answer: A Explanation: Based on the spectrochemical series, the field strength order is CN^- > NH_3 > H_2O > OH^-. Among the given options, H_2O is the weakest field ligand.
Q9. Which of the following is the correct electronic configuration for chromium? A. [Ar] 3d4 4s2 B. [Ar] 3d5 4s1 C. [Ar] 3d6 4s0 D. [Ar] 3d3 4s3 Correct Answer: B Explanation: Chromium is an exception to the Aufbau principle. Its configuration is [Ar] 3d5 4s1 due to the extra stability of a half-filled d subshell.
Q10. What is the electron configuration of Cu^{2+}? A. [Ar] 3d10 B. [Ar] 3d9 C. [Ar] 3d8 4s1 D. [Ar] 3d8 4s2 Correct Answer: B Explanation: Neutral Cu is [Ar] 3d10 4s1. When it forms Cu^{2+}, it loses two electrons (one from 4s and one from 3d), giving [Ar] 3d9.
Multiple Choice Questions (MCQs)
- Q1. Which one of the following species has a d10 sub-shell?
- A. Cu^{2+}
- B. Zn^{2+}
- C. Mn
- D. Cr
- Correct Answer: B
- Explanation: Zn^{2+} has the electron configuration [Ar] 3d10, so its d subshell is completely filled.
Q2. Which one is a correct statement about the zinc element? 1. It has a complete 3d sub-shell. 2. It is a transition metal. 3. It forms colourless complexes. 4. It can exist in one stable oxidation state. A. 1 and 2 only B. 1, 3, and 4 only C. 1 and 4 only D. All are correct Correct Answer: B Explanation: Zinc has a complete 3d sub-shell (1), forms colourless complexes (3), and exists in only one stable oxidation state (+2) (4). It is not a transition metal (2 is incorrect).
Q3. The electronic configuration of the iron cation in the complex [Fe(CN)_6]3- is: A. [Ar] 3d6 B. [Ar] 3d5 C. [Ar] 3d6 4s1 D. [Ar] 3d4 4s2 Correct Answer: B Explanation: In [Fe(CN)_6]3-, the iron is in the +3 oxidation state. Neutral Fe is [Ar] 3d6 4s2. Removing three electrons gives [Ar] 3d5.
Q4. The cyanide ligand (CN^-) can form two complexes with iron with formulae, [Fe(CN)_6]3- and [Fe(CN)_6]4-. What is the oxidation state of iron in them? A. +2 and +3 respectively B. +4 and -2 respectively C. +2 and +4 respectively D. +3 and +2 respectively Correct Answer: D Explanation: In [Fe(CN)_6]3-, the overall charge is -3. Six CN⁻ ligands contribute -6, so Fe must be +3. In [Fe(CN)_6]4-, six CN⁻ ligands contribute -6, so Fe must be +2.
Q5. Transition metals are different from alkali metals and alkaline earth metals in many ways. Which one is an incorrect statement about transition metals when compared with group 1 and group 2 metals? A. They form coloured compounds. B. They show variable oxidation states. C. They have high melting points. D. They are more reactive. Correct Answer: D Explanation: Transition metals are generally less reactive than Group 1 and Group 2 metals. The other statements are true.
Q6. Which one of the following properties of transition metals is held responsible for their catalytic behaviour? A. They form coloured complexes. B. They show variable oxidation states. C. They have high melting points. D. They have high density. Correct Answer: B Explanation: The ability to easily change between different oxidation states allows transition metals to act as catalysts by providing an alternative reaction pathway with a lower activation energy.
- Q7. The highest oxidation state shown by first-row transition metals is shown by:
- A. Copper
- B. Manganese
- C. Nickel
- D. Cobalt
- Correct Answer: B
- Explanation: Manganese has the electron configuration [Ar] 3d5 4s2 and can show a maximum oxidation state of +7 (e.g., in KMnO_4).
Q8. The weakest field ligand among the following is: A. H_2O B. CN^- C. NH_3 D. OH^- Correct Answer: A Explanation: Based on the spectrochemical series, the field strength order is CN^- > NH_3 > H_2O > OH^-. Among the given options, H_2O is the weakest field ligand.
Q9. Which of the following is the correct electronic configuration for chromium? A. [Ar] 3d4 4s2 B. [Ar] 3d5 4s1 C. [Ar] 3d6 4s0 D. [Ar] 3d3 4s3 Correct Answer: B Explanation: Chromium is an exception to the Aufbau principle. Its configuration is [Ar] 3d5 4s1 due to the extra stability of a half-filled d subshell.
Q10. What is the electron configuration of Cu^{2+}? A. [Ar] 3d10 B. [Ar] 3d9 C. [Ar] 3d8 4s1 D. [Ar] 3d8 4s2 Correct Answer: B Explanation: Neutral Cu is [Ar] 3d10 4s1. When it forms Cu^{2+}, it loses two electrons (one from 4s and one from 3d), giving [Ar] 3d9.
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Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.6.1 Electron Configurations of the d-block elements
Sticky Notes · Quick Revision
Key Point 1
- Cr: [Ar] 3d5 4s1 (exception).
Key Point 2
- Cu: [Ar] 3d10 4s1 (exception).
Key Point 3
- Stability: Half-filled (d5) and fully-filled (d10) are stable.
Key Point 4
- Ion formation: Remove from 4s first.
Key Point 5
- Fe^{2+}: [Ar] 3d6.
Key Point 6
- Fe^{3+}: [Ar] 3d5.
Key Point 7
- Zn: [Ar] 3d10 4s2 (not a transition element).
Key Point 8
- Sc: [Ar] 3d1 4s2 (transition element).
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