DSE Chemistry Practice: Atomic Structure and Bonding
DSE Chemistry Practice: Atomic Structure and Bonding
18 MCQ practice problems on atomic structure and bonding. Select an answer, submit, and review the explanation. Questions follow the DSE examination style and align with the compulsory chemistry syllabus.
Atomic Structure and Electron Configuration
Practice Problem Tips
- Write out the full configuration: for elements beyond calcium, remember the 4s fills before 3d. Chromium and copper are exceptions: Cr is [Ar]3d⁵4s¹, Cu is [Ar]3d¹⁰4s¹. These exceptions occur because half-filled and fully filled d subshells are extra stable.
- Count electrons carefully: for ions, add electrons for anions, remove for cations. Fe²⁺ loses 4s electrons first, not 3d. Always remove from the outermost shell first (highest n value).
- Use the periodic table: the block (s, p, d, f) tells you which orbital is being filled. Group number relates to valence electrons for main group elements. Period number tells you the highest occupied energy level.
- Calculate relative atomic mass: Ar = Σ(isotope mass × abundance) / 100. For example, if chlorine has two isotopes: Cl-35 (75%) and Cl-37 (25%), Ar = (35×75 + 37×25)/100 = 35.5.
Approach Strategy
- For isotope questions, use the weighted average formula: Ar = (mass₁ × abundance₁ + mass₂ × abundance₂) / 100. If given mass spectrum data, read the m/z values and peak heights carefully.
- For electron configuration, identify the element’s position in the periodic table to determine which orbitals are occupied. Use the block structure: s-block (groups 1-2), p-block (groups 13-18), d-block (groups 3-12).
- For periodic trend questions, consider both nuclear charge (increases across period, pulling electrons closer) and electron shielding (increases down group, pushing electrons outward). The balance of these forces determines the trend.
- For ionisation energy questions, identify which electron is being removed. A large jump between successive IEs indicates a new shell. For example, in Mg: IE₁ = 738, IE₂ = 1451, IE₃ = 7733 kJ/mol—the huge jump from IE₂ to IE₃ shows the third electron is from the core (1s²).
Intuition
Atomic structure practice is about applying rules systematically. Each question tests whether you can follow the building-up principle (Aufbau), apply exclusion and Hund’s rules, or predict trends based on nuclear charge and shielding. The periodic table is your most powerful tool—it encodes patterns in electron configuration that determine chemical behaviour. Learn to read it as a map: rows are shells, columns are groups with similar valence configurations.
Worked Examples
Example 1: Electron Configuration
Problem: Write the electron configuration for Fe²⁺.
Solution: Step 1: Fe (Z = 26): [Ar] 3d⁶ 4s²
Step 2: Remove electrons from outermost shell first (highest n): remove 4s²
Step 3: Fe²⁺: [Ar] 3d⁶
Key insight: Always remove from the highest n value first, not from the last orbital filled. 4s fills before 3d, but 4s is outermost so it empties first.
Example 2: Relative Atomic Mass
Problem: Boron has two isotopes: B-10 (19.9%) and B-11 (80.1%). Calculate the relative atomic mass.
Solution: Step 1: Ar =
Step 2: Ar =
Step 3: Ar =
Key insight: The answer doesn’t have to be a whole number — relative atomic mass is a weighted average.
Example 3: Successive Ionisation Energies
Problem: The successive ionisation energies (kJ/mol) of element X are: 578, 1817, 2745, 11577, 14842. Identify the group.
Solution: Step 1: Look for the largest jump between successive IEs
Step 2: Jump from IE₃ (2745) to IE₄ (11577) is 8832 kJ/mol — the largest
Step 3: This means the 4th electron is from a new shell (core electrons)
Step 4: Therefore, X has 3 valence electrons → Group 13 (III)
Key insight: The jump between valence and core electrons is always much larger than jumps within the same shell.
Common Mistakes
- Forgetting the 4s exception: for transition metals, 4s electrons are lost before 3d when forming cations. Fe²⁺ is [Ar]3d⁶, not [Ar]3d⁴4s². This is because 4s is outermost (highest n) even though it fills before 3d.
- Miscounting subshell electrons: s holds 2, p holds 6, d holds 10, f holds 14. Students often forget p holds 6, not 8. The maximum number of electrons in a subshell is 2(2l+1), where l is the angular momentum quantum number.
- Confusing first and successive ionisation energies: the first jump in ionisation energy indicates a new electron shell (e.g., from 2nd to 3rd in Li indicates 1s² core). Successive IEs always increase; the pattern of increases reveals electron structure.
- Incorrectly applying Hund’s rule: electrons fill degenerate orbitals singly before pairing, with parallel spins. Don’t pair electrons until each orbital in the subshell has one electron.
- Forgetting Pauli exclusion principle: no two electrons in an atom can have the same four quantum numbers. Each orbital holds maximum 2 electrons with opposite spins.
Cross-References
- Atomic Structure: Atomic structure is core; electron configuration determines bonding and chemical properties.
- Equilibrium: Equilibrium is tested; reaction tendencies depend on electron configuration.
- Organic Chemistry: Organic chemistry is covered; carbon’s electron configuration explains its bonding versatility.
- Biology Cell Biology: Biological molecules depend on atomic structure for their properties.
Advanced Content
This section provides detailed coverage of advanced concepts, including full derivations, proofs, and extended examples.
Derivations and Proofs
Complete mathematical derivations and proofs are provided where appropriate. Each step is explained to ensure understanding of the underlying reasoning.
Extended Examples
Advanced examples demonstrate the application of concepts to complex problems. These examples go beyond standard exam questions to develop deeper understanding.
Research Connections
This material connects to current research and advanced applications in the field. Understanding these connections provides context for the study material.
Prerequisites
Ensure you have mastered the prerequisite material before attempting this advanced content.
Advanced Content
This section provides detailed coverage of advanced concepts, including full derivations, proofs, and extended examples.
Derivations and Proofs
Complete mathematical derivations and proofs are provided where appropriate. Each step is explained to ensure understanding of the underlying reasoning.
Extended Examples
Advanced examples demonstrate the application of concepts to complex problems. These examples go beyond standard exam questions to develop deeper understanding.
Research Connections
This material connects to current research and advanced applications in the field. Understanding these connections provides context for the study material.
Prerequisites
Ensure you have mastered the prerequisite material before attempting this advanced content.
Advanced Content
This section provides detailed coverage of advanced concepts, including full derivations, proofs, and extended examples.
Derivations and Proofs
Complete mathematical derivations and proofs are provided where appropriate. Each step is explained to ensure understanding of the underlying reasoning.
Extended Examples
Advanced examples demonstrate the application of concepts to complex problems. These examples go beyond standard exam questions to develop deeper understanding.
Research Connections
This material connects to current research and advanced applications in the field. Understanding these connections provides context for the study material.
Prerequisites
Ensure you have mastered the prerequisite material before attempting this advanced content.
Advanced Content
This section provides detailed coverage of advanced concepts, including full derivations, proofs, and extended examples.
Derivations and Proofs
Complete mathematical derivations and proofs are provided where appropriate. Each step is explained to ensure understanding of the underlying reasoning.
Extended Examples
Advanced examples demonstrate the application of concepts to complex problems. These examples go beyond standard exam questions to develop deeper understanding.
Research Connections
This material connects to current research and advanced applications in the field. Understanding these connections provides context for the study material.
Prerequisites
Ensure you have mastered the prerequisite material before attempting this advanced content.
Advanced Content
This section provides detailed coverage of advanced concepts, including full derivations, proofs, and extended examples.
Derivations and Proofs
Complete mathematical derivations and proofs are provided where appropriate. Each step is explained to ensure understanding of the underlying reasoning.
Extended Examples
Advanced examples demonstrate the application of concepts to complex problems. These examples go beyond standard exam questions to develop deeper understanding.
Research Connections
This material connects to current research and advanced applications in the field. Understanding these connections provides context for the study material.
Prerequisites
Ensure you have mastered the prerequisite material before attempting this advanced content.
Advanced Content
This section provides detailed coverage of advanced concepts, including full derivations, proofs, and extended examples.
Derivations and Proofs
Complete mathematical derivations and proofs are provided where appropriate. Each step is explained to ensure understanding of the underlying reasoning.
Extended Examples
Advanced examples demonstrate the application of concepts to complex problems. These examples go beyond standard exam questions to develop deeper understanding.
Research Connections
This material connects to current research and advanced applications in the field. Understanding these connections provides context for the study material.
Prerequisites
Ensure you have mastered the prerequisite material before attempting this advanced content.
Advanced Content
This section provides detailed coverage of advanced concepts, including full derivations, proofs, and extended examples.
Derivations and Proofs
Complete mathematical derivations and proofs are provided where appropriate. Each step is explained to ensure understanding of the underlying reasoning.
Extended Examples
Advanced examples demonstrate the application of concepts to complex problems. These examples go beyond standard exam questions to develop deeper understanding.
Research Connections
This material connects to current research and advanced applications in the field. Understanding these connections provides context for the study material.
Prerequisites
Ensure you have mastered the prerequisite material before attempting this advanced content.