General Chemistry Study Guide: Electronic Structure, Bonding, Stoichiometry, Reactions, and Thermochemistry

Q1. What is the condensed electron configuration of Pb (Z=82)?

Background

Topic: Electronic Structure (Chapter 6)

This question tests your understanding of electron configurations, specifically how to write the condensed (noble gas) electron configuration for a heavy main-group element.

Key Terms and Formulas:

• Condensed (noble gas) electron configuration: Uses the symbol of the previous noble gas in brackets to represent core electrons.

• Order of orbital filling: Follows the Aufbau principle (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p).

• Atomic number (Z): Number of protons/electrons in a neutral atom.

Step-by-Step Guidance

1. Identify the noble gas preceding Pb (Z=82) on the periodic table. This will be your starting point for the condensed configuration.

2. Write the electron configuration for the electrons beyond the noble gas core, following the order of orbital filling.

3. Be careful with the filling order for the 6s, 4f, 5d, and 6p orbitals, as these are relevant for elements in the sixth period.

4. Stop before writing the final configuration; make sure you have accounted for all 82 electrons.

Try solving on your own before revealing the answer!

Q2. Write the condensed electron configurations for the following ions/atoms: Fe³⁺, Pd²⁺, N³⁻, K, I

Background

Topic: Electronic Structure (Chapter 6)

This question tests your ability to write condensed electron configurations for both neutral atoms and ions, including transition metals and main-group elements.

Key Terms and Formulas:

• Condensed electron configuration: Uses noble gas core notation.

• For ions: Add electrons for anions, remove electrons for cations (remove from highest energy orbital first).

• Transition metals: Remove s electrons before d electrons when forming cations.

Step-by-Step Guidance

1. For each species, determine the number of electrons (adjust for charge).

2. Identify the noble gas core for each element.

3. Write the configuration for the remaining electrons, following the correct order of filling.

4. For cations, remove electrons from the highest principal quantum number first (for transition metals, remove s before d).

5. Stop before writing the final configurations for each species.

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Q3. Draw the Lewis structures, determine the molecular geometry, and predict the bond angles for ClO₂⁻ and ClO₃⁻

Background

Topic: Lewis Structures, Molecular Geometry, Bond Angles (Chapters 7 & 8)

This question tests your ability to draw Lewis structures for polyatomic ions, apply VSEPR theory to determine molecular geometry, and predict bond angles.

Key Terms and Formulas:

• Lewis structure: Shows valence electrons as dots and bonds as lines.

• VSEPR theory: Predicts molecular geometry based on electron pair repulsion.

• Bond angles: Determined by geometry (e.g., bent, trigonal pyramidal, tetrahedral, etc.).

Step-by-Step Guidance

1. Count the total number of valence electrons for each ion (add for negative charge).

2. Draw the skeletal structure with Cl as the central atom and O atoms around it.

3. Distribute electrons to satisfy the octet rule for each atom, starting with outer atoms.

4. Apply VSEPR theory to determine the electron domain geometry and molecular shape.

5. Predict the bond angles based on the geometry identified.

6. Stop before drawing the final structure or stating the geometry/bond angles.

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Q4. For the Bohr hydrogen atom, which transitions have negative ΔE?

Background

Topic: Atomic Structure, Bohr Model (Chapter 6)

This question tests your understanding of energy changes during electron transitions in the hydrogen atom, specifically the sign of ΔE for absorption and emission.

Key Terms and Formulas:

• ΔE: Change in energy during a transition.

• Bohr equation: $\Delta E = -R_H \left( \frac{1}{n_f^2} - \frac{1}{n_i^2} \right)$, where $R_H$ is the Rydberg constant, $n_i$ is the initial energy level, $n_f$ is the final energy level.

• Negative ΔE: Indicates emission of energy (electron moves to lower energy level).

Step-by-Step Guidance

1. Recall that ΔE is negative when an electron transitions from a higher to a lower energy level ($n_i > n_f$).

2. For each possible transition, identify the initial and final quantum numbers.

3. Apply the Bohr equation to determine the sign of ΔE for each transition.

4. Stop before listing which transitions have negative ΔE.

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Q5. Draw the Lewis structures with resonance for CO₃²⁻ and NO₃⁻

Background

Topic: Lewis Structures, Resonance (Chapter 7)

This question tests your ability to draw resonance structures for polyatomic ions, showing delocalization of electrons.

Key Terms and Formulas:

• Resonance structures: Multiple valid Lewis structures differing only in the placement of electrons.

• Octet rule: Atoms (except H) should have 8 electrons around them.

• Formal charge: Used to determine the most stable resonance structure.

Step-by-Step Guidance

1. Count the total number of valence electrons for each ion (add for negative charge).

2. Draw the skeletal structure with the central atom (C or N) and three O atoms.

3. Distribute electrons to satisfy the octet rule for each atom.

4. Draw all possible resonance structures by moving double bonds among the O atoms.

5. Stop before drawing the final resonance structures.

Try solving on your own before revealing the answer!