⟵ 01 Building Blocks (Problem Set) · Contents · 02 The Hero of Heat Transfer ⟶

Building Blocks: Answer Key

1. Atoms bond because the resulting electron arrangement places the system at a lower total energy. Incomplete valence shells are high-energy states; bonding allows atoms to fill or empty their outer shells and settle into stability.

2. In ionic bonding, electrons are transferred from one atom to another, creating oppositely charged ions that attract. In covalent bonding, electrons are shared between atoms. The distinction is transfer vs. sharing.

3. CO₂ is linear - its two polar bonds point in opposite directions and cancel. Water is bent at 104.5°, so its polar bonds point in the same general direction and the charge imbalance survives. Geometry determines whether local polarity becomes molecular polarity.

4. The Hydrogen-Bond Network is the persistent web of intermolecular attractions between water molecules, formed by the partial positive charge on hydrogen attracting the partial negative charge on a neighboring oxygen. It is not a true chemical bond - it is roughly 1/10th the strength of a covalent bond. Because each molecule can form up to four hydrogen bonds, the network is continuous and interconnected, giving water its collective behavior.

5. (a) 10 gal × 8.34 lb/gal = 83.4 lb. (b) 83.4 lb × 453.6 g/lb = 37,830 g.

6. Ca: 40.08 g/mol, C: 12.01 g/mol, O: 16.00 g/mol. (40.08)+(12.01)+ (3×16.00) = 100.09 g/mol. 50 g ÷ 100.09 g/mol = 0.50 mol.

7. 5,000 gal × 8.34 lb/gal = 41,700 lb. 41,700 lb × 453.6 g/lb = 18,912,120 g. 18,912,120 g ÷ 18.015 g/mol = 1,050,000 mol (1.05 × 10⁶ mol).

8. Covalent bonds: the three C–O bonds within the carbonate ion (CO₃²⁻), forming a rigid, symmetric internal framework. Ionic bond: the electrostatic attraction between Ca²⁺ and CO₃²⁻. The rigid covalent unit does not deform, and the strong ionic attraction locks these units into a dense, interlocking crystal lattice - exceptionally stable and difficult to dissolve.

9. Sodium (Group 1) has one valence electron and donates it to achieve a full outer shell, producing Na⁺. Calcium (Group 2) has two valence electrons and must donate both to reach stability, producing Ca²⁺. Ion charge directly reflects the number of valence electrons an atom must lose.

10. Oxygen carries two lone electron pairs that repel the bonding pairs shared with hydrogen, forcing the molecule into a bent shape (104.5°) rather than a straight line. This asymmetry means the two polar O–H bonds do not cancel, making the molecule permanently polar. The permanent separation of partial positive and partial negative charge is the “stored electrical tension” - it is locked into the geometry by the lone pairs. This is the basis of hydrogen bonding in water.

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⟵ 01 Building Blocks (Problem Set) · Contents · 02 The Hero of Heat Transfer ⟶