AP Chemistry › Unit 2
Why do some atoms share electrons while others transfer them? This unit covers ionic, covalent, and metallic bonding, potential energy curves, and the structure of ionic solids, metals, and alloys. The exam asks you to draw Lewis diagrams, use formal charge and resonance, and predict shape, bond angle, hybridization, and polarity with VSEPR. Common errors include miscounting valence electrons and breaking the octet rule where it applies. Students also mix up a molecule's geometry with its bond angles, or forget that lone pairs change both.
These are taken from the Regular set, with the answer and explanation shown.
1. The H–F bond has an electronegativity difference of about 1.8, which is large. Yet HF is a gas made of separate molecules. How is the H–F bond best classified?
Answer: D. H and F are both nonmetals, and HF exists as molecules. That shows the atoms share electrons, not transfer them. F pulls harder, so the sharing is unequal: polar covalent.
Common trap: Choosing ionic from the number alone. The observed properties (a molecular gas) show the electrons are shared.
2. A copper sheet bends when struck with a hammer, but a crystal of CuCl₂ shatters. Which statement explains the difference?
Answer: C. In a metal, the electron sea surrounds the cations. When layers slide, the cations stay surrounded by electrons, so the metal bends. In an ionic solid, sliding puts like charges side by side, and they repel.
Common trap: Choosing "ionic bonds are too strong to bend." The issue is not strength. It is the repulsion created when like charges line up.
3. Acrylonitrile has the structure H₂C=CH–C≡N. How many sigma (σ) and pi (π) bonds are in one molecule?
Answer: A. There are six bonded pairs of atoms: three C–H, C=C, C–C and C≡N. Each has one σ bond, so 6 σ. The C=C adds 1 π and the C≡N adds 2 π, so 3 π.
Common trap: Choosing 9 σ by counting every line as a σ bond. Only the first bond between two atoms is σ.